Datasheet LTC2411, LTC2411-1 Datasheet (LINEAR TECHNOLOGY)

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LTC2411/LTC2411-1

FEATURES

■

24-Bit ADC in an MS10 Package

■

Low Supply Current (200µA in Conversion Mode and 4µA in Autosleep Mode)

■

Differential Input and Differential Reference with GND to VCC Common Mode Range

■

2ppm INL, No Missing Codes

■

4ppm Full-Scale Error and 1ppm Offset

■

0.29ppm Noise

■

No Latency: Digital Filter Settles in a Single Cycle. Each Conversion Is Accurate, Even After an Input Step

■

Single Supply 2.7V to 5.5V Operation

■

Internal Oscillator—No External Components Required

■

110dB Min, Pin Selectable 50Hz/60Hz Notch Filter (LTC2411)

■

Simultaneous 50Hz/60Hz Rejection (LTC2411-1)

APPLICATIO S

■

Direct Sensor Digitizer

■

Weight Scales

■

Direct Temperature Measurement

■

Gas Analyzers

■

Strain Gauge Transducers

■

Instrumentation

■

Data Acquisition

■

Industrial Process Control

■

6-Digit DVMs

24-Bit No Latency ∆Σ

ADC

with Differential Input and

Reference in MSOP

DESCRIPTIO

The LTC®2411/LTC2411-1 are 2.7V to 5.5V micropower 24-bit differential ∆Σ analog-to-digital converters with an integrated oscillator, 2ppm INL and 0.29ppm RMS noise. They use delta-sigma technology and provide single cycle settling time for multiplexed applications. Through a single pin, the LTC2411 can be configured for better than 110dB differential mode rejection at 50Hz or 60Hz ±2%, and the LTC2411-1 can provide better than 87dB input differential mode rejection over the range of 49Hz to

61.2Hz, or they can be driven by an external oscillator for a user-defined rejection frequency. The LTC2411 and LTC2411-1 are identical when driven by an external oscillator. The internal oscillator requires no external frequency setting components.

The converters accept any external differential reference voltage from 0.1V to VCC for flexible ratiometric and remote sensing measurement configurations. The full-

scale differential input range is from –0.5V The reference common mode voltage, V input common mode voltage, V

, may be indepen-

INCM

dently set anywhere within the GND to VCC range of the LTC2411/LTC2411-1. The DC common mode input rejection is better than 140dB.

The LTC2411/LTC2411-1 communicate through a flexible 3-wire digital interface that is compatible with SPI and MICROWIRETM protocols.

, LTC and LT are registered trademarks of Linear Technology Corporation.

No Latency ∆Σ is a trademark of Linear Technology Corporation. MICROWIRE is a trademark of National Semiconductor Corporation.

to 0.5V

REF

REFCM

REF

, and the

TYPICAL APPLICATIO

2.7V TO 5.5V

1µF

110

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

TO 0.5V

–0.5V

REF

GND

LTC2411/

LTC2411-1

–

SCK

SDO

2411 TA01

= INTERNAL OSC/50Hz REJECTION (LTC2411) = EXTERNAL CLOCK SOURCE = INTERNAL OSC/60Hz REJECTION (LTC2411) = SIMULTANEOUS 50Hz/60Hz REJECTION (LTC2411-1)

3-WIRE SPI INTERFACE

BRIDGE

IMPEDANCE

100Ω TO 10kΩ

1µF

REF+V

LTC2411/

–

LTC2411-1

–

REF

GND F

610

2411 TA02

SCK

3-WIRE

SDO

SPI INTERFACE

LTC2411/LTC2411-1

1 2 3 4 5

REF

–

10 9 8 7 6

SCK SDO CS GND

TOP VIEW

MS10 PACKAGE

10-LEAD PLASTIC MSOP

WWWU

ABSOLUTE AXI U RATI GS

(Notes 1, 2)

Supply Voltage (VCC) to GND.......................–0.3V to 7V

Analog Input Pins Voltage

to GND.................................... –0.3V to (VCC + 0.3V)

Reference Input Pins Voltage

to GND.................................... –0.3V to (VCC + 0.3V)

Digital Input Voltage to GND........ –0.3V to (VCC + 0.3V)

Digital Output Voltage to GND ..... –0.3V to (VCC + 0.3V)

Operating Temperature Range

LTC2411C ............................................... 0°C to 70°C

LTC2411I............................................ –40°C to 85°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec)..................300°C

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)

The ● denotes specifications which apply over the full operating

PACKAGE/ORDER I FOR ATIO

Consult LTC Marketing for parts specified with wider operating temperature ranges.

= 125°C, θJA = 120°C/W

JMAX

ORDER PART NUMBER

LTC2411CMS LTC2411IMS LTC2411-1CMS LTC2411-1IMS

MS10 PART MARKING

LTNS LTNT LTWV LTNN

PARAMETER CONDITIONS MIN TYP MAX UNITS

Resolution (No Missing Codes) 0.1V ≤ V Integral Nonlinearity 4.5V ≤ VCC ≤ 5.5V, REF+ = 2.5V, REF– = GND, V

5V ≤ VCC ≤ 5.5V, REF+ = 5V, REF– = GND, V REF+ = 2.5V, REF– = GND, V

Offset Error 2.5V ≤ REF+ ≤ VCC, REF– = GND, ● 520 µV

GND ≤ IN

Offset Error Drift 2.5V ≤ REF+ ≤ VCC, REF– = GND, 20 nV/°C

GND ≤ IN

Positive Full-Scale Error 2.5V ≤ REF+ ≤ VCC, REF– = GND, ● 4 12 ppm of V

IN+ = 0.75REF+, IN– = 0.25 • REF

Positive Full-Scale Error Drift 2.5V ≤ REF+ ≤ VCC, REF– = GND, 0.04 ppm of V

Negative Full-Scale Error 2.5V ≤ REF+ ≤ VCC, REF– = GND, ● 4 12 ppm of V

Negative Full-Scale Error Drift 2.5V ≤ REF+ ≤ VCC, REF– = GND, 0.04 ppm of V

Total Unadjusted Error 4.5V ≤ VCC ≤ 5.5V, REF+ = 2.5V, REF– = GND, V

Output Noise 5V ≤ VCC ≤ 5.5V, REF+ = 5V, V

IN+ = 0.25 • REF+, IN– = 0.75 • REF

5V ≤ VCC ≤ 5.5V, REF+ = 5V, REF– = GND, V REF+ = 2.5V, REF– = GND, V

GND ≤ IN– = IN+ ≤ 5V, (Note 13)

≤ VCC, –0.5 • V

REF

= IN– ≤ VCC (Note 14)

= IN– ≤ V

= 0.75REF+, IN– = 0.25 • REF

= 0.25 • REF+, IN– = 0.75 • REF

≤ VIN ≤ 0.5 • V

REF

= 1.25V (Note 6) 6 ppm of V

INCM

= 1.25V 6 ppm of V

INCM

REF

(Note 5) ● 24 Bits

REF

= 1.25V (Note 6) 1 ppm of V

INCM

= 2.5V (Note 6) ● 2 14 ppm of V

INCM

REF

= 1.25V 3 ppm of V

INCM

= 2.5V 3 ppm of V

INCM

– = GND, 1.45 µV

REF

REF REF REF

REF

/°C

REF

/°C

REF REF REF

RMS

LTC2411/LTC2411-1

CO VERTER CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)

PARAMETER CONDITIONS MIN TYP MAX UNITS

Input Common Mode Rejection DC 2.5V ≤ REF+ ≤ VCC, REF– = GND, ● 130 140 dB

GND ≤ IN

Input Common Mode Rejection 2.5V ≤ REF+ ≤ VCC, REF– = GND, ● 140 dB 60Hz ±2% (LTC2411) GND ≤ IN

Input Common Mode Rejection 2.5V ≤ REF+ ≤ VCC, REF– = GND, ● 140 dB 50Hz ±2% (LTC2411) GND ≤ IN

Input Common Mode Rejection 2.5V < REF+ < VCC, REF– = GND, ● 140 dB 49Hz to 61.2Hz (LTC2411-1) GND < IN

Input Normal Mode Rejection (Note 7) ● 110 140 dB 60Hz ±2% (LTC2411)

Input Normal Mode Rejection (Note 8) ● 110 140 dB 50Hz ±2% (LTC2411)

Input Normal Mode Rejection (Note 15) ● 87 dB 49Hz to 61.2Hz (LTC2411-1)

Reference Common Mode 2.5V ≤ REF+ ≤ VCC, GND ≤ REF– ≤ 2.5V, ● 130 140 dB Rejection DC V

Power Supply Rejection, DC REF+ = 2.5V, REF– = GND, IN– = IN+ = GND 110 dB Power Supply Rejection, 60Hz ±2% REF+ = 2.5V, REF– = GND, IN– = IN+ = GND, (Note 7) 120 dB

(LTC2411) Power Supply Rejection, 50Hz ±2% REF+ = 2.5V, REF– = GND, IN– = IN+ = GND, (Note 8) 120 dB

(LTC2411) Power Supply Rejection, REF+ = 2.5V, REF– = GND, IN– = IN+ = GND, (Note 15) 120 dB

49Hz to 61.2Hz (LTC2411-1)

–

= IN+ ≤ 5V

–

= IN+ ≤ 5V, (Note 7)

–

= IN+ ≤ 5V, (Note 8)

–

= IN+ < VCC (Note 15)

= 2.5V, IN– = IN+ = GND

REF

The ● denotes specifications which apply over the full operating

A ALOG I PUT A D REFERE CE

The ● denotes specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. (Note 3)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

–

IN V

REF

–

REF V

REF

CS (IN+)IN CS (IN–)IN CS (REF+)REF CS (REF–)REF I I I I

(IN+)IN+ DC Leakage Current CS = VCC = 5.5V, IN+ = GND ● –10 1 10 nA

DC_LEAK

(IN–)IN– DC Leakage Current CS = VCC = 5.5V, IN– = GND ● –10 1 10 nA

DC_LEAK

(REF+)REF+ DC Leakage Current CS = VCC = 5.5V, REF+ = 5V ● –10 1 10 nA

DC_LEAK

(REF–)REF– DC Leakage Current CS = VCC = 5.5V, REF– = GND ● –10 1 10 nA

DC_LEAK

Absolute/Common Mode IN+ Voltage ● GND – 0.3V VCC + 0.3V V Absolute/Common Mode IN– Voltage ● GND – 0.3V VCC + 0.3V V Input Differential Voltage Range ● –V

– IN–)

(IN Absolute/Common Mode REF+ Voltage ● 0.1 V Absolute/Common Mode REF– Voltage ● GND VCC – 0.1V V Reference Differential Voltage Range ● 0.1 V

– REF–)

(REF

Sampling Capacitance 6 pF

–

Sampling Capacitance 6 pF

–

Sampling Capacitance 6 pF

/2 V

REF

/2 V

REF

LTC2411/LTC2411-1

DIGITAL I PUTS A D DIGITAL OUTPUTS

operating temperature range, otherwise specifications are at TA = 25°C. (Note 3)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

High Level Input Voltage 2.7V ≤ VCC ≤ 5.5V ● 2.5 V CS, F

Low Level Input Voltage 4.5V ≤ VCC ≤ 5.5V ● 0.8 V CS, F

High Level Input Voltage 2.7V ≤ VCC ≤ 5.5V (Note 9) ● 2.5 V SCK 2.7V ≤ V

Low Level Input Voltage 4.5V ≤ VCC ≤ 5.5V (Note 9) ● 0.8 V SCK 2.7V ≤ V

Digital Input Current 0V ≤ VIN ≤ V CS, F

Digital Input Current 0V ≤ VIN ≤ VCC (Note 9) ● –10 10 µA SCK

Digital Input Capacitance 10 pF CS, F

Digital Input Capacitance (Note 9) 10 pF SCK

High Level Output Voltage IO = –800µA ● VCC – 0.5V V SDO

Low Level Output Voltage IO = 1.6mA ● 0.4 V SDO

High Level Output Voltage IO = –800µA (Note 10) ● VCC – 0.5V V SCK

Low Level Output Voltage IO = 1.6mA (Note 10) ● 0.4 V SCK

Hi-Z Output Leakage ● –10 10 µA SDO

2.7V ≤ VCC ≤ 3.3V 2.0 V

2.7V ≤ VCC ≤ 5.5V 0.6 V

≤ 3.3V (Note 9) 2.0 V

≤ 5.5V (Note 9) 0.6 V

The ● denotes specifications which apply over the full

● –10 10 µA

POWER REQUIRE E TS

otherwise specifications are at TA = 25°C. (Note 3)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Supply Voltage ● 2.7 5.5 V Supply Current

Conversion Mode CS = 0V (Note 12) Sleep Mode CS = V

Sleep Mode CS = V

The ● denotes specifications which apply over the full operating temperature range,

● 200 300 µA

(Note 12) ● 410 µA

, 2.7V ≤ VCC ≤ 3.3V (Note 12) 2 µA

LTC2411/LTC2411-1

TI I G CHARACTERISTICS

range, otherwise specifications are at TA = 25°C. (Note 3)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

EOSC

HEO

LEO

CONV

ISCK

ESCK

LESCK

HESCK

DOUT_ISCK

DOUT_ESCK

t2 CS ↑ to SDO High Z ● 0 200 ns t3 CS ↓ to SCK ↓ (Note 10) ● 0 200 ns t4 CS ↓ to SCK ↑ (Note 9) ● 50 ns t

KQMAX

KQMIN

External Oscillator Frequency Range ● 2.56 2000 kHz External Oscillator High Period ● 0.25 390 µs External Oscillator Low Period ● 0.25 390 µs Conversion Time FO = 0V (LTC2411) ● 130.86 133.53 136.20 ms

Internal SCK Frequency Internal Oscillator (LTC2411) (Note 10) 19.2 kHz

Internal SCK Duty Cycle (Note 10) ● 45 55 % External SCK Frequency Range (Note 9) ● 2000 kHz External SCK Low Period (Note 9) ● 250 ns External SCK High Period (Note 9) ● 250 ns Internal SCK 32-Bit Data Output Time Internal Oscillator (LTC2411) (Notes 10, 12) ● 1.64 1.67 1.70 ms

External SCK 32-Bit Data Output Time (Note 9) ● 32/f CS ↓ to SDO Low Z ● 0 200 ns

SCK ↓ to SDO Valid ● 220 ns SDO Hold After SCK ↓ (Note 5) ● 15 ns SCK Set-Up Before CS ↓ ● 50 ns SCK Hold After CS ↓ ● 50 ns

The ● denotes specifications which apply over the full operating temperature

= VCC (LTC2411) ● 157.03 160.23 163.44 ms

= 0V (LTC2411-1) ● 143.78 146.71 149.64 ms

External Oscillator (Note 11)

Internal Oscillator (LTC2411-1) (Note 10) 17.5 kHz External Oscillator (Notes 10, 11) f

Internal Oscillator (LTC2411-1) (Notes 10, 12) External Oscillator (Notes 10, 11)

● 20510/f

● 1.80 1.83 1.86 ms

● 256/f

(in kHz) ms

EOSC

/8 kHz

EOSC

(in kHz) ms

EOSC

(in kHz) ms

ESCK

Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired.

