LINEAR TECHNOLOGY LTC2421, LTC2422 Technical data

FEATURES

1-/2-Channel 20-Bit µPower

No Latency ∆Σ

DESCRIPTIO

LTC2421/LTC2422

TM

ADCs in MSOP-10

U

■

20-Bit ADCs in Tiny MSOP-10 Packages

■

1- or 2-Channel Inputs

■

Single Supply 2.7V to 5.5V Operation

■

Low Supply Current (200µA) and Auto Shutdown

■

Automatic Channel Selection (Ping-Pong) (LTC2422)

■

No Latency: Digital Filter Settles in a
Single Conversion Cycle

■

8ppm INL, No Missing Codes

■

4ppm Full-Scale Error

■

0.5ppm Offset

■

1.2ppm Noise

■

Zero Scale and Full Scale Set for Reference
and Ground Sensing

■

Internal Oscillator—No External Components Required

■

110dB Min, 50Hz/60Hz Notch Filter

■

Reference Input Voltage: 0.1V to V

■

Live Zero—Extended Input Range Accommodates

CC

12.5% Overrange and Underrange

■

Pin Compatible with LTC2401/LTC2402

U

APPLICATIO S

■

Weight Scales

■

Direct Temperature Measurement

■

Gas Analyzers

■

Strain Gauge Transducers

■

Instrumentation

■

Data Acquisition

■

Industrial Process Control

The LTC®2421/LTC2422 are 1- and 2-channel 2.7V to 5.5V
micropower 20-bit analog-to-digital converters with an
integrated oscillator, 8ppm INL and 1.2ppm RMS noise.
These ultrasmall devices use delta-sigma technology and
a new digital filter architecture that settles in a single cycle.
This eliminates the latency found in conventional ∆Σ
converters and simplifies multiplexed applications.

Through a single pin, the LTC2421/LTC2422 can be
configured for better than 110dB rejection at 50Hz or
60Hz ±2%, or can be driven by an external oscillator for
a user defined rejection frequency in the range 1Hz to
120Hz. The internal oscillator requires no external fre­quency setting components.

These converters accept an external reference voltage
from 0.1V to VCC. With an extended input conversion
range of –12.5% V
ZS

), the LTC2421/LTC2422 smoothly resolve the off-

SET

to 112.5% V

REF

REF

(V

REF

= FS

SET

–

set and overrange problems of preceding sensors or
signal conditioning circuits.

The LTC2421/LTC2422 communicate through a 2- or
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.

TYPICAL APPLICATIO

2.7V TO 5.5V

1µF

110

REFERENCE VOLTAGE

ZS

+ 0.1V TO V

SET

ANALOG INPUT RANGE

– 0.12V

SET

+ 0.12V

– ZS

SET

SET

TO

REF

REF

SET

– 100mV

ZS

(V

REF

SET

FS
= FS

0V TO FS

CC

)

V

2

FS

3

CH1 SDO

4

CH0

5

ZS

CC

LTC2422

SET

SET

SCK

GND

F

O

CS

U

Pseudo Differential Bridge Digitizer

2.7V TO 5.5V

V

CC

= INTERNAL OSC/50Hz REJECTION
= EXTERNAL CLOCK SOURCE
= INTERNAL OSC/60Hz REJECTION

9

8

7

6

3-WIRE
SPI INTERFACE

24212 TA01

1

V

CC

LTC2422

2

FS

SET

4

CH0

3

CH1

5

ZS

SET

GND

9

SCK

8

CS

F

O

7

10

3-WIRE
SPI INTERFACE

INTERNAL OSCILLATOR
60Hz REJECTION

24012TA02

24212f

SDO

6

1

LTC2421/LTC2422

1
2
3
4
5

V

CC

FS

SET

CH1
CH0

ZS

SET

10
9
8
7
6

F

O

SCK
SDO
CS
GND

TOP VIEW

MS10 PACKAGE

10-LEAD PLASTIC MSOP

WW

W

ABSOLUTE MAXIMUM RATINGS

U

(Notes 1, 2)

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

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

Reference Input 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)

UUW

PACKAGE/ORDER INFORMATION

TOP VIEW

10

1

V

CC

FS

2

SET

V

3

IN

NC

4

ZS

5

SET

MS10 PACKAGE

10-LEAD PLASTIC MSOP

T

= 125°C, θJA = 130°C/W

JMAX

F

O

SCK

9

SDO

8

CS

7

GND

6

Consult factory for parts specified with wider operating temperature ranges.

