Datasheet LTC2410 Datasheet (Linear Technology)

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LTC2410

Final Electrical Specifications

FEATURES

■

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

■

2ppm INL, No Missing Codes

■

2.5ppm Full-Scale Error

■

0.1ppm Offset

■

0.16ppm Noise

■

Single Conversion Settling Time for Multiplexed Applications

■

Internal Oscillator—No External Components Required

■

110dB Min, 50Hz/60Hz Notch Filter

■

24-Bit ADC in Narrow SSOP-16 Package (SO-8 Footprint)

■

Single Supply 2.7V to 5.5V Operation

■

Low Supply Current (200µA) and Auto Shutdown

■

Fully Differential Version of LTC2400

APPLICATIO S

■

Direct Sensor Digitizer

■

Weight Scales

■

Direct Temperature Measurement

■

Gas Analyzers

■

Strain-Gage Transducers

■

Instrumentation

■

Data Acquisition

■

Industrial Process Control

■

6-Digit DVMs

24-Bit No Latency ∆Σ

ADC

with Differential Input and

Differential Reference

DESCRIPTIO

The LTC®2410 is a 2.7V to 5.5V micropower 24-bit differential ∆Σ analog to digital converter with an inte- grated oscillator, 2ppm INL and 0.16ppm RMS noise. It uses delta-sigma technology and provides single cycle settling time for multiplexed applications. Through a single pin, the LTC2410 can be configured for better than 110dB input differential mode rejection at 50Hz or 60Hz ±2%, or it can be driven by an external oscillator for a user defined rejection frequency. The internal oscillator requires no external frequency setting components.

The converter accepts any external differential reference voltage from 0.1V to VCC for flexible ratiometric and remote sensing measurement configurations. The fullscale differential input range is from –0.5V The reference common mode voltage, V input common mode voltage, V dently set anywhere within the GND to VCC range of the LTC2410. The DC common mode input rejection is better than 140dB.

The LTC2410 communicates through a flexible 3-wire digital interface which 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.

INCM

April 2000

to 0.5V

REF

REFCM

REF

, and the

, may be indepen-

TYPICAL APPLICATIO S

2.7V TO 5.5V

1µF

214

LTC2410

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

TO 0.5V

–0.5V

REF

1, 7, 8, 9, 10, 15, 16

REF REF

IN IN GND

SCK

–

SDO

–

5 6

= INTERNAL OSC/50Hz REJECTION

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.

= EXTERNAL CLOCK SOURCE = INTERNAL OSC/60Hz REJECTION

3-WIRE SPI INTERFACE

2410 TA01

BRIDGE

IMPEDANCE 100Ω TO10k

1µF

REF+V

IN IN

+ –

REF

LTC2410

–

GND F

1, 7, 8 9, 10, 15, 16

SDO

3-WIRE

SCK

2410 TA02

SPI INTERFACE

LTC2410

ABSOLUTE AXI U RATI GS

UUW

PACKAGE/ORDER I FOR ATIO

(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

GND

REF

GND GND

IN IN

LTC2410C ............................................... 0°C to 70°C

LTC2410I............................................ –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 T

PARAMETER CONDITIONS MIN TYP MAX UNITS

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

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

Offset Error 2.5V ≤ REF+ ≤ VCC, REF– = GND, ● 0.5 2.5 µV

GND ≤ IN

Offset Error Drift 2.5V ≤ REF+ ≤ VCC, REF– = GND, 10 nV/°C

GND ≤ IN

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

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

Total Unadjusted Error 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

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

The ● denotes specifications which apply over the full operating

= 25°C. (Notes 3, 4)

≤ VIN ≤ 0.5 • V

REF

= 1.25V, (Note 6) ● 1 ppm of V

INCM

= 1.25V 5 ppm of V

INCM

REF

Consult factory for Military grade parts.

REF

= 2.5V, (Note 6) ● 2 14 ppm of V

INCM

= 2.5V 10 ppm of V

INCM

– = GND, 0.8 µV

TOP VIEW

1 2

–

6 7 8

GN PACKAGE

16-LEAD PLASTIC SSOP

= 125°C, θJA = 95°C/W

JMAX

, (Note 5) ● 24 Bits

GND

SCK

SDO

GND

ORDER PART NUMBER

LTC2410CGN LTC2410IGN

GN PART MARKING

2410 2410I

REF REF

REF

/°C

REF

/°C

REF

REF REF

RMS

LTC2410

CO VERTER CHARACTERISTICS

temperature range, otherwise specifications are at T

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% GND ≤ IN

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

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

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

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 100 dB Power Supply Rejection, 60Hz ±2% REF+ = 2.5V, REF– = GND, IN– = IN+ = GND, (Note 7) 110 dB Power Supply Rejection, 50Hz ±2% REF+ = 2.5V, REF– = GND, IN– = IN+ = GND, (Note 8) 110 dB