Note 2: All voltage values are with respect to GND. Note 3: VCC = 2.7 to 5.5V unless otherwise specified.

= REF+ – REF–, V

REF

= IN+ – IN–, V

INCM

= (REF+ + REF–)/2;

REFCM

= (IN+ + IN–)/2.

Note 4: FO pin tied to GND or to VCC or to external conversion clock source with f

= 153600Hz unless otherwise specified.

EOSC

Note 5: Guaranteed by design, not subject to test. Note 6: Integral nonlinearity is defined as the deviation of a code from

a straight line passing through the actual endpoints of the transfer curve. The deviation is measured from the center of the quantization band.

Note 7: FO = 0V (internal oscillator) or f

= 153600Hz ±2%

EOSC

(external oscillator).

Note 8: F

= VCC (internal oscillator) or f

= 128000Hz ±2%

EOSC

(external oscillator). Note 9: The converter is in external SCK mode of operation such that

the SCK pin is used as digital input. The frequency of the clock signal driving SCK during the data output is f

and is expressed in kHz.

ESCK

Note 10: The converter is in internal SCK mode of operation such that the SCK pin is used as digital output. In this mode of operation the SCK pin has a total equivalent load capacitance C

Note 11: The external oscillator is connected to the F oscillator frequency, f

, is expressed in kHz.

EOSC

= 20pF.

LOAD

pin. The external

Note 12: The converter uses the internal oscillator. F

= 0V or FO = VCC.

Note 13: The output noise includes the contribution of the internal calibration operations.

Note 14: Guaranteed by design and test correlation. Note 15: F

= 0V (internal oscillator) or f

= 139800Hz ±2%

EOSC

(external oscillator).

LTC2411/LTC2411-1

TYPICAL PERFOR A CE CHARACTERISTICS

Total Unadjusted Error (VCC = 5V, V

)

REF

–1

TUE (ppm OF V

VCC = 5V

= 5V

REF

–

= GND

REF

–2

–3

= 2.5V

INCM

= GND

–2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 2 2.5

REF

= 5V)

TA = 90°C

TA = 25°C

TA = –45°C

VIN (V)

Integral Nonlinearity (VCC = 5V, V

VCC = 5V

= 5V

REF

–

= GND

REF

= 2.5V

INCM

= GND

)

REF

–1

INL (ppm OF V

–2

–3

–2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 2 2.5

REF

= 5V)

TA = 25°C

TA = 90°C

TA = –45°C

VIN (V)

2411 G01

2411 G04

Total Unadjusted Error (VCC = 5V, V

1.5

1.0

)

0.5

REF

–0.5

TUE (ppm OF V

VCC = 5V

= 2.5V

REF

–

= GND

REF

–1.0

–1.5

= 2.5V

INCM

= GND

–1.25 –0.75 –0.25 0.25 0.75 1.25

REF

VIN (V)

= 2.5V)

TA = 25°C

TA = –45°C

TA = 90°C

Integral Nonlinearity (VCC = 5V, V

1.5

1.0

)

0.5

REF

–0.5

INL (ppm OF V

VCC = 5V

= 2.5V

REF

–

= GND

REF

–1.0

–1.5

= 2.5V

INCM

= GND

–1.25 –0.75 –0.25 0.25 0.75 1.25

REF

VIN (V)

= 2.5V)

TA = 25°C

TA = –45°C

TA = 90°C

2411 G02

2411 G01

Total Unadjusted Error (VCC = 2.7V, V

8 6

)

REF

2 0

–2

TUE (ppm OF V

VCC = 2.7V

–4

= 2.5V

REF

–

–6

= GND

REF

= 1.25V

INCM

–8

= GND

–10

–1.25

–0.75

Integral Nonlinearity (VCC = 2.7V, V

8 6

)

REF

2 0

–2

INL (ppm OF V

VCC = 2.7V

–4

= 2.5V

REF

–

–6

= GND

REF

= 1.25V

INCM

–8

= GND

–10

–1.25

–0.75

–0.25

= 2.5V)

REF

TA = –45°C

TA = 25°C

VIN (V)

= 2.5V)

REF

TA = –45°C

TA = 25°C

VIN (V)

0.25

TA = 90°C

0.75

TA = 90°C

0.75

1.25

2411 G03

1.25

2411 G06

Noise Histogram

10,000 CONSECUTIVE READINGS

= 5V

REF

= 0V

= 2.5V

INCM

= GND

= 25°C

NUMBER OF READINGS (%)

–1.5 –1.0 0

–2.0

OUTPUT CODE (ppm OF V

–0.5

GAUSSIAN DISTRIBUTION m = –0.647ppm σ = 0.287ppm

0.5 )

REF

2411 G07

)

REF

–0.5

–1.0

ADC READING (ppm OF V

–1.5

–2.0

Long Term ADC Readings

1.0 VCC = 5V, V

= GND, TA = 25°C, RMS NOISE = 0.29ppm

0.5

REF

10 15 20

= 5V, VIN = 0V, V

25 30 35 40 45 50 55 60

TIME (HOURS)

INCM

= 2.5V,

2411 G08

RMS Noise vs Input Differential Voltage

0.5

0.4

)

REF

0.3

0.2

TA = 25°C

= 5V

RMS NOISE (ppm OF V

0.1 = 5V

REF

= 2.5V

INCM

= GND

–2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 2 2.5

INPUT DIFFERENTIAL VOLTAGE (V)

2411 G09

VCC (V)

2.7

RMS NOISE (µV)

1.50

1.55

1.60

3.9 4.7

2411 G12

1.45

1.40

3.1 3.5

4.3 5.1 5.5

1.35

1.30

REF+ = 2.5V REF

–

= GND

= 0V

= GND

= 25°C

TEMPERATURE (°C)

–45

–3

FULL-SCALE ERROR (ppm OF V

REF

)

–2

–15

30 90

2411 G18

–1

–30 0

VCC = 5V REF

= 5V

REF

–

= GND

= 2.5V

–

= GND

TYPICAL PERFOR A CE CHARACTERISTICS

LTC2411/LTC2411-1

RMS Noise vs V

1.60 VCC = 5V

= 5V

REF REF

–1

–

= GND

RMS NOISE (µV)

1.55

1.50

1.45

1.40

1.35

1.30

RMS Noise vs V

1.60

VCC = 5V

–

= GND

REF

1.55

= 0V

= GND

= 25°C

1.50

1.45

RMS NOISE (µV)

1.40

1.35

1.30

INCM

= 0V

= GND

= 25°C

356

(V)

INCM

2411 G10

REF

1234

(V)

REF

2411 G13

1.60

1.55

1.50

1.45

1.40

RMS NOISE (µV)

1.35

1.30

–0.1

)

–0.2

REF

–0.3 –0.4 –0.5 –0.6 –0.7 –0.8

OFFSET ERROR (ppm OF V

–0.9

–1.0

RMS Noise vs Temperature RMS Noise vs V

VCC = 5V V

= 5V

REF

= 0V

= GND

INCM

= GND

–30 0

–15

–45

Offset Error vs V

VCC = 5V

REF

= 5V

–

REF

= GND

= 0V

= GND

= 25°C

02 5

–1

TEMPERATURE (°C)

INCM

30 90

(V)

2411 G11

2411 G14

Offset Error vs Temperature

VCC = 5V

–0.1

= 5V

REF

)

= 0V

–0.2

INCM

= GND

–30 0

–45

= GND

–15

TEMPERATURE (°C)

REF

–0.3 –0.4 –0.5 –0.6 –0.7 –0.8

OFFSET ERROR (ppm OF V

–0.9 –1.0

30 90

2411 G15

Offset Error vs V

REF+ = 2.5V

–

0.8

REF

)

REF

–0.2 –0.4 –0.6

OFFSET ERROR (ppm OF V

–0.8 –1.0

= GND

= 0V

0.6 V

INCM

= GND

0.4

= 25°C

0.2

3.1 3.9 5.1

2.7

= GND

3.5 VCC (V)

4.3

4.7 5.5

2411 G16

Offset Error vs V

0.8

)

0.6

REF

0.4

0.2 0

–0.2 –0.4 –0.6

OFFSET ERROR (ppm OF V

–0.8

–1.0

REF

(V)

REF

VCC = 5V REF– = GND V

= 0V

= GND

INCM

= GND

= 25°C

2411 G17

+Full-Scale Error vs Temperature

LTC2411/LTC2411-1

FREQUENCY AT VCC (Hz)

7600

–60

–40

7750

2411 G24

–80

–100

7650 7700 7800

–120

–140

–20

REJECTION (dB)

VCC = 4.1V DC ±0.7V

P-P

REF+ = 2.5V REF

–

= GND

–

= GND

= 25°C

FREQUENCY AT VCC (Hz)

6880

–60

–40

7030

2411 G33

–80

–100

6930 6980 7080

–120

–140

–20

REJECTION (dB)

VCC = 4.1V DC ±0.7V REF

= 2.5V

REF

–

= GND

–

= GND

= 25°C

TYPICAL PERFOR A CE CHARACTERISTICS

+Full-Scale Error vs Temperature

VCC = 2.7V

REF

3 2 1

0 –1 –2 –3 –4 –5

–45

= 2.5V

–

= GND

REF

= 1.25V

–

= GND

IN F

= GND

–30 0

–15

30 90

TEMPERATURE (°C)

)

REF

FULL-SCALE ERROR (ppm OF V

PSRR vs Frequency at V (LTC2411)

VCC = 4.1V DC

= 2.5V

REF

–20

–

REF

= GND

–

–40

= GND

= 25°C

–60

–Full-Scale Error vs Temperature

)

REF

VCC = 5V

–1

= 5V

REF

–

= GND

REF

= GND

–2

–

= 2.5V

–FULL-SCALE ERROR (ppm OF V

= GND

2411 G19

–3

–30 0

–15

–45

TEMPERATURE (°C)

PSRR vs Frequency at V

30 90

2411 G20

(LTC2411)

VCC = 4.1V DC ±1.4V

= 2.5V

REF

–40

–60

REF

–

= GND

= 25°C

–

= GND = GND = GND

–20

–Full-Scale Error vs Temperature

5 4

)

REF

3 2 1 0

–1

VCC = 2.7V

–2

= 2.5V

REF

–

REF

= GND

–3

= GND

–

–FULL-SCALE ERROR (ppm OF V

–4 –5

–45

= 1.25V

= GND

–30 0

–15

30 90

TEMPERATURE (°C)

PSRR vs Frequency at V (LTC2411)

2411 G21

–80

REJECTION (dB)

–100

–120

–140

10 100

FREQUENCY AT VCC (Hz)

PSRR vs Frequency at V (LTC2411-1)

VCC = 4.1V DC

REF

REF IN IN F

–

= GND

–

= GND

= GND = 25°C

= 2.5V = GND

10 100

FREQUECY AT VCC (Hz)

–20

–40

–60

–80

REJECTION (dB)

–100

–120

–140

10k 1M

1k 100k

10k 1M

1k 100k

2411 G22

2411 G31

–80

REJECTION (dB)

–100

–120

–140

30 60

FREQUENCY AT VCC (Hz)

PSRR vs Frequency at V (LTC2411-1)

VCC = 4.1V DC ±1.4V

= 2.5V

REF

–20

–

= GND

REF

= GND

–

–40

= GND

= 25°C

–60

–80

REJECTION (dB)

–100

–120

–140

40 60 120 160

20 100 140

80 200180 220

FREQUENCY AT VCC (Hz)

150 210

120 180

2411 G23

2411 G32

240

PSRR vs Frequency at V (LTC2411-1)

OUTPUT DATA RATE (READINGS/SEC)

RESOLUTION (BITS)

20 40 60 80

2411 G30

10010030507090

VCC = 5V REF

–

= GND

INCM

= 2.5V

= 0V

= EXT OSC

RES = LOG

2(VREF

/INL

MAX

)

= 25°C

REF

= 2.5V

REF

= 5V

TYPICAL PERFOR A CE CHARACTERISTICS

LTC2411/LTC2411-1

Conversion Current vs Temperature

240

FO = GND CS = GND

230

SCK = NC SDO = NC

220

210

200

190

180

CONVERSION CURRENT (µA)

170

160

–30 90

–45

–15

TEMPERATURE (°C)

VCC = 5.5V

VCC = 2.7V

Offset Error vs Output Data Rate

)

REF

–20

–40

–60

VCC = 5V

–

= GND

REF

–80

OFFSET ERROR (ppm OF V

–100

–120

= 2.5V

INCM

= 0V

= EXT OSC

= 25°C

20 40 60 80

OUTPUT DATA RATE (READINGS/SEC)

REF

VCC = 5V

VCC = 3V

= 2.5V

= 5V

REF

Conversion Current vs Output Data Rate

650

REF+ = V

600 550 500 450 400 350 300

SUPPLY CURRENT (µA)

250 200

2411 G25

150

REF– = GND

= GND

–

= GND

IN T

= 25°C

SCK = NC SDO = NC CS = GND F

= EXT OSC

2010

OUTPUT DATA RATE (READINGS/SEC)

4030

Resolution (NOISE

= 5V

VCC = 3V

60 70 90

RMS

≤ 1LSB)

100

2411 G26

vs Output Data Rate

= 5V

REF

= 2.5V

REF

VCC = 5V

–

= GND

REF

10010030507090

2411 G28

RESOLUTION (BITS)

= 2.5V

INCM

= 0V

= EXT OSC

RES = LOG T

= 25°C

OUTPUT DATA RATE (READINGS/SEC)

/NOISE

2(VREF

)

RMS

2411 G29

100

Sleep Mode Current vs Temperature

SLEEP MODE CURRENT (µA)

–30 90

–45

–15

Resolution (INL

TEMPERATURE (°C)

MAX

vs Output Data Rate

FO = GND CS = V

SCK = NC SDO = NC

VCC = 5.5V

VCC = 5V

VCC = 3V

VCC = 2.7V

≤ 1LSB)

2411 G27

PI FU CTIO S

VCC (Pin 1): Positive Supply Voltage. Bypass to GND (Pin␣ 6) with a 10µF tantalum capacitor in parallel with

0.1µF ceramic capacitor as close to the part as possible. REF+ (Pin 2), REF– (Pin 3): Differential Reference Input.

The voltage on these pins can have any value between GND and VCC as long as the reference positive input, REF+, is more positive than the reference negative input, REF–, by at least 0.1V.

IN+ (Pin 4), IN– (Pin 5): Differential Analog Input. The voltage on these pins can have any value between

GND – 0.3V and VCC + 0.3V. Within these limits, the converter bipolar input range (VIN = IN+ – IN–) extends from –0.5 • (V range, the converter produces unique overrange and underrange output codes.