ORDER PART NUMBER

LTC2421CMS
LTC2421IMS

MS10 PART MARKING

LTUX
LTUY

Operating Temperature Range

LTC2421/LTC2422C ................................ 0°C to 70°C

LTC2421/LTC2422I ............................ –40°C to 85°C

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

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

ORDER PART NUMBER

LTC2422CMS
LTC2422IMS

MS10 PART MARKING

T

= 125°C, θJA = 130°C/W

JMAX

LTUZ
LTVA

U

CONVERTER CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Resolution ● 20 Bits
No Missing Codes Resolution 0.1V ≤ FS
Integral Nonlinearity FS

FS

SET
SET

Offset Error 2.5V ≤ FS
Offset Error Drift 2.5V ≤ FS
Full-Scale Error 2.5V ≤ FS
Full-Scale Error Drift 2.5V ≤ FS
Total Unadjusted Error FS

FS

SET
SET

Output Noise VIN = 0V (Note 13) 6 µV
Normal Mode Rejection 60Hz ±2% (Note 7) ● 110 130 dB
Normal Mode Rejection 50Hz ±2% (Note 8) ● 110 130 dB
Power Supply Rejection, DC FS
Power Supply Rejection, 60Hz ±2% FS
Power Supply Rejection, 50Hz ±2% FS

SET

SET

SET

≤ VCC, ZS

SET

= 2.5V, ZS
= 5V, ZS

≤ VCC, ZS

SET

≤ VCC, ZS

SET

≤ VCC, ZS

SET

≤ VCC, ZS

SET

SET

= 2.5V, ZS
= 5V, ZS

SET

= 2.5V, ZS
= 2.5V, ZS
= 2.5V, ZS

SET

= 0V (Note 6) ● 8 20 ppm of V

SET

= 0V 16 ppm of V

SET

SET

SET

The ● denotes specifications which apply over the full operating

= FS

REF

= 0V (Note 5) ● 20 Bits

SET

= 0V (Note 6) ● 4 10 ppm of V

= 0V ● 0.5 10 ppm of V

SET

= 0V 0.04 ppm of V

SET

= 0V ● 4 10 ppm of V

SET

= 0V 0.04 ppm of V

SET

= 0V 8 ppm of V

= 0V, VIN = 0V 100 dB
= 0V, VIN = 0V, (Notes 7, 15) 110 dB
= 0V, VIN = 0V, (Notes 8, 15) 110 dB

SET

– ZS

. (Notes 3, 4)

SET

REF

REF

REF
REF

REF

/°C

REF

/°C

REF
REF

RMS

2

24212f

LTC2421/LTC2422

UU

U

A ALOG I PUT A D REFERE CE

temperature range, otherwise specifications are at TA = 25°C. V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

IN

FS

SET

ZS

SET

C

S(IN)

C

S(REF)

I

IN(LEAK)

I

REF(LEAK)

Input Voltage Range (Note 14) ● ZS
Full-Scale Set Range ● 0.1 + ZS
Zero-Scale Set Range ● 0FS
Input Sampling Capacitance 1 pF
Reference Sampling Capacitance 1.5 pF
Input Leakage Current CS = V
Reference Leakage Current V

U

CC

= 2.5V, CS = V

REF

The ● denotes specifications which apply over the full operating

REF

= FS

CC

SET

– ZS

. (Note 3)