–

= IN+ ≤ 5V

–

= IN+ ≤ 5V, (Note 7)

–

= IN+ ≤ 5V, (Note 8)

= 2.5V, IN– = IN+ = GND

REF

The ● denotes specifications which apply over the full operating

= 25°C. (Notes 3, 4)

UUU

A ALOG I PUT AUD REFERE CE

temperature range, otherwise specifications are at T

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, IN+ = GND ● –10 1 10 nA

DC_LEAK

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

DC_LEAK

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

DC_LEAK

(REF–)REF– DC Leakage Current CS = VCC, 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 18 pF

–

Sampling Capacitance 18 pF

–

Sampling Capacitance 18 pF

= 25°C. (Note 3)

The ● denotes specifications which apply over the full operating

/2 V

REF

/2 V

REF

LTC2410

DIGITAL I PUTS A D DIGITAL OUTPUTS

operating temperature range, otherwise specifications are at T

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 ● 0.4V V SDO

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

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

= 25°C. (Note 3)

≤ 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 T

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

= 25°C. (Note 3)

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

● 200 300 µA

(Note 12) ● 20 30 µA

LTC2410

TI I G CHARACTERISTICS

range, otherwise specifications are at T

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

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

= REF+ – REF–, V

REF

= IN+ – IN–, V

Note 4: F source with f

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 (external oscillator).

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 ● 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 32-Bit Data Output Time Internal Oscillator (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

= 2.7 to 5.5V unless otherwise specified.

INCM

pin tied to GND or to VCC or to external conversion clock

= 153600Hz unless otherwise specified.

EOSC

= (REF+ + REF–)/2;

REFCM

= (IN+ + IN–)/2.

= 25°C. (Note 3)

= 153600Hz ±2%

EOSC

The ● denotes specifications which apply over the full operating temperature

= V

External Oscillator (Note 11)

External Oscillator (Notes 10, 11) f

External Oscillator (Notes 10, 11)

Note 8: FO = VCC (internal oscillator) or f (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

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 FO pin. The external oscillator frequency, f

Note 12: The converter uses the internal oscillator.

= 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.

● 157.03 160.23 163.44 ms

● 20510/f

● 256/f

, is expressed in kHz.

EOSC

(in kHz) ms

EOSC

/8 kHz

EOSC

(in kHz) ms

EOSC

(in kHz) ms

ESCK

= 128000Hz ±2%

EOSC

and is expressed in kHz.

ESCK

= 20pF.

LOAD

LTC2410

UUU

PI FU CTIO S

GND (Pins 1, 7, 8, 9, 10, 15, 16): Ground. Multiple ground pins internally connected for optimum ground current flow and VCC decoupling. Connect each one of these pins to a ground plane through a low impedance connection.

VCC (Pin 2): Positive Supply Voltage. Bypass to GND

(Pin␣ 1) with a 10µF tantalum capacitor in parallel with

0.1µF ceramic capacitor as close to the part as possible.

REF+ (Pin 3), REF– (Pin 4): 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 maintained more positive than the reference negative input, REF–, by at least 0.1V.

IN+ (Pin 5), IN– (Pin 6): 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 the converter produces unique overrange and underrange output codes.

CS (Pin 11): 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-to-HIGH transition on CS during the Data Output transfer aborts the data transfer and starts a new conversion.

) to 0.5 • (V

REF

). Outside this input range

REF

SDO (Pin 12): Three-State Digital Output. During the Data Output period this pin is used as 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 13): Bidirectional Digital Clock Pin. In Internal Serial Clock Operation mode, SCK is used as digital output for the internal serial interface clock during the Data Output period. In External Serial Clock Operation mode, SCK is used as digital input for the external serial interface clock during the Data Output period. A weak internal pullup 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 14): 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 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. When F is driven by an external clock signal with a frequency f the converter uses this signal as its system clock and the digital filter first null is located at a frequency f

EOSC

/2560.

LTC2410

1.69k

SDO

2410 TA04

Hi-Z TO V

VOH TO V

VOL TO Hi-Z

LOAD

= 20pF

CONVERT

SLEEP

DATA OUTPUT

2410 F01

TRUE

FALSE

CS = LOW

AND

SCK

FU CTIO AL BLOCK DIAGRA

GND

–

+ –

∫∫∫

∑

REF

–

REF

–+

DAC

TEST CIRCUITS

SDO

ADC

AUTOCALIBRATION

AND CONTROL

DECIMATING FIR

INTERNAL

OSCILLATOR

SERIAL

INTERFACE

(INT/EXT)

SDO

SCK

2410 FD

1.69k

Hi-Z TO V VOL TO V

VOH TO Hi-Z

= 20pF

LOAD

2410 TA03

WUU

APPLICATIO S I FOR ATIO

CONVERTER OPERATION

Converter Operation Cycle

The LTC2410 is a low power, delta-sigma analog-todigital converter with an easy to use 3-wire serial interface. Its 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 1). The 3-wire interface consists of serial data output (SDO), serial clock (SCK) and chip select (CS).