GND (Pin 6): Ground. Connect this pin to a ground plane through a low impedance connection.

CS (Pin 7): Active LOW Digital Input. A LOW on this pin enables the SDO digital output and wakes up the ADC. Following each conversion the ADC automatically enters

) to 0.5 • (V

REF

). Outside this input

REF

LTC2411/LTC2411-1

PI FU CTIO S

the Sleep mode and remains in this low power state as long as CS is HIGH. A LOW-to-HIGH transition on CS during the Data Output transfer aborts the data transfer and starts a new conversion.

SDO (Pin 8): Three-State Digital Output. During the Data Output period, this pin is used as the serial data output. When the chip select CS is HIGH (CS = VCC), the SDO pin is in a high impedance state. During the Conversion and Sleep periods, this pin is used as the conversion status output. The conversion status can be observed by pulling CS LOW.

SCK (Pin 9): Bidirectional Digital Clock Pin. In Internal Serial Clock Operation mode, SCK is used as the digital output for the internal serial interface clock during the Data Output period. In External Serial Clock Operation mode, SCK is used as the digital input for the external serial interface clock during the Data Output period. A weak

internal pull-up is automatically activated in Internal Serial Clock Operation mode. The Serial Clock Operation mode is determined by the logic level applied to the SCK pin at power up or during the most recent falling edge of CS.

FO (Pin 10): Frequency Control Pin. Digital input that controls the ADC’s notch frequencies and conversion time. For the LTC2411, when the FO pin is connected to V

(FO = VCC), the converter uses its internal oscillator and the digital filter first null is located at 50Hz. When the FO pin is connected to GND (FO = OV), the converter uses its internal oscillator and the digital filter first null is located at 60Hz. For the LTC2411-1, the converter provides simultaneous 50Hz/60Hz rejection with the FO pin connected to GND. When FO is driven by an external clock signal with a frequency f

, the converters use this signal as their

EOSC

system clock and the digital filter first null is located at a frequency f

EOSC

/2560.

FU CTIO AL BLOCK DIAGRA

GND

REF

–

+ –

–+

DAC

∫∫∫

TEST CIRCUITS

SDO

1.69k

Hi-Z TO V VOL TO V VOH TO Hi-Z

= 20pF

LOAD

2411 TA03

∑

Figure 1

ADC

AUTOCALIBRATION

AND CONTROL

DECIMATING FIR

SDO

Hi-Z TO V VOH TO V VOL TO Hi-Z

INTERNAL

OSCILLATOR

(INT/EXT)

SDO

SERIAL

INTERFACE

1.69k

= 20pF

LOAD

2411 TA04

OL OL

SCK

2411 FD

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

CONVERTER OPERATION

Converter Operation Cycle

The LTC2411/LTC2411-1 are low power, delta-sigma analog-to-digital converters with an easy-to-use 3-wire serial interface (see Figure 1). Their operation is made up of three states. The converter operating cycle begins with the conversion, followed by the low power sleep state and ends with the data output (see Figure 2). The 3-wire interface consists of serial data output (SDO), serial clock (SCK) and chip select (CS).

Initially, the LTC2411/LTC2411-1 perform a conversion. Once the conversion is complete, the devices enter the sleep state. While in this sleep state, power consumption is reduced by an order of magnitude. The parts remain in the sleep state as long as CS is HIGH. The conversion result is held indefinitely in a static shift register while the converter is in the sleep state.

Once CS is pulled LOW, the devices begin outputting the conversion result. There is no latency in the conversion result. The data output corresponds to the conversion just performed. This result is shifted out on the serial data out pin (SDO) under the control of the serial clock (SCK). Data is updated on the falling edge of SCK allowing the user to reliably latch data on the rising edge of SCK (see Figure 3). The data output state is concluded once 32 bits are read out of the ADC or when CS is brought HIGH. The devices automatically initiate a new conversion and the cycle repeats.

CONVERT

SLEEP

FALSE

CS = LOW

AND

SCK

TRUE

DATA OUTPUT

2411 F02

Figure 2. LTC2411/LTC2411-1 State Transition Diagram

Through timing control of the CS and SCK pins, the LTC2411/LTC2411-1 offer several flexible modes of operation (internal or external SCK and free-running conversion modes). These various modes do not require programming configuration registers; moreover, they do not disturb the cyclic operation described above. These modes of operation are described in detail in the Serial Interface Timing Modes section.

Conversion Clock

A major advantage the delta-sigma converter offers over conventional type converters is an on-chip digital filter (commonly implemented as a Sinc or Comb filter). For high resolution, low frequency applications, this filter is typically designed to reject line frequencies of 50 or 60Hz plus their harmonics. The filter rejection performance is directly related to the accuracy of the converter system clock. The LTC2411/LTC2411-1 incorporate a highly accurate on-chip oscillator. This eliminates the need for external frequency setting components such as crystals or oscillators. Clocked by the on-chip oscillator, the LTC2411 achieves a minimum of 110dB rejection at the line frequency (50Hz or 60Hz ±2%) and the LTC2411-1 achieves a minimum of 87dB rejection over 49Hz to 61.2Hz.

Ease of Use

The LTC2411/LTC2411-1 data output has no latency, filter settling delay or redundant data associated with the conversion cycle. There is a one-to-one correspondence between the conversion and the output data. Therefore, multiplexing multiple analog voltages is easy.

The LTC2411/LTC2411-1 perform offset and full-scale calibrations in every conversion cycle. This calibration is transparent to the user and has no effect on the cyclic operation described above. The advantage of continuous calibration is extreme stability of offset and full-scale readings with respect to time, supply voltage change and temperature drift.

Power-Up Sequence

The LTC2411/LTC2411-1 automatically enter an internal reset state when the power supply voltage VCC drops below approximately 1.9V. This feature guarantees the

LTC2411/LTC2411-1

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APPLICATIO S I FOR ATIO

integrity of the conversion result and of the serial interface mode selection. (See the 2-wire I/O sections in the Serial Interface Timing Modes section.)

When the VCC voltage rises above this critical threshold, the converter creates an internal power-on-reset (POR) signal with a duration of approximately 1ms. The POR signal clears all internal registers. Following the POR signal, the LTC2411/LTC2411-1 start a normal conversion cycle and follow the succession of states described above. The first conversion result following POR is accurate within the specifications of the device if the power supply voltage is restored within the operating range (2.7V to

5.5V) before the end of the POR time interval.

Reference Voltage Range

The LTC2411/LTC2411-1 accept a truly differential external reference voltage. The absolute/common mode voltage specification for the REF+ and REF– pins covers the entire range from GND to VCC. For correct converter operation, the REF+ pin must always be more positive than the REF– pin.

The LTC2411/LTC2411-1 can accept a differential reference voltage from 0.1V to VCC. The converter output noise is determined by the thermal noise of the front-end circuits, and, as such, its value in nanovolts is nearly constant with reference voltage. A decrease in reference voltage will not significantly improve the converter’s effective resolution. On the other hand, a reduced reference voltage will improve the converter’s overall INL performance. A reduced reference voltage will also improve the converter performance when operated with an external conversion clock (external FO signal) at substantially higher output data rates.

Input Voltage Range

The analog input is truly differential with an absolute/ common mode range for the IN+ and IN– input pins extending from GND – 0.3V to VCC + 0.3V. Outside these limits, the ESD protection devices begin to turn on and the errors due to input leakage current increase rapidly. Within these limits, the LTC2411/LTC2411-1 convert the bipolar differential input signal, VIN = IN+ – IN–, from –FS = –0.5 • V REF+ – REF–. Outside this range the converter indicates

to +FS = 0.5 • V

REF

where V

REF

the overrange or the underrange condition using distinct output codes.

Input signals applied to IN+ and IN– pins may extend by 300mV below ground and above VCC. In order to limit any fault current, resistors of up to 5k may be added in series with the IN+ and IN– pins without affecting the performance of the device. In the physical layout, it is important to maintain the parasitic capacitance of the connection between these series resistors and the corresponding pins as low as possible; therefore, the resistors should be located as close as practical to the pins. In addition, series resistors will introduce a temperature dependent offset error due to the input leakage current. A 1nA input leakage current will develop a 1ppm offset error on a 5k resistor if V

= 5V. This error has a very strong temperature

REF

dependency.

Output Data Format

The LTC2411/LTC2411-1 serial output data stream is 32 bits long. The first 3 bits represent status information indicating the sign and conversion state. The next 24 bits are the conversion result, MSB first. The remaining 5 bits are sub LSBs beyond the 24-bit level that may be included in averaging or discarded without loss of resolution. The third and fourth bits together are also used to indicate an underrange condition (the differential input voltage is below –FS) or an overrange condition (the differential input voltage is above +FS).

Bit 31 (first output bit) is the end of conversion (EOC) indicator. This bit is available at the SDO pin during the conversion and sleep states whenever the CS pin is LOW. This bit is HIGH during the conversion and goes LOW when the conversion is complete.

Bit 30 (second output bit) is a dummy bit (DMY) and is always LOW.

Bit 29 (third output bit) is the conversion result sign indicator (SIG). If VIN is >0, this bit is HIGH. If VIN is <0, this bit is LOW.

Bit 28 (fourth output bit) is the most significant bit (MSB) of the result. This bit in conjunction with Bit 29 also provides the underrange or overrange indication. If both Bit 29 and Bit 28 are HIGH, the differential input voltage is

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

above +FS. If both Bit 29 and Bit 28 are LOW, the differential input voltage is below –FS.

The function of these bits is summarized in Table 1.

Table 1. LTC2411/LTC2411-1 Status Bits

Bit 31 Bit 30 Bit 29 Bit 28

Input Range EOC DMY SIG MSB

VIN ≥ 0.5 • V 0V ≤ VIN < 0.5 • V –0.5 • V VIN < –0.5 • V

REF

≤ VIN < 0V 0 0 0 1

REF

0011 0010

0000

Bits 28-5 are the 24-bit conversion result MSB first. Bit 5 is the least significant bit (LSB). Bits 4-0 are sub LSBs below the 24-bit level. Bits 4-0 may

be included in averaging or discarded without loss of resolution.

Data is shifted out of the SDO pin under control of the serial clock (SCK), see Figure 3. Whenever CS is HIGH, SDO remains high impedance and any externally generated SCK clock pulses are ignored by the internal data out shift register.

In order to shift the conversion result out of the device, CS must first be driven LOW. EOC is seen at the SDO pin of the device once CS is pulled LOW. EOC changes real time from HIGH to LOW at the completion of a conversion. This signal may be used as an interrupt for an external microcontroller. Bit 31 (EOC) can be captured on the first rising edge of SCK. Bit 30 is shifted out of the device on the first falling edge of SCK. The final data bit (Bit 0) is shifted out on the falling edge of the 31st SCK and may be latched on the rising edge of the 32nd SCK pulse. On the falling edge of the 32nd SCK pulse, SDO goes HIGH indicating the initiation of a new conversion cycle. This bit serves as EOC (Bit 31) for the next conversion cycle. Table 2 summarizes the output data format.

BIT 31

SDO

Hi-Z

SCK

EOC

1 2 3 4 5 262732

SLEEP DATA OUTPUT CONVERSION

BIT 28BIT 29BIT 30

MSBSIG“0”

LSB

BIT 0BIT 27 BIT 5

2411 F03

Figure 3. Output Data Timing

Table 2. LTC2411/LTC2411-1 Output Data Format

Differential Input Voltage Bit 31 Bit 30 Bit 29 Bit 28 Bit 27 Bit 26 Bit 25 … Bit 0 V

* EOC DMY SIG MSB

VIN* ≥ 0.5 • V

0.5 • V

REF

0.25 • V

REF

0.25 • V

REF

0 00100 0 0…0 –1LSB 0 0011 1 1…1 –0.25 • V –0.25 • V –0.5 • V VIN* < –0.5 • V *The differential input voltage VIN = IN+ – IN–.

**The differential reference voltage V

** 00110 0 0…0

REF

** – 1LSB 00101 11…1

** 0 0101 0 0…0 ** – 1LSB 00100 1 1…1

** 00011 0 0…0

REF

** – 1LSB 00010 1 1…1

REF

** 00010 0 0…0

REF

** 00001 1 1…1

REF

= REF+ – REF–.

REF

LTC2411/LTC2411-1

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APPLICATIO S I FOR ATIO

As long as the voltage on the IN+ and IN– pins is maintained within the –0.3V to (VCC + 0.3V) absolute maximum operating range, a conversion result is generated for any differential input voltage VIN from –FS = –0.5 • V +FS = 0.5 • V

. For differential input voltages greater than

REF

+FS, the conversion result is clamped to the value corresponding to the +FS + 1LSB. For differential input voltages below –FS, the conversion result is clamped to the value corresponding to –FS – 1LSB.

Frequency Rejection Selection (FO) (LTC2411 Only)

The LTC2411 internal oscillator provides better than 110dB normal mode rejection at the line frequency and all its harmonics for 50Hz ±2% or 60Hz ±2%. For 60Hz rejection, FO should be connected to GND while for 50Hz rejection the FO pin should be connected to VCC.

The selection of 50Hz or 60Hz rejection can also be made by driving FO to an appropriate logic level. A selection change during the sleep or data output states will not disturb the converter operation. If the selection is made during the conversion state, the result of the conversion in progress may be outside specifications but the following conversions will not be affected.

When a fundamental rejection frequency different from 50Hz or 60Hz is required or when the converter must be synchronized with an outside source, the LTC2411 can operate with an external conversion clock. The converter automatically detects the presence of an external clock signal at the FO pin and turns off the internal oscillator. The frequency f

of the external signal must be at least

EOSC

2560Hz (1Hz notch frequency) to be detected. The external clock signal duty cycle is not significant as long as the minimum and maximum specifications for the high and low periods t

HEO

and t

are observed.

LEO

While operating with an external conversion clock of a frequency f normal mode rejection in a frequency range f

, the LTC2411 provides better than 110dB

EOSC

/2560

±4% and its harmonics. The normal mode rejection as a function of the input frequency deviation from f

EOSC

/2560

is shown in Figure 4. Whenever an external clock is not present at the FO pin, the

converter automatically activates its internal oscillator and

–80 –85 –90

–95 –100 –105 –110 –115 –120 –125

NORMAL MODE REJECTION (dB)

–130 –135 –140

–12–8–404812

DIFFERENTIAL INPUT SIGNAL FREQUENCY

DEVIATION FROM NOTCH FREQUENCY f

Figure 4. LTC2411 Normal Mode Rejection When Using an External Oscillator of Frequency f

EOSC

/2560(%)

2411 F04

EOSC

enters the Internal Conversion Clock mode. The LTC2411 operation will not be disturbed if the change of conversion clock source occurs during the sleep state or during the data output state while the converter uses an external serial clock. If the change occurs during the conversion state, the result of the conversion in progress may be outside specifications but the following conversions will not be affected. If the change occurs during the data output state and the converter is in the Internal SCK mode, the serial clock duty cycle may be affected but the serial data stream will remain valid.