SET

– 0.12V

SET

● –100 1 100 nA

● –100 1 100 nA

REF

SET

FS

+ 0.12V

SET

SET

V

CC

– 0.1 V

REF

V
V

UU

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

V

IH

V

IL

V

IH

V

IL

I

IN

I

IN

C

IN

C

IN

V

OH

V

OL

V

OH

V

OL

I

OZ

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

O

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

O

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

O

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

Digital Input Capacitance 10 pF
CS, F

O

Digital Input Capacitance (Note 9) 10 pF
SCK

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

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

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

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

High-Z Output Leakage ● –10 10 µA
SDO

2.7V ≤ VCC ≤ 3.3V 2.0 V

2.7V ≤ VCC ≤ 5.5V 0.6 V

The ● denotes specifications which apply over the full

≤ 3.3V (Note 9) 2.0 V

CC

≤ 5.5V (Note 9) 0.6 V

CC

CC

● –10 10 µA

WU

POWER REQUIRE E TS

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

V

CC

I

CC

Supply Voltage ● 2.7 5.5 V
Supply Current

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

(Note 12) ● 20 30 µA

CC

● 200 300 µA

24212f

3

LTC2421/LTC2422

UW

TI I G CHARACTERISTICS

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

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

f

EOSC

t

HEO

t

LEO

t

CONV

f

ISCK

D

ISCK

f

ESCK

t

LESCK

t

HESCK

t

DOUT_ISCK

t

DOUT_ESCK

t

1

t

2

t

3

t

4

t

KQMAX

t

KQMIN

t

5

t

6

External Oscillator Frequency Range ● 2.56 307.2 kHz
External Oscillator High Period ● 0.5 390 µs
External Oscillator Low Period ● 0.5 390 µs
Conversion Time FO = 0V ● 130.86 133.53 136.20 ms

Internal SCK Frequency Internal Oscillator (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 24-Bit Data Output Time Internal Oscillator (Notes 10, 12) ● 1.23 1.25 1.28 ms

External SCK 24-Bit Data Output Time (Note 9) ● 24/f
CS ↓ to SDO Low Z ● 0 150 ns
CS ↑ to SDO High Z ● 0 150 ns
CS ↓ to SCK ↓ (Note 10) ● 0 150 ns
CS ↓ to SCK ↑ (Note 9) ● 50 ns
SCK ↓ to SDO Valid ● 200 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

= V

F

O

CC

External Oscillator (Note 11)

External Oscillator (Notes 10, 11) f

External Oscillator (Notes 10, 11)

● 157.03 160.23 163.44 ms

● 20510/f

● 192/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: V

= 2.7 to 5.5V unless otherwise specified. Input source

CC

resistance = 0Ω.
Note 4: Internal Conversion Clock source with the F

to GND or to V
f

= 153600Hz unless otherwise specified.

EOSC

or to external conversion clock source with

CC

pin tied

O

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

O

= 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

LOAD

= 20pF.

Note 11: The external oscillator is connected to the FO pin. The external
oscillator frequency, f

, is expressed in kHz.

EOSC

Note 12: The converter uses the internal oscillator.

= 0V or FO = VCC.

F

O

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

= FS

Note 14: V

REF

SET

– ZS

to –0.3V and the maximum to V
Note 15: VCC (DC) = 4.1V, VCC (AC) = 2.8V

. The minimum input voltage is limited

SET

+ 0.3V.

CC

.

P-P

24212f

4

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Total Unadjusted Error (3V Supply) INL (3V Supply)

10

VCC = 3V

8

= 2.5V

V

REF

6
4
2
0

–2

ERROR (ppm)

–10

–4
–6
–8

0

TA = –55°C, –45°C, 25°C, 90°C

0.5
INPUT VOLTAGE (V)

1.0

1.5

Positive Extended Input Range
Total Unadjusted Error (3V Supply)

VCC = 3V

8

= 2.5V

V

REF

6
4
2
0

–2

ERROR (ppm)

–4
–6
–8

–10

TA = –55°C, –45°C, 25°C, 90°C

2.50

2.55

2.60

INPUT VOLTAGE (V)

2.65 2.70

2.0

24212 G01

2.75102.80

24212 G04

2.5

10

VCC = 3V

8

= 2.5V

V

REF

6
4

2
0

–2

ERROR (ppm)

–4
–6
–8

–10

0

TA = –55°C, –45°C, 25°C, 90°C

0.5
INPUT VOLTAGE (V)

1.0

1.5

2.0

Total Unadjusted Error (5V Supply) INL (5V Supply)

10

VCC = 5V

8

V

= 5V

REF

6
4

2
0

–2

ERROR (ppm)

–4
–6
–8

–10

TA = –55°C, –45°C, 25°C, 90°C

1

0

2

INPUT VOLTAGE (V)

3

4

24212 G02

24212 G05

2.5

5

LTC2421/LTC2422

Negative Extended Input Range
Total Unadjusted Error (3V Supply)

VCC = 3V

8

V

REF

6
4
2
0

–2

ERROR (ppm)

–4
–6
–8

–10

0

10

VCC = 5V

8

V

REF

6
4

2
0

–2

ERROR (ppm)

–4
–6
–8

–10

0

= 2.5V

–0.05

= 5V

TA = –55°C, –45°C, 25°C, 90°C

1

TA = 90°C

–0.15 –0.20

–0.10

INPUT VOLTAGE (V)

3

2

INPUT VOLTAGE (V)