Initially, the LTC2410 performs 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. The part remains 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.

Figure 1. LTC2410 State Transition Diagram

LTC2410

WUU

APPLICATIO S I FOR ATIO

Once CS is pulled LOW, 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 3). The data output state is concluded once 32 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 LTC2410 offers 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 LTC2410 incorporates a highly accurate onchip oscillator. This eliminates the need for external frequency setting components such as crystals or oscillators. Clocked by the on-chip oscillator, the LTC2410 achieves a minimum of 110dB rejection at the line frequency (50Hz or 60Hz ±2%).

Ease of Use

The LTC2410 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 LTC2410 performs offset and full-scale calibrations 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 LTC2410 automatically enters an internal reset state when the power supply voltage VCC drops below approximately 2.2V. This feature guarantees the 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 0.5ms. The POR signal clears all internal registers. Following the POR signal, the LTC2410 starts a normal conversion cycle and follows 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

This converter accepts 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 LTC2410 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 (see the Output Data Rate section).

LTC2410

WUU

APPLICATIO S I FOR ATIO

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 LTC2410 converts the bipolar differential input signal, VIN = IN+ – IN–, from –FS = – 0.5 • V REF+ – REF–. Outside this range the converter indicates 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. The effect of the series resistance on the converter accuracy can be evaluated from the curves presented in the Input Current/ Reference Current sections. 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 This error has a very strong temperature dependency.

Output Data Format

The LTC2410 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 bit 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.

to +FS = 0.5 • V

REF

where V

REF

= 5V.

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 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. LTC2410 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

LTC2410

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APPLICATIO S I FOR ATIO

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.

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.

REF

Frequency Rejection Selection (FO)

The LTC2410 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

BIT 31

SDO

Hi-Z

SCK

SLEEP DATA OUTPUT CONVERSION

Table 2. LTC2410 Output Data Format

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

* EOC DMY SIG MSB

VIN* ≥ 0.5 • V

0.5 • V

0.25 • V

0.25 • V 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

REF

** – 1LSB 0 0101 1 1…1

REF

** 00101 0 0…0

REF

** – 1LSB 00100 1 1…1

REF

** 00011 0 0…0

REF

** – 1LSB 00010 1 1…1

REF

** 00010 0 0…0

REF

EOC

12345 262732

** 00110 0 0…0

** 00001 1 1…1

= REF+ – REF–.

REF

BIT 28BIT 29BIT 30

MSBSIG“0”

Figure 3. Output Data Timing

LSB

BIT 0BIT 27 BIT 5

2410 F03

LTC2410

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APPLICATIO S I FOR ATIO

synchronized with an outside source, the LTC2410 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 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

While operating with an external conversion clock of a frequency f normal mode rejection in a frequency range f ±4% and its harmonics. The normal mode rejection as a function of the input frequency deviation from f is shown in Figure 4.

Whenever an external clock is not present at the FO pin the converter automatically activates its internal oscillator and enters the Internal Conversion Clock mode. The LTC2410 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.

of the external signal must be at least

EOSC

and t

HEO

, the LTC2410 provides better than 110dB

EOSC

are observed.

LEO

EOSC

/2560

–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. LTC2410 Normal Mode Rejection When Using an External Oscillator of Frequency f

EOSC

/2560(%)

2410 F04

EOSC

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

SERIAL INTERFACE PINS

The LTC2410 transmits the conversion results and receives 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 converter status and during the data output state it is used to read the conversion result.

Table 3. LTC2410 State Duration

State Operating Mode Duration

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

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

(50Hz 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 As Long As CS = LOW But Not Longer Than 1.67ms

(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

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APPLICATIO S I FOR ATIO

Serial Clock Input/Output (SCK)

The serial clock signal present on SCK (Pin 13) 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 LTC2410 creates its 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 powerup 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 12), 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 11) 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 11), 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 LTC2410 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).

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. Tying a capacitor to CS will reduce the output rate and power dissipation by a factor proportional to the capacitor’s value, see Figures 12 to 14.

SERIAL INTERFACE TIMING MODES

The LTC2410’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 and autostart. 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 FO = HIGH) or an external oscillator connected to the FO pin. Refer to Table␣ 4 for a summary.

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.