Table 3 summarizes the duration of each state and the achievable output data rate as a function of FO.

Simultaneous Frequency Rejection (LTC2411-1 Only)

The LTC2411-1 internal oscillator provides better than 87dB normal mode rejection over the range of 49Hz to

61.2Hz. For this simultaneous 50/60Hz rejection, FO should be connected to GND. The performance of the LTC2411-1 is the same as the LTC2411 when driven by an external conversion clock at FO pin.

SERIAL INTERFACE PINS

The LTC2411/LTC2411-1 transmit the conversion results and receive the start of conversion command through a synchronous 3-wire interface. During the conversion and sleep states, this interface can be used to assess the

LTC2411/LTC2411-1

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APPLICATIO S I FOR ATIO

Table 3. LTC2411/LTC2411-1 State Duration

State Operating Mode Duration

CONVERT Internal Oscillator FO = LOW (LTC2411) 133ms, Output Data Rate ≤ 7.5 Readings/s

(60Hz Rejection) FO = HIGH (LTC2411) 160ms, Output Data Rate ≤ 6.2 Readings/s

(50Hz Rejection) FO = LOW (LTC2411-1) 147ms, Output Data Rate ≤ 6.8 Readings/s

(Simultaneous 50Hz/60Hz Rejection)

External Oscillator FO = External Oscillator 20510/f

with Frequency f

/2560 Rejection)

EOSC

SLEEP As Long As CS = HIGH Until CS = LOW and SCK DATA OUTPUT Internal Serial Clock FO = LOW/HIGH (LTC2411) As Long As CS = LOW But Not Longer Than 1.67ms

(Internal Oscillator) (32 SCK cycles) FO = LOW (LTC2411-1) As Long As CS = LOW But Not Longer Than 1.83ms

(Internal Oscillator) (32 SCK cycles) FO = External Oscillator with As Long As CS = LOW But Not Longer Than 256/f

Frequency f

External Serial Clock with As Long As CS = LOW But Not Longer Than 32/f Frequency f

kHz (32 SCK cycles)

SCK

EOSC

kHz

EOSC

kHz (32 SCK cycles)

s, Output Data Rate ≤ f

EOSC

/20510 Readings/s

EOSC

SCK

EOSC

converter status and during the data output state it is used to read the conversion result.

Serial Clock Input/Output (SCK)

The serial clock signal present on SCK (Pin 9) is used to synchronize the data transfer. Each bit of data is shifted out the SDO pin on the falling edge of the serial clock.

In the Internal SCK mode of operation, the SCK pin is an output and the LTC2411/LTC2411-1 create their own serial clock by dividing the internal conversion clock by 8. In the External SCK mode of operation, the SCK pin is used as input. The internal or external SCK mode is selected on power-up and then reselected every time a HIGH-to-LOW transition is detected at the CS pin. If SCK is HIGH or floating at power-up or during this transition, the converter enters the internal SCK mode. If SCK is LOW at power-up or during this transition, the converter enters the external SCK mode.

Serial Data Output (SDO)

The serial data output pin, SDO (Pin 8), provides the result of the last conversion as a serial bit stream (MSB first) during the data output state. In addition, the SDO pin is

used as an end of conversion indicator during the conversion and sleep states.

When CS (Pin 7) is HIGH, the SDO driver is switched to a high impedance state. This allows sharing the serial interface with other devices. If CS is LOW during the convert or sleep state, SDO will output EOC. If CS is LOW during the conversion phase, the EOC bit appears HIGH on the SDO pin. Once the conversion is complete, EOC goes LOW. The device remains in the sleep state until the first rising edge of SCK occurs while CS = LOW.

Chip Select Input (CS)

The active LOW chip select, CS (Pin 7), is used to test the conversion status and to enable the data output transfer as described in the previous sections.

In addition, the CS signal can be used to trigger a new conversion cycle before the entire serial data transfer has been completed. The LTC2411/LTC2411-1 will abort any serial data transfer in progress and start a new conversion cycle anytime a LOW-to-HIGH transition is detected at the CS pin after the converter has entered the data output state (i.e., after the first rising edge of SCK occurs with CS␣=␣LOW).

LTC2411/LTC2411-1

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APPLICATIO S I FOR ATIO

Table 4. LTC2411/LTC2411-1 Interface Timing Modes

Conversion Data Connection

SCK Cycle Output and

Configuration Source Control Control Waveforms

External SCK, Single Cycle Conversion External CS and SCK CS and SCK Figures 5, 6 External SCK, 2-Wire I/O External SCK SCK Figure 7 Internal SCK, Single Cycle Conversion Internal CS ↓ CS ↓ Figures 8, 9 Internal SCK, 2-Wire I/O, Continuous Conversion Internal Continuous Internal Figure 10

2.7V TO 5.5V

1µF

110

REF

2 3

4 5 6

SDO

SCK

(EXTERNAL)

CONVERSION

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

TO 0.5V

–0.5V

REF

TEST EOCTEST EOC

SLEEP DATA OUTPUT CONVERSION

BIT 31

EOC

Figure 5. External Serial Clock, Single Cycle Operation

Finally, CS can be used to control the free-running modes of operation, see Serial Interface Timing Modes section. Grounding CS will force the ADC to continuously convert at the maximum output rate selected by FO.

SERIAL INTERFACE TIMING MODES

The LTC2411/LTC2411-1’s 3-wire interface is SPI and MICROWIRE compatible. This interface offers several flexible modes of operation. These include internal/external serial clock, 2- or 3-wire I/O, single cycle conversion. The following sections describe each of these serial interface timing modes in detail. In all these cases, the converter can use the internal oscillator (FO = LOW or F

= HIGH) or an external oscillator connected to the FO pin. Refer to Table␣ 4 for a summary.

= 50Hz REJECTION (LTC2411)

LTC2411/

LTC2411-1

REF

–

REF

–

IN GND

SCK

SDO

= EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2411) = SIMULTANEOUS 50Hz/60Hz REJECTION (LTC2411-1)

3-WIRE SPI INTERFACE

BIT 5BIT 27 BIT 26BIT 28BIT 29BIT 30

LSB

SUB LSBMSBSIG

BIT 0

TEST EOC

Hi-ZHi-ZHi-Z

External Serial Clock, Single Cycle Operation (SPI/MICROWIRE Compatible)

This timing mode uses an external serial clock to shift out the conversion result and a CS signal to monitor and control the state of the conversion cycle, see Figure 5.

The serial clock mode is selected on the falling edge of CS. To select the external serial clock mode, the serial clock pin (SCK) must be LOW during each CS falling edge.

The serial data output pin (SDO) is Hi-Z as long as CS is HIGH. At any time during the conversion cycle, CS may be pulled LOW in order to monitor the state of the converter. While CS is pulled LOW, EOC is output to the SDO pin. EOC␣ =␣ 1 while a conversion is in progress and EOC = 0 if the device is in the sleep state. Independent of CS, the

2411 F05

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

device automatically enters the low power sleep state once the conversion is complete.

When the device is in the sleep state (EOC = 0), its conversion result is held in an internal static shift register. The device remains in the sleep state until the first rising edge of SCK is seen while CS is LOW. Data is

shifted out the SDO pin on each falling edge of SCK. This enables external circuitry to latch the output on the rising edge of SCK. EOC can be latched on the first rising edge of SCK and the last bit of the conversion result can be latched on the 32nd rising edge of SCK. On the 32nd falling edge of SCK, the device begins a new conversion. SDO goes HIGH (EOC = 1) indicating a conversion is in progress.

At the conclusion of the data cycle, CS may remain LOW and EOC monitored as an end-of-conversion interrupt. Alternatively, CS may be driven HIGH setting SDO to Hi-Z. As described above, CS may be pulled LOW at any time in order to monitor the conversion status.

Typically, CS remains LOW during the data output state. However, the data output state may be aborted by pulling CS HIGH anytime between the first rising edge and the 32nd falling edge of SCK, see Figure 6. On the rising edge of CS, the device aborts the data output state and immediately initiates a new conversion. This is useful for systems not

requiring all 32 bits of output data, aborting an invalid conversion cycle or synchronizing the start of a conversion.

External Serial Clock, 2-Wire I/O

This timing mode utilizes a 2-wire serial I/O interface. The conversion result is shifted out of the device by an externally generated serial clock (SCK) signal, see Figure 7. CS may be permanently tied to ground, simplifying the user interface or isolation barrier.

The external serial clock mode is selected at the end of the power-on reset (POR) cycle. The POR cycle is concluded approximately 1ms after VCC exceeds 1.9V. The level applied to SCK at this time determines if SCK is internal or external. SCK must be driven LOW prior to the end of POR in order to enter the external serial clock timing mode.

Since CS is tied LOW, the end-of-conversion (EOC) can be continuously monitored at the SDO pin during the convert and sleep states. EOC may be used as an interrupt to an external controller indicating the conversion result is ready. EOC = 1 while the conversion is in progress and EOC␣ =␣ 0 once the conversion enters the low power sleep state. On the falling edge of EOC, the conversion result is loaded into an internal static shift register. The device remains in the sleep state until the first rising edge of SCK.

SDO

SCK

(EXTERNAL)

2.7V TO 5.5V

1µF

110

REF

GND

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

–0.5V

TO 0.5V

REF

TEST EOC TEST EOC

EOC

Hi-Z

CONVERSIONSLEEP SLEEP

DATA OUTPUT

BIT 31BIT 0

EOC

Hi-Z Hi-ZHi-Z

LTC2411/

LTC2411-1

–

SCK

SDO

DATA OUTPUT

= 50Hz REJECTION (LTC2411) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2411) = SIMULTANEOUS 50Hz/60Hz REJECTION (LTC2411-1)

3-WIRE SPI INTERFACE

BIT 8BIT 27 BIT 9BIT 28BIT 29BIT 30

MSBSIG

TEST EOC

CONVERSION

2411 F06

Figure 6. External Serial Clock, Reduced Data Output Length

LTC2411/LTC2411-1

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APPLICATIO S I FOR ATIO

Data is shifted out the SDO pin on each falling edge of SCK enabling external circuitry to latch data on the rising edge of SCK. EOC can be latched on the first rising edge of SCK. On the 32nd falling edge of SCK, SDO goes HIGH (EOC␣ =␣ 1) indicating a new conversion has begun.

2.7V TO 5.5V

1µF

110

2 3

REF

5 6

MSBSIG

SDO

SCK

(EXTERNAL)

CONVERSION

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

TO 0.5V

–0.5V

REF

BIT 31

EOC

SLEEP DATA OUTPUT CONVERSION

Internal Serial Clock, Single Cycle Operation

This timing mode uses an internal serial clock to shift out the conversion result and a CS signal to monitor and control the state of the conversion cycle, see Figure 8.

= 50Hz REJECTION (LTC2411)

LTC2411/

LTC2411-1

REF

–

REF

–

IN GND

SCK

SDO

= EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2411) = SIMULTANEOUS 50Hz/60Hz REJECTION (LTC2411-1)

2-WIRE I/O

BIT 5BIT 27 BIT 26BIT 28BIT 29BIT 30

LSB

BIT 0

2411 F07

SDO

SCK

(INTERNAL)

Figure 7. External Serial Clock, CS = 0 Operation

2.7V TO 5.5V

1µF

110

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

TO 0.5V

–0.5V

REF

EOCtest

TEST EOC

Hi-Z Hi-Z Hi-Z Hi-Z

BIT 31

EOC

SLEEP DATA OUTPUT CONVERSIONCONVERSION

REF

GND

MSBSIG

LTC2411/

LTC2411-1

–

BIT 27 BIT 26BIT 28BIT 29BIT 30

SCK

SDO

= 50Hz REJECTION (LTC2411) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2411) = SIMULTANEOUS 50Hz/60Hz REJECTION (LTC2411-1)

3-WIRE SPI INTERFACE

BIT 5

LSB

BIT 0

10k

TEST EOC

2411 F08

Figure 8. Internal Serial Clock, Single Cycle Operation

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

In order to select the internal serial clock timing mode, the serial clock pin (SCK) must be floating (Hi-Z) or pulled HIGH prior to the falling edge of CS. The device will not enter the internal serial clock mode if SCK is driven LOW on the falling edge of CS. An internal weak pull-up resistor is active on the SCK pin during the falling edge of CS; therefore, the internal serial clock timing mode is automatically selected if SCK is not externally driven.

The serial data output pin (SDO) is Hi-Z as long as CS is HIGH. At any time during the conversion cycle, CS may be pulled LOW in order to monitor the state of the converter. Once CS is pulled LOW, SCK goes LOW and EOC is output to the SDO pin. EOC = 1 while a conversion is in progress and EOC = 0 if the device is in the sleep state.

When testing EOC, if the conversion is complete (EOC = 0), the device will exit the sleep state and enter the data output state if CS remains LOW. In order to prevent the device from exiting the low power sleep state, CS must be pulled HIGH before the first rising edge of SCK. In the internal SCK timing mode, SCK goes HIGH and the device begins outputting data at time t (if EOC = 0) or t

after EOC goes LOW (if CS is LOW

EOCtest

during the falling edge of EOC). The value of t

after the falling edge of CS

EOCtest

is 23µs

if the device is using its internal oscillator for the LTC2411 (FO = logic LOW or HIGH) and 26µs for the LTC2411-1 (FO = logic LOW). If FO is driven by an external oscillator of frequency f HIGH before time t

EOSC

, then t

EOCtest

is 3.6/f

. If CS is pulled

EOSC

, the device remains in the sleep state. The conversion result is held in the internal static shift register.

If CS remains LOW longer than t

, the first rising

EOCtest

edge of SCK will occur and the conversion result is serially shifted out of the SDO pin. The data output cycle begins on this first rising edge of SCK and concludes after the 32nd rising edge. Data is shifted out the SDO pin on each falling edge of SCK. The internally generated serial clock is output to the SCK pin. This signal may be used to shift the conversion result into external circuitry. EOC can be latched on the first rising edge of SCK and the last bit of the conversion result on the 32nd rising edge of SCK. After the 32nd rising edge, SDO goes HIGH (EOC = 1), SCK stays HIGH and a new conversion starts.