TA = 25°C

TA = –45°C

TA = –55°C

–0.2510–0.30

4

24212 G03

5

24212 G06

Negative Extended Input Range
Total Unadjusted Error (5V Supply)

VCC = 5V

8

V

REF

6
4
2
0

–2

ERROR (ppm)

–4
–6
–8

–10

0

= 5V

–0.05

–0.10
INPUT VOLTAGE (V)

TA = 25°C
TA = 90°C
TA = –45°C

–0.15 –0.20

TA = –55°C

–0.2510–0.30

24212 G07

Positive Extended Input Range
Total Unadjusted Error (5V Supply)

VCC = 5V

8

= 5V

V

REF

6
4
2
0

–2

ERROR (ppm)

–4

–6

–10

–8

5.00

5.05

TA = 25°CTA = 90°C

5.15 5.20

5.10

INPUT VOLTAGE (V)

TA = –55°C

TA = –45°C

5.25105.30

24212 G08

Offset Error vs Reference Voltage

150

120

90

60

OFFSET ERROR (ppm)

30

0

1

0

REFERENCE VOLTAGE (V)

3

2

VCC = 5V

= 25°C

T

A

4

24212 G09

5

24212f

5

LTC2421/LTC2422

UW

TYPICAL PERFOR A CE CHARACTERISTICS

RMS Noise vs Reference Voltage Offset Error vs V

60

50

)

REF

40

30

20

RMS NOISE (ppm OF V

10

0

0

1234

REFERENCE VOLTAGE (V)

VCC = 5V

= 25°C

T

A

24212 G10

5

10

V

= 2.5V

REF

T

= 25°C

A

5

0

OFFSET ERROR (ppm)

–5

–10

2.7

3.2 3.7 4.2 4.7

CC

V

(V)

CC

5.2 5.5

24212 G11

RMS Noise vs V

10.0
V

= 2.5V

REF

T

= 25°C

A

7.5

5.0

RMS NOISE (ppm)

2.5

0

2.7

3.2 3.7 4.2 4.7

CC

VCC (V)

Noise Histogram RMS Noise vs Code Out Offset Error vs Temperature

350

VCC = 5

= 5

V

REF

300

= 0

V

IN

250

200

150

100

NUMBER OF READINGS

50

5.00
VCC = 5V

= 5V

V

REF

V

= 0.3V TO 5.3V

IN

= 25°C

T

A

3.75

2.50

RMS NOISE (ppm)

1.25

10

VCC = 5V

= 5V

V

REF

= 0V

V

IN

5

0

OFFSET ERROR (ppm)

–5

5.2 5.5

24212 G12

0

–2 0

26

OUTPUT CODE (ppm)

Full-Scale Error vs Temperature

10

VCC = 5V

= 5V

V

REF

= 5V

V

IN

5

0

–5

FULL-SCALE ERROR (ppm)

–10

–55

–30 –5 20 45

TEMPERATURE (°C)

4

24212 G13

70 95 120

24212 G16

0

0 7FFFF FFFFF

CODE OUT (HEX)

24212 G14

–10

–55

–30 –5 20 45

TEMPERATURE (°C)

Full-Scale Error
vs Reference Voltage Full-Scale Error vs V

0

–25

–50

–75

–100

FULL-SCALE ERROR (ppm)

–125

–150

0

1234

REFERENCE VOLTAGE (V)

VCC = 5V

= V

V

IN

24212 G17

REF

5

10

V

= 2.5V

REF

= 2.5V

V

IN

= 25°C

T

A

5

0

–5

FULL-SCALE ERROR (ppm)

–10

2.7

3.2 3.7 4.2 4.7

70 95 120

24212 G15

CC

5.2 5.5

VCC (V)

24212 G18

24212f

6

UW

FREQUENCY AT VIN (Hz)

1

–120

REJECTION (dB)

–100

–80

–60

–40

–20

0

50 100 150 200

24212 G24

250

VCC = 5V
V

REF

= 5V

V

IN

= 2.5V

F

O

= 0

INPUT FREQUENCY

0

–60

–40

0

24212 G27

–80

–100

fS/2 f

S

–120

–140

–20

REJECTION (dB)

TYPICAL PERFOR A CE CHARACTERISTICS

Conversion Current
vs Temperature

230

220

210

200

190

180

SUPPLY CURRENT (µA)

170

160

150

–30 –5 45

–55

VCC = 5.5V

VCC = 4.1V

VCC = 2.7V

20

TEMPERATURE (°C)

70 95 120

24212 G19

Sleep Current vs Temperature

30

20

10

SUPPLY CURRENT (µA)

0

–30 –5 20 45

–55

VCC = 2.7V

VCC = 5V

TEMPERATURE (°C)

70 95 120

24212 G20

LTC2421/LTC2422

Rejection vs Frequency at V

0

VCC = 4.1V

= 0V

V

IN

–20

= 25°C

T

A

= 0

F

O

–40

–60

REJECTION (dB)

–80

–100

–120

1

100 10k 1M

FREQUENCY AT VCC (Hz)

CC

24212 G23

Rejection vs Frequency at V

–20

VCC = 4.1V
V

= 0V

IN

= 25°C

T

A

–40

F

= 0

O

–60

–80

REJECTION (dB)

–100

–120

50

1

FREQUENCY AT VCC (Hz)

100

150

Rejection vs Frequency at V

–60

–70

–80

–90

–100

–110

REJECTION (dB)

–120

–130

–140

–12–8–404812

INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)

200

CC

250

24212 G21

IN

24212 G25

Rejection vs Frequency at V

0

VCC = 4.1V

= 0V

V

IN

–20

= 25°C

T

A

= 0

F

O

–40

–60

REJECTION (dB)

–80

–100

–120

15200

15300 15350 15400

15250

FREQUENCY AT VCC (Hz)

Rejection vs Frequency at V

0

VCC = 5V

= 5V

V

REF

–20

= 2.5V

V

IN

= 0

F

O

–40

–60

REJECTION (dB)

–80

–100

–120

SAMPLE RATE = 15.36kHz ±2%

15100

15200 15300 15400 15500

FREQUENCY AT VIN (Hz)

CC

15450 15500

24212 G22

IN

24212 G26

Rejection vs Frequency at V

Rejection vs Frequency at V

IN

IN

24212f

7

LTC2421/LTC2422

UW

TYPICAL PERFOR A CE CHARACTERISTICS

INL vs Output Rate

20

VCC = 5V

= 5V

V

REF

= EXTERNAL

F

O

18

16

14

TUE RESOLUTION (BITS)

12

10

0

10

TA = –45°C

30

20

OUTPUT RATE (Hz)

TA = 25°C

40

50 60 70 80 90

TA = 90°C

100

24212 G28

INL vs Output Rate

20

VCC = 3V

= 2.5V

V

REF

= EXTERNAL

F

O

18

16

TA = –45°C

14

TA = 25°C

TUE RESOLUTION (BITS)

12

10

102030

0

OUTPUT RATE (Hz)

40

UUU

PIN FUNCTIONS

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.

Resolution vs Output Rate

24

22

20

TA = 90°C

50 60 70 80 90

24212 G29

100

VCC = 5V

18

V

= 5V

REF

EFFECTIVE RESOLUTION (BITS)

= EXTERNAL

f

O

STANDARD DEVIATION
OF 100 SAMPLES

16

0 7.5

25

50

OUTPUT RATE (Hz)

TA = 25°C

= 90°C

T

A

T

= –45°C

A

75

100

24212 G30

be connected directly to a ground plane through a mini­mum length trace or it should be the single-point-ground
in a single-point grounding system.

FS

(Pin 2): Full-Scale Set Input. This pin defines the

SET

full-scale input value. When VIN = FS

, the ADC outputs

SET

full scale (FFFFFH). The total reference voltage is
FS

SET

– ZS

SET

.

CH0, CH1 (Pins 4, 3): Analog Input Channels. The input
voltage range is –0.125 • V
V

> 2.5V, the input voltage range may be limited by the

REF

to 1.125 • V

REF

REF

. For

absolute maximum rating of – 0.3V to VCC + 0.3V. Conver­sions are performed alternately between CH0
and CH1 for the LTC2422. Pin 4 is a No Connect (NC) on
the LTC2421.

ZS

(Pin 5): Zero-Scale Set Input. This pin defines the

SET

zero-scale input value. When VIN = ZS

, the ADC

SET

outputs zero scale (00000H).
GND (Pin 6): Ground. Shared pin for analog ground,

digital ground, reference ground and signal ground. Should

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
the Sleep mode and remains in this low power state as
long as CS is HIGH. A LOW on CS wakes up the ADC. A
LOW-to-HIGH transition on this pin disables the SDO
digital output. 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 for 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 can be used as a conversion status out­put. The conversion status can be observed by pulling CS
LOW.