Typically, CS remains LOW during the data output state. However, the data output state may be aborted by pulling CS HIGH anytime between the first and 32nd rising edge of SCK, see Figure 9. On the rising edge of CS, the device

SDO

SCK

(INTERNAL)

2.7V TO 5.5V

1µF

110

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

TO 0.5V

–0.5V

REF

EOCtest

BIT 0

EOC

Hi-Z Hi-Z Hi-Z Hi-Z Hi-Z

DATA OUTPUT

EOCtest

TEST EOCTEST EOC

BIT 31

EOC

SLEEP DATA OUTPUT

REF

GND

LTC2411/

LTC2411-1

–

SCK

SDO

MSBSIG

= 50Hz REJECTION (LTC2411) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2411) = SIMULTANEOUS 50Hz/60Hz REJECTION (LTC2411-1)

3-WIRE SPI INTERFACE

BIT 27 BIT 26BIT 28BIT 29BIT 30

BIT 8

Figure 9. Internal Serial Clock, Reduced Data Output Length

CONVERSIONCONVERSIONSLEEP

TEST EOC

2411 F09

10k

LTC2411/LTC2411-1

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APPLICATIO S I FOR ATIO

aborts the data output state and immediately initiates a new conversion. This is useful for systems not requiring all 32 bits of output data, aborting an invalid conversion cycle, or synchronizing the start of a conversion. If CS is pulled HIGH while the converter is driving SCK LOW, the internal pull-up is not available to restore SCK to a logic HIGH state. This will cause the device to exit the internal serial clock mode on the next falling edge of CS. This can be avoided by adding an external 10k pull-up resistor to the SCK pin or by never pulling CS HIGH when SCK is LOW.

Whenever SCK is LOW, the LTC2411/LTC2411-1’s internal pull-up at pin SCK is disabled. Normally, SCK is not externally driven if the device is in the internal SCK timing mode. However, certain applications may require an external driver on SCK. If this driver goes Hi-Z after outputting a LOW signal, the LTC2411/LTC2411-1’s internal pull-up remains disabled. Hence, SCK remains LOW. On the next falling edge of CS, the device is switched to the external SCK timing mode. By adding an external 10k pullup resistor to SCK, this pin goes HIGH once the external driver goes Hi-Z. On the next CS falling edge, the device will remain in the internal SCK timing mode.

A similar situation may occur during the sleep state when CS is pulsed HIGH-LOW-HIGH in order to test the conversion status. If the device is in the sleep state (EOC = 0), SCK will go LOW. Once CS goes HIGH (within the time period defined above as t

), the internal pull-up is

EOCtest

activated. For a heavy capacitive load on the SCK pin, the internal pull-up may not be adequate to return SCK to a HIGH level before CS goes low again. This is not a concern under normal conditions where CS remains LOW after detecting EOC = 0. This situation is easily overcome by adding an external 10k pull-up resistor to the SCK pin.

Internal Serial Clock, 2-Wire I/O, Continuous Conversion

This timing mode uses a 2-wire, all output (SCK and SDO) interface. The conversion result is shifted out of the device by an internally generated serial clock (SCK) signal, see Figure 10. CS may be permanently tied to ground, simplifying the user interface or isolation barrier.

SDO

SCK

(INTERNAL)

2.7V TO 5.5V

1µF

110

REF

GND

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

–0.5V

TO 0.5V

REF

BIT 31

EOC

SLEEP

MSBSIG

LTC2411/

LTC2411-1

–

DATA OUTPUT CONVERSIONCONVERSION

SCK

SDO

= 50Hz REJECTION (LTC2411) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2411) = SIMULTANEOUS 50Hz/60Hz REJECTION (LTC2411-1)

2-WIRE I/O

BIT 5 BIT 0BIT 27 BIT 26BIT 28BIT 29BIT 30

LSB

2411 F10

Figure 10. Internal Serial Clock, CS = 0 Continuous Operation

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

The internal serial clock mode is selected at the end of the power-on reset (POR) cycle. The POR cycle is concluded approximately 1ms after VCC exceeds 1.9V. An internal weak pull-up is active during the POR cycle; therefore, the internal serial clock timing mode is automatically selected if SCK is not externally driven LOW (if SCK is loaded such that the internal pull-up cannot pull the pin HIGH, the external SCK mode will be selected).

During the conversion, the SCK and the serial data output pin (SDO) are HIGH (EOC = 1). Once the conversion is complete, SCK and SDO go LOW (EOC = 0) indicating the conversion has finished and the device has entered the low power sleep state. The part remains in the sleep state a minimum amount of time (1/2 the internal SCK period) then immediately begins outputting data. The data output cycle begins on the first rising edge of SCK and ends after the 32nd rising edge. Data is shifted out the SDO pin on each falling edge of SCK. The internally generated serial clock is output to the SCK pin. This signal may be used to shift the conversion result into external circuitry. EOC can be latched on the first rising edge of SCK and the last bit of the conversion result can be latched on the 32nd rising edge of SCK. After the 32nd rising edge, SDO goes HIGH (EOC = 1) indicating a new conversion is in progress. SCK remains HIGH during the conversion.

PRESERVING THE CONVERTER ACCURACY

The LTC2411/LTC2411-1 are designed to reduce as much as possible the conversion result sensitivity to device decoupling, PCB layout, antialiasing circuits, line frequency perturbations and so on. Nevertheless, in order to preserve the extreme accuracy capability of this part, some simple precautions are desirable.

Digital Signal Levels

The LTC2411/LTC2411-1’s digital interface is easy to use. Its digital inputs (FO, CS and SCK in External SCK mode of operation) accept standard TTL/CMOS logic levels and the internal hysteresis receivers can tolerate edge rates as slow as 100µs. However, some considerations are required to take advantage of the exceptional accuracy and low supply current of this converter.

The digital output signals (SDO and SCK in Internal SCK mode of operation) are less of a concern because they are not generally active during the conversion state.

While a digital input signal is in the range 0.5V to (VCC␣ –␣ 0.5V), the CMOS input receiver draws additional current from the power supply. It should be noted that, when any one of the digital input signals (FO, CS and SCK in External SCK mode of operation) is within this range, the LTC2411/LTC2411-1 power supply current may increase even if the signal in question is at a valid logic level. For micropower operation, it is recommended to drive all digital input signals to full CMOS levels [VIL < 0.4V and VOH > (VCC – 0.4V)].

During the conversion period, the undershoot and/or overshoot of a fast digital signal connected to the LTC2411/ LTC2411-1 pins may severely disturb the analog to digital conversion process. Undershoot and overshoot can occur because of the impedance mismatch at the converter pin when the transition time of an external control signal is less than twice the propagation delay from the driver to LTC2411/LTC2411-1. For reference, on a regular FR-4 board, signal propagation velocity is approximately 183ps/inch for internal traces and 170ps/inch for surface traces. Thus, a driver generating a control signal with a minimum transition time of 1ns must be connected to the converter pin through a trace shorter than 2.5 inches. This problem becomes particularly difficult when shared control lines are used and multiple reflections may occur. The solution is to carefully terminate all transmission lines close to their characteristic impedance.

Parallel termination near the LTC2411/LTC2411-1 pin will eliminate this problem but will increase the driver power dissipation. A series resistor between 27Ω and 56Ω placed near the driver or near the LTC2411/LTC2411-1 pin will also eliminate this problem without additional power dissipation. The actual resistor value depends upon the trace impedance and connection topology.

An alternate solution is to reduce the edge rate of the control signals. It should be noted that using very slow edges will increase the converter power supply current during the transition time. The differential input and reference architecture reduce substantially the converter’s sensitivity to ground currents.

LTC2411/LTC2411-1

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APPLICATIO S I FOR ATIO

Particular attention must be given to the connection of the FO signal when the LTC2411/LTC2411-1 are used with an external conversion clock. This clock is active during the conversion time and the normal mode rejection provided by the internal digital filter is not very high at this frequency. A normal mode signal of this frequency at the converter reference terminals may result into DC gain and INL errors. A normal mode signal of this frequency at the converter input terminals may result into a DC offset error. Such perturbations may occur due to asymmetric capacitive coupling between the FO signal trace and the converter input and/or reference connection traces. An immediate solution is to maintain maximum possible separation between the FO signal trace and the input/reference signals. When the FO signal is parallel terminated near the converter, substantial AC current is flowing in the loop formed by the FO connection trace, the termination and the ground return path. Thus, perturbation signals may be inductively coupled into the converter input and/or reference. In this situation, the user must reduce to a minimum the loop area for the FO signal as well as the loop area for the differential input and reference connections.

Driving the Input and Reference

The input and reference pins of the LTC2411/LTC2411-1 converter are directly connected to a network of sampling capacitors. Depending upon the relation between the differential input voltage and the differential reference voltage, these capacitors are switching between these four pins transfering small amounts of charge in the process. A simplified equivalent circuit is shown in Figure 11.

For a simple approximation, the source impedance R

driving an analog input pin (IN+, IN–, REF+ or REF–) can be considered to form, together with RSW and CEQ (see Figure␣ 11), a first order passive network with a time constant τ = (RS + RSW) • CEQ. The converter is able to sample the input signal with better than 1ppm accuracy if the sampling period is at least 14 times greater than the input circuit time constant τ. The sampling process on the four input analog pins is quasi-independent so each time constant should be considered by itself and, under worstcase circumstances, the errors may add.

When using the internal oscillator (FO = LOW or HIGH), the LTC2411 (LTC2411-1)’s front-end switched-capacitor network is clocked at 76800Hz (69900Hz) corresponding to

REF

IIN+

VIN+

IIN–

VIN–

–

REF

–

REF

SWITCHING FREQUENCY

= 76800Hz INTERNAL OSCILLATOR (FO = LOW OR HIGH)

= 0.5 • f

LEAK

EXTERNAL OSCILLATOR

EOSC

LEAK

RSW (TYP)

20k

RSW (TYP)

20k

RSW (TYP)

20k

RSW (TYP)

20k

Figure 11. LTC2411/LTC2411-1 Equivalent Analog Input Circuit

2411 F11

6pF (TYP)

IIN

I REF

where V REF REF

VININ

R M INTERNAL OSCILLATOR Hz Notch F LOW LTC R M INTERNAL OSCILLATOR Hz Notch F HIGH LTC R M INTERNAL OSCILLATOR F LOW LTC

R f EXTERNAL OSCILLATOR

()

AVG

−

()

AVG

()

AVG

−

()

AVG

=−

REF

REFCM

+−

=−

INCM

10 8 60 2411

EQ O

13 0 50 2411

EQ O

11 9

EQ O

=•

167 10

()

EQ EOSC

•

−+ −

VV V

−• − +

+−



REF REF





+−



IN IN

−





Ω

Ω 24112411 1

IN INCM REFCM

•

•− +

VV V

REF INCM REFCM

•

VV V

REF INCM REFCM

•

+−







 



−

•

REF EQ

•

REF EQ

()() ()()

()

−

VV V

+−

IN INCM REFCM

WUUU

APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

a 13µs (14.2µs) sampling period. Thus, for settling errors of less than 1ppm, the driving source impedance should be chosen such that τ ≤ 13µs/14 = 920ns (1.02µs). When an external oscillator of frequency f sampling period is 2/f than 1ppm, τ ≤ 0.14/f

and, for a settling error of less

EOSC

is used, the

EOSC

Input Current

If complete settling occurs on the input, conversion results will be unaffected by the dynamic input current. An incomplete settling of the input signal sampling process may result in gain and offset errors, but it will not degrade the INL performance of the converter. Figure 11 shows the mathematical expressions for the average bias currents flowing through the IN+ and IN– pins as a result of the sampling charge transfers when integrated over a substantial time period (longer than 64 internal clock cycles).

The effect of this input dynamic current can be analyzed using the test circuit of Figure 12. The C

capacitor

PAR

includes the LTC2411/LTC2411-1 pin capacitance (5pF typical) plus the capacitance of the test fixture used to obtain the results shown in Figures 13 and 14. A careful implementation can bring the total input capacitance (C + C

) closer to 5pF thus achieving better performance

PAR

than the one predicted by Figures 13 and 14. The effect of the input dynamic current is almost the same for the LTC2411 and the LTC2411-1 and measurements of the LTC2411 with FO = GND are plotted out as a typical case. For simplicity, two distinct situations can be considered.

or relatively small values of input capacitance (CIN <

0.01µF), the voltage on the sampling capacitor settles almost completely and relatively large values for the source impedance result in only small errors. Such values for CIN will deteriorate the converter offset and gain performance without significant benefits of signal filtering and the user is advised to avoid them. Nevertheless, when small values of CIN are unavoidably present as parasitics of input multiplexers, wires, connectors or sensors, the LTC2411/LTC2411-1 can maintain their exceptional accuracy while operating with relative large values of source resistance as shown in Figures 13 and

14. These measured results may be slightly different from the first order approximation suggested earlier because

they include the effect of the actual second order input network together with the nonlinear settling process of the input amplifiers. For small CIN values, the settling on IN+ and IN– occurs almost independently and there is little benefit in trying to match the source impedance for the two pins.

SOURCE

+ 0.5V

INCM

– 0.5V

SOURCE

Figure 12. An RC Network at IN+ and IN

VCC = 5V

= 5V

REF

–

= GND

REF

)

= 5V

–

REF

+FS ERROR (ppm OF V

= 2.5V

= GND

= 25°C

1 10 100 1k 10k 100k

IN =

0.001µF

IN =

Figure 13. +FS Error vs R

VCC = 5V

= 5V

REF

–

= GND

REF

–10

)

= GND

–

REF

= GND

–20

= 25°C

–30

–FS ERROR (ppm OF V

–40

–50

1 10 100 1k 10k 100k

= 2.5V

IN =

0.001µF

IN =

Figure 14. –FS Error vs R

PAR

≅20pF

PAR

≅20pF

at IN+ or IN– (Small CIN)

0.01µF

100pF

0pF

SOURCE

0pF

100pF

0.01µF

SOURCE

(Ω)

2411 F13

2411 F14

LTC2411/

LTC2411-1

–

2411 F12

–

LTC2411/LTC2411-1

WUUU

APPLICATIO S I FOR ATIO

Larger values of input capacitors (CIN > 0.01µF) may be required in certain configurations for antialiasing or general input signal filtering. Such capacitors will average the input sampling charge and the external source resistance will see a quasi constant input differential impedance. For the LTC2411, when FO = LOW (internal oscillator and 60Hz notch), the typical differential input resistance is 5.4MΩ which will generate a gain error of approximately 0.093ppm for each ohm of source resistance driving IN+ or IN–. When FO = HIGH (internal oscillator and 50Hz notch), the typical differential input resistance is 6.5MΩ which will generate a gain error of approximately 0.077ppm for each ohm of source resistance driving IN+ or IN–. For the LTC2411-1, the typical differential input resistance is 6MΩ which will generate a gain error of approximately

0.084ppm for each ohm of source resistance driving IN

or IN– (FO = LOW). When FO is driven by an external oscillator with a frequency f

(external conversion

EOSC

clock operation), the typical differential input resistance is

0.83 • 1012/f driving IN+ or IN– will result in 0.59 • 10–6 • f

Ω and each ohm of source resistance

EOSC

ppm gain

EOSC

error. The effect of the source resistance on the two input pins is additive with respect to this gain error. The typical +FS and –FS errors as a function of the sum of the source resistance seen by IN+ and IN– for large values of CIN are shown in Figure 15.

reference common mode voltages. While the input drive circuit nonzero source impedance combined with the converter average input current will not degrade the INL performance, indirect distortion may result from the modulation of the offset error by the common mode component of the input signal. Thus, when using large CIN capacitor values, it is advisable to carefully match the source impedance seen by the IN+ and IN– pins. For the LTC2411, when FO = LOW (internal oscillator and 60Hz notch), every 1Ω mismatch in source impedance transforms a full-scale common mode input signal into a differential mode input signal of 0.093ppm. When FO = HIGH (internal oscillator and 50Hz notch), every 1Ω mismatch in source impedance transforms a full-scale common mode input signal into a differential mode input signal of 0.077ppm. For the LTC2411-1, when internal oscillator is used (FO = LOW), every 1Ω mismatch in source impedance transforms a full-scale common mode input signal into a differential mode input signal of 0.084ppm. When FO is driven by an external oscillator with a frequency f

, every 1Ω mis-

EOSC

match in source impedance transforms a full-scale common mode input signal into a differential mode input signal of 0.59 • 10–6 • f

ppm. Figure 16 shows the

EOSC

typical offset error due to input common mode voltage for various values of source resistance imbalance between the IN+ and IN– pins when large CIN values are used.