8

24212f

LTC2421/LTC2422

3.4k

SDO

24212 TC02

Hi-Z TO V

OL

VOH TO V

OL

VOL TO Hi-Z

C

LOAD

= 20pF

V

CC

U

UU

PIN FUNCTIONS

SCK (Pin 9): Bidirectional Digital Clock Pin. In the Internal
Serial Clock Operation mode, SCK is used as digital output
for the internal serial interface clock during the data output
period. In the External Serial Clock Operation mode, SCK
is used as digital input for the external serial interface. An
internal pull-up current source is automatically activated
in Internal Serial Clock Operation mode. The Serial Clock
mode is determined by the level applied to SCK at power
up and the falling edge of CS.

UU

W

FUNCTIONAL BLOCK DIAGRA

V

CC

GND

FO (Pin 10): Frequency Control Pin. Digital input that
controls the ADC’s notch frequencies and conversion
time. When the FO pin is connected to VCC (FO = VCC), the
converter uses its internal oscillator and the digital filter’s
first null is located at 50Hz. When the FO pin is connected
to GND (FO = 0V), the converter uses its internal oscillator
and the digital filter’s first null is located at 60Hz. When F
is driven by an external clock signal with a frequency f

EOSC

O

,
the converter uses this signal as its clock and the digital
filter first null is located at a frequency f

OSCILLATOR

AUTOCALIBRATION

AND CONTROL

EOSC

INTERNAL

/2560.

(INT/EXT)

F

O

V

IN

V

REF

TEST CIRCUITS

SDO

DAC

3.4k

Hi-Z TO V

OH

VOL TO V

OH

VOH TO Hi-Z

C

LOAD

∫∫∫

= 20pF

24212 TC01

∑

ADC

DECIMATING FIR

SERIAL

INTERFACE

SDO

SCK

CS

24212 FD

24212f

9

LTC2421/LTC2422

CONVERT

SLEEP

DATA OUTPUT

24212 F02

0

1

CS AND

SCK

WUUU

APPLICATIO S I FOR ATIO

The LTC2421/LTC2422 are pin compatible with the
LTC2401/LTC2402. The devices are designed to allow the
user to incorporate either device in the same design with
no modifications. While the LTC2421/LTC2422 output word
length is 24 bits (as opposed to the 32-bit output of the
LTC2401/LTC2402), its output clock timing can be identi­cal to the LTC2401/LTC2402. As shown in Figure 1, the
LTC2421/LTC2422 data output is concluded on the falling
edge of the 24th serial clock (SCK). In order to maintain
drop-in compatibility with the LTC2401/LTC2402, it is
possible to clock the LTC2421/LTC2422 with an additional
8 serial clock pulses. This results in 8 additional output bits
which are always logic HIGH.

Converter Operation Cycle

The LTC2421/LTC2422 are low power, delta-sigma ana­log-to-digital converters with an easy to use 3-wire serial
interface. Their operation is simple and made up of three
states. The converter operating cycle begins with the con­version, followed by the sleep state and concluded with the
data output (see Figure 2). The 3-wire interface consists of
serial data output (SDO), a serial clock (SCK) and a chip
select (CS).

Once CS is pulled LOW and SCK rising edge is applied, the
device begins 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 4. The data output
state is concluded once 24 bits are read out of the ADC or
when CS is brought HIGH. The device automatically
initiates a new conversion and the cycle repeats.

Through timing control of the CS and SCK pins, the
LTC2421/LTC2422 offer several flexible modes of opera­tion (internal or external SCK and free-running conver­sion modes). These various modes do not require
program

ming 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.

Initially, the LTC2421/LTC2422 perform a conversion. Once
the conversion is complete, the device enters the sleep
state. While in this sleep state, power consumption is re­duced by an order of magnitude if CS is HIGH. The part
remains in the sleep state as long as CS is logic HIGH. The
conversion result is held indefinitely in a static shift regis­ter while the converter is in the sleep state.

CS

SCK

EOC = 0

SDO

EOC = 1

CONVERSION SLEEP

Figure 1. LTC2421/LTC2422 Compatible Timing with the LTC2401/LTC2402

Figure 2. LTC2421/LTC2422 State Transition Diagram

8 8 8 8 (OPTIONAL)

DATA OUT

4 STATUS BITS 20 DATA BITS

DATA OUTPUT

EOC = 1

LAST 8 BITS ALWAYS 1

CONVERSION

24212 F01

24212f

10