In addition to this gain error, an offset error term may also appear. The offset error is proportional with the mismatch between the source impedance driving the two input pins IN+ and IN– and with the difference between the input and

120

VCC = 5V

= 5V

REF

–

100

)

REF

+FS ERROR (ppm OF V

Figure 15a. +FS Error vs R

= GND

REF

= 3.75V

–

= 1.25V

= GND

= 25°C

100 200 300 400 500 600 700 800 9001000

SOURCE

IN =

0.01µF

IN =

(Ω)

at IN+ or IN– (Large CIN)

IN =

0.1µF

10µF

IN =

1µF

2411 F15a

If possible, it is desirable to operate with the input signal common mode voltage very close to the reference signal common mode voltage as is the case in the ratiometric measurement of a symmetric bridge. This configuration

0.01µF

–20

)

REF

–40

–60

VCC = 5V

= 5V

REF

–80

–

= GND

REF

= 1.25V

–FS ERROR (ppm OF V

–

= 3.75V

–100

= GND

= 25°C

–120

100 200 300 400 500 600 700 800 9001000

SOURCE

Figure 15b. –FS Error vs R

SOURCE

IN =

0.1µF

IN =

10µF

IN =

1µF

IN =

(Ω)

2411 F15b

at IN+ or IN– (Large CIN)

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

)

REF

–10

–20 –30

OFFSET ERROR (ppm OF V

–40 –50

0.5

A: ∆RIN = +400Ω B: ∆R C: ∆R D: ∆R

= +200Ω

= +100Ω

= 0Ω

VCC = 5V

REF

= 5V

–

= GND

REF

= IN– = V

FO = GND

= 25°C

SOURCEIN

= 10µF

21.5 V

INCM

– = 500Ω

3 3.5 4.5

2.5 (V)

E: ∆RIN = –100Ω

= –200Ω

F: ∆R

= –400Ω

G: ∆R

2411 F16

Figure 16. Offset Error vs Common Mode Voltage (V

= IN+ = IN–) and Input Source Resistance Imbalance

(∆R

INCM

= R

SOURCEIN

– R

SOURCEIN

–

) for Large CIN Values (CIN ≥ 1µF)

eliminates the offset error caused by mismatched source impedances.

The magnitude of the dynamic input current depends upon the size of the very stable internal sampling capacitors and upon the accuracy of the converter sampling clock. The accuracy of the internal clock over the entire temperature and power supply range is typically better than 1%. Such a specification can also be easily achieved by an external clock. When relatively stable resistors (50ppm/°C) are used for the external source impedance seen by IN+ and IN–, the expected drift of the dynamic current, offset and gain errors will be insignificant (about 1% of their respective values over the entire temperature and voltage range). Even for the most stringent applications, a one-time calibration operation may be sufficient.

In addition to the input sampling charge, the input ESD protection diodes have a temperature dependent leakage current. This current, nominally 1nA (±10nA max), results in a small offset shift. A 100Ω source resistance will create a 0.1µV typical and 1µV maximum offset voltage.

Reference Current

In a similar fashion, the LTC2411/LTC2411-1 sample the differential reference pins REF+ and REF– transfering small amount of charge to and from the external driving circuits thus producing a dynamic reference current. This current

does not change the converter offset, but it may degrade the gain and INL performance. The effect of this current can be analyzed in the same two distinct situations.

For relatively small values of the external reference capacitors (C

< 0.01µF), the voltage on the sampling capacitor

REF

settles almost completely and relatively large values for the source impedance result in only small errors. Such values for C

will deteriorate the converter offset and

REF

gain performance without significant benefits of reference filtering and the user is advised to avoid them.

Larger values of reference capacitors (C

> 0.01µF)

REF

may be required as reference filters in certain configurations. Such capacitors will average the reference sampling charge and the external source resistance will see a quasi constant reference differential impedance. For the LTC2411, when FO = LOW (internal oscillator and 60Hz notch), the typical differential reference resistance is

3.9MΩ which will generate a gain error of approximately

0.13ppm for each ohm of source resistance driving REF

or REF–. When FO = HIGH (internal oscillator and 50Hz notch), the typical differential reference resistance is

4.68MΩ which will generate a gain error of approximately

0.11ppm for each ohm of source resistance driving REF

or REF–. For the LTC2411-1, when internal oscillator is used (FO = LOW), the typical differential reference resistance is 4.29MΩ which will generate a gain error of approximately 0.12ppm for each ohm of source resistance driving REF+ or REF–. When FO is driven by an external oscillator with a frequency f

(external con-

EOSC

version clock operation), the typical differential reference resistance is 0.60 • 1012/f

Ω and each ohm of source

EOSC

resistance drving REF+ or REF– will result in 0.823 • 10–6 • f

ppm gain error. The effect of the source

EOSC

resistance on the two reference pins is additive with respect to this gain error. The typical FS errors for various combinations of source resistance seen by the REF+ and REF– pins and external capacitance C

connected to

REF

these pins are shown in Figures 17 and 18. Typical –FS errors are similar to +FS errors with opposite polarity.

In addition to this gain error, the converter INL performance is degraded by the reference source impedance. For LTC2411, when FO = LOW (internal oscillator and 60Hz notch), every 100Ω of source resistance driving REF+ or

LTC2411/LTC2411-1

INDIF/VREFDIF

–0.5–0.4– 0.3–0.2– 0.1 0 0.1 0.2 0.3 0.4 0.5

INL (ppm OF V

REF

)

9 6 4 2

0 –2 –4 –6 –8

–10

VCC = 5V REF

= 5V

REF

–

= GND

INCM

= 0.5 • (IN+ + IN–) = 2.5V

FO = GND C

REF

= 10µF

= 25°C

SOURCE

= 2k

SOURCE

= 1k

SOURCE

= 500Ω

2411 F19

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APPLICATIO S I FOR ATIO

VCC = 5V

= 5V

REF

–

= GND

REF

–10

)

= 3.75V

–

REF

+FS ERROR (ppm OF V

= 1.25V

= GND

–20

= 25°C

0pF

REF =

100pF

0.001µF

0.01µF

SOURCE

(Ω)

–30

–40

–50

1 10 100 1k 10k 100k

2411 F17a

VCC = 5V

= 5V

REF

–

= GND

REF

)

–FS ERROR (ppm OF V

= 1.25V

–

REF

= 3.75V

= GND

= 25°C

0pF

REF =

100pF

0.001µF

0.01µF

SOURCE

(Ω)

1 10 100 1k 10k 100k

2411 F17b

Figure 17a. +FS Error vs R

–20

)

–40

REF

= 10µF

REF

–60

–80

VCC = 5V

–100

–120

+FS ERROR (ppm OF V

–140

–160

= 5V

REF

–

= GND

REF

= 3.75V

–

= 1.25V

= GND

= 25°C

200 400 600 1000700100 300 500 900

Figure 18a. +FS Error vs R

REF– translates into about 0.45ppm additional INL error. When FO = HIGH (internal oscillator and 50Hz notch), every 100Ω of source resistance driving REF+ or REF translates into about 0.37ppm additional INL error. For the LTC2411-1, when FO = LOW, every 100Ω of source resistance driving REF+ or REF– translates into about

0.41ppm additional INL error. When FO is driven by an external oscillator with a frequency f source resistance driving REF+ or REF– translates into about 2.91 • 10–6 • f ure␣ 19 shows the typical INL error due to the source resistance driving the REF+ or REF– pins when large C values are used. The effect of the source resistance on the two reference pins is additive with respect to this INL error. In general, matching of source impedance for the REF and REF– pins does not help the gain or the INL error. The

at REF+ or REF– (Small CIN)

SOURCE

= 0.01µF

REF

= 0.1µF

REF

= 1µF

REF

800

(Ω)

SOURCE

at REF+ or REF– (Large CIN)

SOURCE

ppm additional INL error. Fig-

EOSC

2411 F18a

, every 100Ω of

EOSC

REF

Figure 17b. –FS Error vs R

160

VCC = 5V

REF

140

)

120

REF

100

–FS ERROR (ppm OF V

= 5V

–

= GND

REF

= 1.25V

–

= 3.75V

= GND

= 25°C

200 400 600 1000700100 300 500 900

Figure 18b. –FS Error vs R

at REF+ or REF– (Small CIN)

SOURCE

= 10µF

REF

= 1µF

REF

= 0.1µF

REF

= 0.01µF

REF

800

(Ω)

SOURCE

at REF+ or REF– (Large CIN)

SOURCE

–

Figure 19. INL vs Differential Input Voltage (VIN = IN+ = IN–)

and Reference Source Resistance (R for Large C

Values (C

REF

≥ 1µF)

SOURCE

2411 F18b

at REF+ and REF–)

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

user is thus advised to minimize the combined source impedance driving the REF+ and REF– pins rather than to try to match it.

The magnitude of the dynamic reference current depends upon the size of the very stable internal sampling capacitors and upon the accuracy of the converter sampling clock. The accuracy of the internal clock over the entire temperature and power supply range is typical better than 1%. Such a specification can also be easily achieved by an external clock. When relatively stable resistors (50ppm/°C) are used for the external source impedance seen by REF and REF–, the expected drift of the dynamic current gain error will be insignificant (about 1% of its value over the entire temperature and voltage range). Even for the most stringent applications, a one-time calibration operation may be sufficient.

In addition to the reference sampling charge, the reference pins ESD protection diodes have a temperature dependent leakage current. This leakage current, nominally 1nA (±10nA max), results in a small gain error. A 100Ω source resistance will create a 0.05µV typical and 0.5µV maxi- mum full-scale error.

Output Data Rate

When using its internal oscillator, the LTC2411 can produce up to 7.5 readings per second with a notch frequency of 60Hz (FO = LOW) and 6.25 readings per second with a notch frequency of 50Hz (FO = HIGH) and the LTC2411-1 can produce up to 6.8 readings per second with FO = LOW. The actual output data rate will depend upon the length of the sleep and data output phases which are controlled by the user and which can be made insignificantly short. When operated with an external conversion clock (F connected to an external oscillator), the LTC2411/LTC24111 output data rate can be increased as desired. The duration of the conversion phase is 20510/f = 153600Hz, the converter behaves as if the internal oscillator is used and the notch is set at 60Hz. There is no significant difference in the LTC2411/LTC2411-1 performance between these two operation modes.

EOSC

. If f

EOSC

accompanied by three potential effects, which must be carefully considered.

First, a change in f in the internal notch position and in a reduction of the converter differential mode rejection at the power line frequency. In many applications, the subsequent performance degradation can be substantially reduced by relying upon the LTC2411/LTC2411-1’s exceptional common mode rejection and by carefully eliminating common mode to differential mode conversion sources in the input circuit. The user should avoid single-ended input filters and should maintain a very high degree of matching and symmetry in the circuits driving the IN+ and IN– pins.

Second, the increase in clock frequency will increase proportionally the amount of sampling charge transferred through the input and the reference pins. If large external input and/or reference capacitors (CIN, C previous section provides formulae for evaluating the effect of the source resistance upon the converter performance for any value of f or reference capacitors (CIN, C the external source resistance upon the LTC2411/ LTC2411-1 typical performance can be inferred from Figures 13, 14 and 17 in which the horizontal axis is scaled by 153600/f

Third, an increase in the frequency of the external oscillator above 460800Hz (a more than 3× increase in the output data rate) will start to decrease the effectiveness of the internal autocalibration circuits. This will result in a progressive degradation in the converter accuracy and linearity. Typical measured performance curves for output data rates up to 100 readings per second are shown in Figures␣ 20 to

27. In order to obtain the highest possible level of accuracy from this converter at output data rates above 20 readings per second, the user is advised to maximize the power supply voltage used and to limit the maximum ambient operating temperature. In certain circumstances, a reduction of the differential reference voltage may be beneficial.

Input Bandwidth

EOSC

will result in a proportional change

EOSC

) are used, the

REF

. If small external input and/

EOSC

) are used, the effect of

REF

An increase in f translate into a proportional increase in the maximum output data rate. This substantial advantage is nevertheless

over the nominal 153600Hz will

EOSC

The combined effect of the internal sinc4 digital filter and of the analog and digital autocalibration circuits determines the LTC2411/LTC2411-1 input bandwidth. When

LTC2411/LTC2411-1

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APPLICATIO S I FOR ATIO

120

VCC = 5V V

= 5V

REF

= 2.5V

)

INCM

= 0V

REF

= EXT OSC

–40

OFFSET ERROR (ppm OF V

–80

TA = 85°C

TA = 25°C

250

VCC = 5V

= 5V

REF

200

= 3.75V

–

)

150

100

= 1.25V

= EXT OSC

REF

+FS ERROR (ppm OF V

TA = 85°C

TA = 25°C

–120

10 20 30 40 50 100

OUTPUT DATA RATE (READINGS/SEC)

60 70 80 90

2411 F20

Figure 20. Offset Error vs Output Data Rate and Temperature

100

TA = 25°C

)

REF

TA = 85°C

–50

VCC = 5V

= 5V

–FS ERROR (ppm OF V

REF

–100

–150

= 1.25V

–

= 3.75V

= EXT OSC

OUTPUT DATA RATE (READINGS/SEC)

50 60 70 80 90 100

2411 F22

Figure 22. –FS Error vs Output Data Rate and Temperature

–50

20 40 60 80

OUTPUT DATA RATE (READINGS/SEC)

10010030507090

2411 F21

Figure 21. +FS Error vs Output Data Rate and Temperature

TA = 85°C

VCC = 5V

RESOLUTION (BITS)

= 5V

REF

= 2.5V

INCM

= 0V

= EXT OSC

RES = LOG

OUTPUT DATA RATE (READINGS/SEC)

/NOISE

2(VREF

20 40 60 80

RMS

Figure 23. Resolution (Noise

TA = 25°C

)

10010030507090

2411 F23

≤ 1LSB)

RMS

vs Output Data Rate and Temperature

VCC = 5V

RESOLUTION (BITS)

V V V

F RES = LOG

Figure 24. Resolution (INL

TA = 85°C

= 5V

REF

= 2.5V

INCM

= 0V

= EXT OSC

20 40 60 80

OUTPUT DATA RATE (READINGS/SEC)

2(VREF

/INL

MAX

TA = 25°C

)

RMS

2411 F24

≤ 1LSB)

vs Output Data Rate and Temperature

10010030507090

)

REF

–20

–40

–60

VCC = 5V

–

= GND

REF

–80

OFFSET ERROR (ppm OF V

–100

–120

= 2.5V

INCM

= 0V

= EXT OSC

= 25°C

20 40 60 80

OUTPUT DATA RATE (READINGS/SEC)

= 2.5V

REF

Figure 25. Offset Error vs Output Data Rate and Reference Voltage

= 5V

10010030507090

2411 F25

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

= 5V

REF

= 2.5V

REF

VCC = 5V

–

= GND

REF

RESOLUTION (BITS)

Figure 26. Resolution (Noise

= 2.5V

INCM

= 0V

= EXT OSC

RES = LOG T

= 25°C

OUTPUT DATA RATE (READINGS/SEC)

2(VREF

/NOISE

RMS

)

RMS

100

2411 F26

≤ 1LSB)

vs Output Data Rate and Reference Voltage

the internal oscillator is used, the 3dB input bandwidth of the LTC2411 is 3.63Hz for 60Hz notch frequency (FO = LOW) and 3.02Hz for 50Hz notch frequency (FO = HIGH). The 3dB input bandwidth for the LTC2411-1 is 3.30Hz (FO = LOW). If an external conversion clock generator of frequency f bandwidth is 0.236 • 10–6 • f

is connected to the FO pin, the 3dB input

EOSC

Due to the complex filtering and calibration algorithms utilized, the converter input bandwidth is not modeled very accurately by a first order filter with the pole located at the 3dB frequency. When the internal oscillator is used, the shape of the LTC2411/LTC2411-1 input bandwidth is shown in Figure␣ 28. When an external oscillator of frequency f

is used, the shape of the LTC2411/LTC2411-1

EOSC

input bandwidth can be derived from Figure␣ 28, FO = LOW curve of the LTC2411 in which the horizontal axis is scaled by f

EOSC

/153600.

MAX

= 5VV

REF

2411 F27

≤ 1LSB)

10010030507090

= 2.5V

VCC = 5V

–

= GND

REF

RESOLUTION (BITS)

= 2.5V

INCM

= 0V

= EXT OSC

RES = LOG

= 25°C

OUTPUT DATA RATE (READINGS/SEC)

REF

/INL

2(VREF

20 40 60 80

MAX

)

Figure 27. Resolution (INL vs Output Data Rate and Reference Voltage

0.0 –0.5 –1.0 –1.5 –2.0 –2.5 –3.0 –3.5 –4.0 –4.5 –5.0

INPUT SIGNAL ATTENUATION (dB)

–5.5 –6.0

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

FO = HIGH

(LTC2411)

FO = LOW

(LTC2411-1)

FO = LOW (LTC2411)

2411 F28

Figure 28. Input Signal Bandwidth Using the Internal Oscillator

solution is a high gain, low bandwidth amplifier stage followed by a high bandwidth unity-gain buffer.

The conversion noise (1.45µV

typical for V

RMS

REF

= 5V)

can be modeled as a white noise source connected to a noise free converter. The noise spectral density is 70nV/√Hz for an infinite bandwidth source and 126nV/√Hz for a single 0.5MHz pole source. From these numbers, it is clear that particular attention must be given to the design of external amplification circuits. Such circuits face the simultaneous requirements of very low bandwidth (just a few Hz) in order to reduce the output referred noise and relatively high bandwidth (at least 500kHz) necessary to drive the input switched-capacitor network. A possible

When external amplifiers are driving the LTC2411/ LTC2411-1, the ADC input referred system noise calculation can be simplified by Figure 29. The noise of an amplifier driving the LTC2411/LTC2411-1 input pin can be modeled as a band-limited white noise source. Its bandwidth can be approximated by the bandwidth of a single pole lowpass filter with a corner frequency fi. The amplifier noise spectral density is ni. From Figure␣ 29, using fi as the x-axis selector, we can find on the y-axis the noise equivalent bandwidth freqi of the input driving amplifier. This bandwidth includes the band limiting effects of the ADC

LTC2411/LTC2411-1

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APPLICATIO S I FOR ATIO

internal calibration and filtering. The noise of the driving amplifier referred to the converter input and including all these effects can be calculated as N␣ = ni • √freqi. The total system noise (referred to the LTC2411/LTC2411-1 input) can now be obtained by summing as square root of sum of squares the three ADC input referred noise sources: the LTC2411/LTC2411-1 internal noise (1.45µV), the noise of the IN+ driving amplifier and the noise of the IN– driving amplifier.

If the FO pin is driven by an external oscillator of frequency f

, Figure 29 can still be used for noise calculation if the

EOSC

x-axis is scaled by f ratio f

/153600, the Figure 29 plot accuracy begins to

EOSC

/153600. For large values of the

EOSC

decrease, but in the same time the LTC2411/LTC2411-1

1000

100

INPUT REFERRED NOISE

EQUIVALENT BANDWIDTH (Hz)

0.1

0.1 1 INPUT NOISE SOURCE SINGLE POLE

EQUIVALENT BANDWIDTH (Hz)

Figure 29. Input Referred Noise Equivalent Bandwidth of an Input Connected White Noise Source

FO = LOW

FO = HIGH

10 100 1k 10k 100k 1M

2411 G29

noise floor rises and the noise contribution of the driving amplifiers lose significance.

Normal Mode Rejection and Antialiasing

One of the advantages delta-sigma ADCs offer over conventional ADCs is on-chip digital filtering. Combined with a large oversampling ratio, the LTC2411/LTC2411-1 significantly simplifies antialiasing filter requirements.

The sinc4 digital filter provides greater than 120dB normal mode rejection at all frequencies except DC and integer multiples of the modulator sampling frequency (fS). The LTC2411/LTC2411-1’s autocalibration circuits further simplify the antialiasing requirements by additional normal mode signal filtering both in the analog and digital domain. Independent of the operating mode, fS = 256 • fN = 2048

• f

OUTMAX

where fN is the notch frequency and f

OUTMAX

is the maximum output data rate. In the internal oscillator mode, for the LTC2411, FS = 12800Hz with a 50Hz notch setting and fS = 15360Hz with a 60Hz notch setting. For the LTC2411-1, fS = 13980Hz (FO = LOW). In the external oscillator mode, fS = f

EOSC

/10.

The combined normal mode rejection performance is shown in Figure␣ 30 for the internal oscillator with 50Hz notch setting (FO = HIGH) and in Figure␣ 31 for the internal oscillator with FO = LOW and for the external oscillator mode. The regions of low rejection occurring at integer multiples of fS have a very narrow bandwidth. Magnified details of the normal mode rejection curves are shown in

FO = HIGH

–10 –20 –30 –40 –50 –60 –70 –80 –90

–100

INPUT NORMAL MODE REJECTION (dB)

–110 –120

S2fS3fS4fS5fS6fS7fS8fS9fS

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

10fS11fS12f

2411 F30

Figure 30. Input Normal Mode Rejection, Internal Oscillator and 50Hz Notch (LTC2411)

0 –10 –20 –30 –40 –50 –60 –70 –80 –90

–100

INPUT NORMAL MODE REJECTION (dB)

–110 –120

2fS3fS4fS5fS6fS7fS8fS9fS10f

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

FO = LOW OR

= EXTERNAL OSCILLATOR,

EOSC

2411 F31

Figure 31. Input Normal Mode Rejection, Internal Oscillator and FO = LOW or External Oscillator

= 10 • f

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

0 –10 –20 –30 –40 –50 –60 –70 –80 –90

–100

INPUT NORMAL MODE REJECTION (dB)

–110 –120

fN0 2fN3fN4fN5fN6fN7fN8f

INPUT SIGNAL FREQUENCY (Hz)

2411 F32

Figure 32. Input Normal Mode Rejection Figure 33. Input Normal Mode Rejection

Figure␣ 32 (rejection near DC) and Figure␣ 33 (rejection at fS = 256fN) where fN represents the notch frequency. These curves have been derived for the external oscillator mode but they can be used in all operating modes by appropriately selecting the fN value.

The user can expect to achieve in practice this level of performance using the internal oscillator as it is demonstrated by Figures 34 to 36. Typical measured values of the normal mode rejection of the LTC2411 operating with an internal oscillator and a 60Hz notch setting are shown in Figure 34 superimposed over the theoretical calculated curve. Similarly, typical measured values of the normal mode rejection of the LTC2411 operating with an internal oscillator and a 50Hz notch setting are shown in Figure 35 superimposed over the theoretical calculated curve.

0 –10 –20 –30 –40 –50 –60 –70 –80 –90

–100

INPUT NORMAL MODE REJECTION (dB)

–110 –120

250f

252fN254fN256fN258fN260fN262f

INPUT SIGNAL FREQUENCY (Hz)

2411 F33

–20

–40

–60

–80

NORMAL MODE REJECTION (dB)

–100

–120

0 15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240

MEASURED DATA CALCULATED DATA

INPUT FREQUENCY (Hz)

Figure 34. Input Normal Mode Rejection vs Input Frequency (LTC2411)

VCC = 5V V

REF

INCM

IN(P-P)

= GND

= 25°C

= 5V

= 2.5V

= 5V

2411 F34

As a result of these remarkable normal mode specifications, minimal (if any) antialias filtering is required in front of the LTC2411/LTC2411-1. If passive RC components are placed in front of the LTC2411/LTC2411-1, the input dynamic current should be considered (see Input Current section). In cases where large effective RC time constants are used, an external buffer amplifier may be required to minimize the effects of dynamic input current.

Traditional high order delta-sigma modulators, while providing very good linearity and resolution, suffer from potential instabilities at large input signal levels. The proprietary architecture used for the LTC2411/LTC2411-1 third order modulator resolves this problem and guarantees a predictable stable behavior at input signal levels of

–20

–40

–60

–80

NORMAL MODE REJECTION (dB)

–100

–120

0 25 50 75 100 125 150 175

MEASURED DATA CALCULATED DATA

INPUT FREQUENCY (Hz)

Figure 35. Input Normal Mode Rejection vs Input Frequency (LTC2411)

VCC = 5V V

= 5V

REF

INCM

IN(P-P)

= 5V

= 25°C

= 2.5V

= 5V

200

2411 F35

LTC2411/LTC2411-1

INPUT FREQUENCY (Hz)

0 15 30 45 60 75 90 105 120 135 150 165 180 195 210

225 240

NORMAL MODE REJECTION (dB)

2411 F37

–20

–40

–60

–80

–100

–120

VCC = 5V V

REF

= 5V

INCM

= 2.5V

= GND

= 25°C

IN(P-P)

= 5V

IN(P-P)

= 7.5V

(150% OF FULL SCALE)

INPUT FREQUENCY (Hz)

0 20 40 60 80 100 120 140 160 180 200 220

NORMAL MODE REJECTION (dB)

2411 F39

–20

–40

–60

–80

–100

–120

VCC = 5V V

REF

= 5V

REF

–

= GND

INCM

= 2.5V

= GND

= 25°C

IN(P-P)

= 5V

IN(P-P)

= 7.5V

(150% OF FULL SCALE)

INPUT FREQUENCY (Hz)

NORMAL MODE REJECTION (dB)

2411 F38

–20

–40

–60

–80

–100

–120

VCC = 5V V

REF

= 5V

INCM

= 2.5V

= 5V

= 25°C

IN(P-P)

= 5V

IN(P-P)

= 7.5V

(150% OF FULL SCALE)

25 50 75 100 125 150 175 200

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APPLICATIO S I FOR ATIO

–20

–40

–60

–80

NORMAL MODE REJECTION (dB)

–100

–120

0 20 40 60 80 100 120 140 160 180 200 220

MEASURED DATA CALCULATED DATA

INPUT FREQUENCY (Hz)

VCC = 5V V

= 5V

REF

–

REF

= GND

= 2.5V

INCM

IN(P-P)

= GND

= 25°C

= 5V

2411 F36

Figure 36. Input Normal Mode Rejection vs Input Frequency (LTC2411-1)

up to 150% of full scale. In many industrial applications, it is not uncommon to have to measure microvolt level signals superimposed over volt level perturbations and LTC2411/LTC2411-1 are eminently suited for such tasks. When the perturbation is differential, the specification of interest is the normal mode rejection for large input signal levels. With a reference voltage V

␣ =␣ 5V, the LTC2411/

REF

LTC2411-1 have a full-scale differential input range of 5V peak-to-peak. Figures 37 and 38 show measurement results for the LTC2411 normal mode rejection ratio with a

7.5V peak-to-peak (150% of full scale) input signal superimposed over the more traditional nor

mal mode rejection ratio results obtained with a 5V peak-to-peak (full scale) input signal and Figure 39 shows the corresponding measurement result for the LTC2411-1. It is clear that the LTC2411/LTC2411-1 rejection performance is maintained with no compromises in this extreme situation. When operating with large input signal levels, the user must observe that such signals do not violate the device absolute maximum ratings.

Figure 37. Measured Input Normal Mode Rejection vs Input Frequency (LTC2411)

Figure 38. Measured Input Normal Mode Rejection vs Input Frequency (LTC2411)

BRIDGE APPLICATIONS

Typical strain gauge based bridges deliver only 2mV/Volt of excitation. As the maximum reference voltage of the LTC2411/LTC2411-1 is 5V, remote sensing of applied excitation without additional circuitry requires that excitation be limited to 5V. This gives only 10mV full scale, which can be resolved to 1 part in 5000 without averaging. For many solid state sensors, this is comparable to the sensor.

Figure 39. Measured Input Normal Mode Rejection vs Input Frequency (LTC2411-1)

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APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

Averaging 64 samples however reduces the noise level by a factor of eight, bringing the resolving power to 1 part in 40000, comparable to better weighing systems. Hysteresis and creep effects in the load cells are typically much greater than this. Most applications that require strain measurements to this level of accuracy are measuring slowly changing phenomena, hence the time required to average a large number of readings is usually not an issue. For those systems that require accurate measurement of a small incremental change on a significant tare weight, the lack of history effects in the LTC2400 family is of great benefit.

For those applications that cannot be fulfilled by the LTC2411/LTC2411-1 alone, compensating for error in external amplification can be done effectively due to the “no latency” feature of the LTC2411/LTC2411-1. No latency operation allows samples of the amplifier offset and gain to be interleaved with weighing measurements. The use of correlated double sampling allows suppression of 1/f noise, offset and thermocouple effects within the bridge. Correlated double sampling involves alternating the polarity of excitation and dealing with the reversal of input polarity mathematically. Alternatively, bridge excitation can be increased to as much as ±10V, if one of several precision attenuation techniques is used to produce a precision divide operation on the reference signal.

Another option is the use of a reference within the 5V input range of the LTC2411/LTC2411-1 and developing excitation via fixed gain, or LTC1043 based voltage multiplication, along with remote feedback in the excitation amplifiers, as shown in Figures 45 and 46.

Figure 40 shows an example of a simple bridge connection. Note that it is suitable for any bridge application where measurement speed is not of the utmost importance. For many applications where large vessels are weighed, the average weight over an extended period of time is of concern and short term weight is not readily determined due to movement of contents, or mechanical resonance. Often, large weighing applications involve load cells located at each load bearing point, the output of which can be summed passively prior to the signal processing circuitry, actively with amplification prior to the ADC, or can be digitized via multiple ADC channels and summed mathematically. The mathematical summation of the output of multiple LTC2411/LTC2411-1’s provide the benefit of a root square reduction in noise. The low power consumption of the LTC2411/LTC2411-1 make it attractive for multidrop communication schemes where the ADC is located within the load-cell housing.

A direct connection to a load cell is perhaps best incorporated into the load-cell body, as minimizing the distance to

LT1019

BRIDGE

R1 AND R2 CAN BE USED TO INCREASE TOLERABLE AC COMPONENT ON REF SIGNALS

Figure 40. Simple Bridge Connection

3350Ω

REF

–

REF

LTC2411/

LTC2411-1

–

GND

SDO

SCK

2411 F40

LTC2411/LTC2411-1

WUU

APPLICATIO S I FOR ATIO

the sensor largely eliminates the need for protection devices, RFI suppression and wiring. The LTC2411/ LTC2411-1 exhibit extremely low temperature dependent drift. As a result, exposure to external ambient temperature ranges does not compromise performance. The incorporation of any amplification considerably complicates thermal stability, as input offset voltages and currents, temperature coefficient of gain settling resistors all become factors.

The circuit in Figure 41 shows an example of a simple amplification scheme. This example produces a differential output with a common mode voltage of 2.5V, as determined by the bridge. The use of a true three amplifier instrumentation amplifier is not necessary, as the LTC2411/ LTC2411-1 have common mode rejection far beyond that of most amplifiers. The LTC1051 is a dual autozero amplifier that can be used to produce a gain of 30 before its input referred noise dominates the LTC2411/LTC2411-1 noise. This example shows a gain of 34, that is determined by a feedback network built using a resistor array containing eight individual resistors. The resistors are organized to optimize temperature tracking in the presence of thermal gradients. The second LTC1051 buffers the low noise

input stage from the transient load steps produced during conversion.

The gain stability and accuracy of this approach is very good, due to a statistical improvement in resistor matching due to individual error contribution being reduced. A gain of 34 may seem low, when compared to common practice in earlier generations of load-cell interfaces, however the accuracy of the LTC2411/LTC2411-1 changes the rationale. Achieving high gain accuracy and linearity at higher gains may prove difficult, while providing little benefit in terms of noise reduction.

At a gain of 100, the gain error that could result from typical open-loop gain of 160dB is –1ppm, however, worst-case is at the minimum gain of 116dB, giving a gain error of –158ppm. Worst-case gain error at a gain of 34, is –54ppm. The use of the LTC1051A reduces the worstcase gain error to –33ppm. The advantage of gain higher than 34, then becomes dubious, as the input referred noise sees little improvement1 and gain accuracy is potentially compromised.

Input referred noise for AV = 34 is approximately 0.05µV

0.048µV

RMS

, whereas at a gain of 50, it would be

RMS

350Ω

BRIDGE

U1A

–

RN1

RN1 = 5k × 8 RESISTOR ARRAY U1A, U1B, U2A, U2B = 1/2 LTC1051

611

–

U1B

0.1µF

710

512

0.1µF

–

U2A

–

U2B

Figure 41. Using Autozero Amplifiers to Reduce Input Referred Noise

REF

0.1µF

REF

–

LTC2411/

LTC2411-1

–

GND

SDO

SCK

2411 F41

WUUU

APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

Note that this 4-amplifier topology has advantages over the typical integrated 3-amplifier instrumentation amplifier in that it does not have the high noise level common in the output stage that usually dominates when an instrumentation amplifier is used at low gain. If this amplifier is used at a gain of 10, the gain error is only 10ppm and input referred noise is reduced to 0.15µV

. The buffer stages

RMS

can also be configured to provide gain of up to 50 with high gain stability and linearity.

Figure 42 shows an example of a single amplifier used to produce single-ended gain. This topology is best used in applications where the gain setting resistor can be made to match the temperature coefficient of the strain gauges. If the bridge is composed of precision resistors, with only one or two variable elements, the reference arm of the bridge can be made to act in conjunction with the feedback resistor to determine the gain. If the feedback resistor is incorporated into the design of the load cell, using resistors which match the temperature coefficient of the loadcell elements, good results can be achieved without the need for resistors with a high degree of absolute accuracy. The common mode voltage in this case, is again a function of the bridge output. Differential gain as used with a 350Ω bridge is:

995

+Ω

1 175

Common mode gain is half the differential gain. The maximum differential signal that can be used is 1/4 V as opposed to 1/2 V

in the 2-amplifier topology above.

REF

Remote Half Bridge Interface

As opposed to full bridge applications, typical half bridge applications must contend with nonlinearity in the bridge output, as signal swing is often much greater. Applications include RTD’s, thermistors and other resistive elements that undergo significant changes over their span. For single variable element bridges, the nonlinearity of the half

2.7V TO 5.5V

REF

–

LTC2411/

LTC2411-1

GND

2411 F43

25.5k

0.1%

100Ω

RTD

PLATINUM

Figure 43. Remote Half Bridge Interface

350Ω

BRIDGE

0.1µV

1µF

= 9.95 =

4.99k

R1 + R2

R1 + 175Ω

LTC1050S8

–

46.4k

175Ω

1µF

20k

Figure 42. Bridge Amplification Using a Single Amplifier

10µF

2 3

REF

–

REF

LTC2411/

LTC2411-1

–

GND

0.1µF

2411 F42

LTC2411/LTC2411-1

WUUU

APPLICATIO S I FOR ATIO

bridge output can be eliminated completely; if the reference arm of the bridge is used as the reference to the ADC, as shown in Figure 43. The LTC2411/LTC2411-1 can accept inputs up to 1/2 V

. Hence, the reference resistor

REF

R1 must be at least 2× the highest value of the variable resistor.

In the case of 100Ω platinum RTD’s, this would suggest a value of 800Ω for R1. Such a low value for R1 is not advisable due to self-heating effects. A value of 25.5k is shown for R1, reducing self-heating effects to acceptable levels for most sensors.

The basic circuit shown in Figure 43 shows connections for a full 4-wire connection to the sensor, which may be located remotely. The differential input connections will reject induced or coupled 60Hz interference, however, the reference inputs do not have the same rejection. If 60Hz or other noise is present on the RTD, a low pass filter is recommended as shown in Figure 44. Note that you cannot place a large capacitor directly at the junction of R1 and R2, as it will store charge from the sampling process. A better approach is to produce a low pass filter decoupled from the input lines with a high value resistor (R3).

The use of a third resistor in the half bridge, between the variable and fixed elements gives essentially the same

result as the two resistor version, but has a few benefits. If, for example, a 25k reference resistor is used to set the excitation current with a 100Ω RTD, the negative reference input is sampling the same external node as the positive input, but may result in errors if used with a long cable. For short cable applications, the errors may be acceptably low. If instead the single 25k resistor is replaced with a 10k 5% and a 10k 0.1% reference resistor, the noise level introduced at the reference, at least at higher frequencies, will be reduced. A filter can be introduced into the network, in the form of one or more capacitors, or ferrite beads, as long as the sampling pulses are not translated into an error. The reference voltage is also reduced, but this is not undesirable, as it will decrease the value of the LSB, although, not the input referred noise level.

The circuit shown in Figure 44 shows a more rigorous example of Figure 43, with increased noise suppression and more protection for remote applications.

Figure 45 shows an example of gain in the excitation circuit and remote feedback from the bridge. The LTC1043s provide voltage multiplication, providing ±10V from a 5V reference with only 1ppm error. The amplifiers are used at unity-gain and, hence, introduce a very little error due to

10k

0.1% R3

1µF

10k, 5%

PLATINUM

100Ω

RTD

Figure 44. Remote Half Bridge Sensing with Noise Suppression on Reference

10k 5%

LTC1050

–

560Ω

10k

2 3

REF REF

LTC2411/

LTC2411-1

–

GND

2411 F44

WUUU

APPLICATIO S I FOR ATIO

LTC2411/LTC2411-1

350Ω

BRIDGE

2N3904

33Ω

10V

2N3906

–15V

–10V

33Ω

20Ω

U2 LTC1043

15V

47µF 0.1µF

10µF

0.1µF

LT1236-5

+ +

LTC2411/

LTC2411-1

REF

–

REF

–

GND

10V

15V15V

LTC1150

–15V

15V

LTC1150

–15V

1µF

–

0.1µF

200Ω

10V 5V

U1 LTC1043

0.1µF

–10V

1µF

FILM

–10V

200Ω

U2 LTC1043

*FLYING CAPACITORS ARE 1µF FILM (MKP OR EQUIVALENT)

SEE LTC1043 DATA SHEET FOR DETAILS ON UNUSED HALF OF U1

2411 F45

Figure 45. LTC1043 Provides Precise 4× Reference for Excitation Voltages

LTC2411/LTC2411-1

WUUU

APPLICATIO S I FOR ATIO

gain error or due to offset voltages. A 1µV/°C offset voltage drift translates into 0.05ppm/°C gain error. Simpler alternatives, with the amplifiers providing gain using resistor arrays for feedback, can produce results that are similar to bridge sensing schemes via attenuators. Note that the amplifiers must have high open-loop gain or gain error will be a source of error. The fact that input offset voltage has relatively little effect on overall error may lead one to use low performance amplifiers for this application. Note that the gain of a device such as an LF156, (25V/mV over temperature) will produce a worst-case error of –180ppm at a noise gain of 3, such as would be encountered in an inverting gain of 2, to produce –10V from a 5V reference.

The error associated with the 10V excitation would be –80ppm. Hence, overall reference error could be as high as 130ppm, the average of the two.

Figure 47 shows a similar scheme to provide excitation using resistor arrays to produce precise gain. The circuit is configured to provide 10V and –5V excitation to the bridge, producing a common mode voltage at the input to

the LTC2411/LTC2411-1 of 2.5V, maximizing the AC input range for applications where induced 60Hz could reach amplitudes up to 2V

RMS

The circuits in Figures 45 and 47 could be used where multiple bridge circuits are involved and bridge output can be multiplexed onto a single LTC2411/LTC2411-1, via an inexpensive multiplexer such as the 74HC4052.

Figure 46 shows the use of an LTC2411/LTC2411-1 with a differential multiplexer. This is an inexpensive multiplexer that will contribute some error due to leakage if used directly with the output from the bridge, or if resistors are inserted as a protection mechanism from overvoltage. Although the bridge output may be within the input range of the A/D and multiplexer in normal operation, some thought should be given to fault conditions that could result in full excitation voltage at the inputs to the multiplexer or ADC. The use of amplification prior to the multiplexer will largely eliminate errors associated with channel leakage developing error voltages in the source impedance.

TO OTHER

DEVICES

12 14 15 11

74HC4052

1 5

2 4

47µF

13 3

2 3

4 5

REF

–

REF

LTC2411/

LTC2411-1

–

GND

A0 A1

2411 F46

Figure 46. Use a Differential Multiplexer to Expand Channel Capability

PACKAGE DESCRIPTIO

MS10 Package

10-Lead Plastic MSOP

(Reference LTC DWG # 05-08-1661)

0.118 ± 0.004* (3.00 ± 0.102)

LTC2411/LTC2411-1

8910

0.193 ± 0.006

(4.90 ± 0.15)

0.043

(1.10)

0.007 (0.18)

0.021

± 0.006

(0.53 ± 0.015)

* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH,

PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.

INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

° – 6° TYP

SEATING

PLANE

MAX

0.007 – 0.011 (0.17 – 0.27)

0.0197 (0.50)

BSC

0.118 ± 0.004** (3.00 ± 0.102)

0.034 (0.86)

REF

0.005

± 0.002

(0.13 ± 0.05)

MSOP (MS10) 1100

Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

LTC2411/LTC2411-1

TYPICAL APPLICATIO

350Ω BRIDGE

TWO ELEMENTS

VARYING

2N3904

22Ω

10V

33Ω

×2

Q2, Q3

2N3906

×2

–15V

15V

–5V

20Ω

RN1

10k

RN1

10k

20Ω

0.1µF

1/2

LT1112

15V

1/2

LT1112

–15V

RN1

10k

–

LT1236-5

C3 47µF

LTC2411/

LTC2411-1

REF

–

REF

–

RN1 10k

RN1 IS CADDOCK T914 10K-010-02

2411 F47

GND

0.1µF

Figure 47. Use Resistor Arrays to Provide Precise Matching in Excitation Amplifier

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LT1019 Precision Bandgap Reference, 2.5V, 5V 3ppm/°C Drift, 0.05% Max Initial Accuracy LT1025 Micropower Thermocouple Cold Junction Compensator 80µA Supply Current, 0.5°C Initial Accuracy LTC1050 Precision Chopper Stabilized Op Amp No External Components 5µV Offset, 1.6µV LT1236A-5 Precision Bandgap Reference, 5V 0.05% Max Initial Accuracy, 5ppm/°C Drift LT1460 Micropower Series Reference 0.075% Max Initial Accuracy, 10ppm/°C Max Drift LTC2400 24-Bit, No Latency ∆Σ ADC in SO-8 0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA LTC2401/LTC2402 1-/2-Channel, 24-Bit, No Latency ∆Σ ADC in MSOP 0.6ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA LTC2404/LTC2408 4-/8-Channel, 24-Bit, No Latency ∆Σ ADC 0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA LTC2410 24-Bit, Fully Differential, No Latency ∆Σ ADC 0.16ppm Noise, 2ppm INL, 10ppm Total Unadjusted Error, 200µA LTC2413 24-Bit, Fully Differential, No Latency ∆Σ ADC Simultaneous 50Hz and 60Hz Rejection, 800nV LTC2415 24-Bit, No Latency ∆Σ ADC with 15Hz Output Rate Pin Compatible with the LTC2410 LTC2420 20-Bit, No Latency ∆Σ ADC in SO-8 1.2ppm Noise, 8ppm INL, Pin Compatible with LTC2400 LTC2424/LTC2428 4-/8-Channel, 20-Bit, No Latency ∆Σ ADC 1.2ppm Noise, Pin Compatible with LTC2404/LTC2408

P-P

Noise

RMS

Noise

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear.com

24111f LT/TP 0601 2K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 2000

Datasheet LTC2411, LTC2411-1 Datasheet (LINEAR TECHNOLOGY)

Specifications and Main Features

Frequently Asked Questions

User Manual