Table 4. LTC2410 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 Internal SCK, Autostart Conversion Internal C

EXT

Internal Figure 11

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APPLICATIO S I FOR ATIO

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 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 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.

shifted

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 0.5ms after VCC exceeds 2.2V. 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.

SDO

SCK

(EXTERNAL)

CONVERSION

2.7V TO 5.5V

1µF

214

LTC2410

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

TO 0.5V

–0.5V

REF

1, 7, 8, 9, 10, 15, 16

TEST EOCTEST EOC

SLEEP DATA OUTPUT CONVERSION

BIT 31

EOC

REF

–

IN GND

SCK

–

SDO

= 50Hz REJECTION = EXTERNAL OSCILLATOR = 60Hz REJECTION

3-WIRE SPI INTERFACE

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

LSB

BIT 0

SUB LSBMSBSIG

Figure 5. External Serial Clock, Single Cycle Operation

TEST EOC

Hi-ZHi-ZHi-Z

2410 F05

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APPLICATIO S I FOR ATIO

ANALOG INPUT RANGE

–0.5V

REF

TEST EOC TEST EOC

SDO

SCK

(EXTERNAL)

EOC

CONVERSIONSLEEP SLEEP

DATA OUTPUT

Hi-Z

Hi-Z Hi-ZHi-Z

2.7V TO 5.5V

1µF

REFERENCE

VOLTAGE

0.1V TO V

TO 0.5V

REF

1, 7, 8, 9, 10, 15, 16

BIT 31BIT 0

EOC

214

REF

IN GND

LTC2410

–

SCK

SDO

DATA OUTPUT

= 50Hz REJECTION = EXTERNAL OSCILLATOR = 60Hz REJECTION

3-WIRE SPI INTERFACE

MSBSIG

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

TEST EOC

CONVERSION

2410 F06

Figure 6. External Serial Clock, Reduced Data Output Length

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. 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.

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.

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 (F0 = logic LOW or HIGH). If FO is driven by an external oscillator of

LTC2410

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APPLICATIO S I FOR ATIO

ANALOG INPUT RANGE

–0.5V

BIT 31

SDO

SCK

(EXTERNAL)

CONVERSION

SLEEP DATA OUTPUT CONVERSION

EOC

Figure 7. External Serial Clock, CS = 0 Operation

2.7V TO 5.5V

1µF

REFERENCE

VOLTAGE

0.1V TO V

TO 0.5V

REF

1, 7, 8, 9, 10, 15, 16

214

REF

IN GND

MSBSIG

LTC2410

–

SCK

SDO

3-WIRE SPI INTERFACE

= 50Hz REJECTION = EXTERNAL OSCILLATOR = 60Hz REJECTION

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

LSB

BIT 0

2410 F07

SDO

SCK

(INTERNAL)

2.7V TO 5.5V

1µF

214

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

–0.5V

TO 0.5V

REF

1, 7, 8, 9, 10, 15, 16

EOCtest

TEST EOC

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

BIT 31

EOC

SLEEP DATA OUTPUT CONVERSIONCONVERSION

REF

IN GND

MSBSIG

LTC2410

–

SCK

SDO

BIT 27 BIT 26BIT 28BIT 29BIT 30

= 50Hz REJECTION = EXTERNAL OSCILLATOR = 60Hz REJECTION

3-WIRE SPI INTERFACE

BIT 5

LSB

10k

BIT 0

TEST EOC

2410 F08

Figure 8. Internal Serial Clock, Single Cycle Operation

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APPLICATIO S I FOR ATIO

frequency f HIGH before time t state. The conversion result is held in the internal static shift register.

If CS remains LOW longer than t 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 aborts the data output state and immediately initiates a

EOSC

, then t

EOCtest

is 3.6/f

. If CS is pulled

EOSC

, the device remains in the sleep

, the first rising

EOCtest

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 LTC2410’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 LTC2410’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 pull-up 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.

SDO

SCK

(INTERNAL)

2.7V TO 5.5V

1µF

214

REF

IN GND

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

TO 0.5V

–0.5V

REF

1, 7, 8, 9, 10, 15, 16

EOCtest

BIT 0

EOC

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

DATA OUTPUT

TEST EOCTEST EOC

EOCtest

BIT 31

EOC

SLEEP DATA OUTPUT

LTC2410

–

SCK

SDO

MSBSIG

= 50Hz REJECTION = EXTERNAL OSCILLATOR = 60Hz REJECTION

3-WIRE SPI INTERFACE

BIT 27 BIT 26BIT 28BIT 29BIT 30

BIT 8

10k

TEST EOC

CONVERSIONCONVERSIONSLEEP

2410 F09

Figure 9. Internal Serial Clock, Reduced Data Output Length

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APPLICATIO S I FOR ATIO

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 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.

The internal serial clock mode is selected at the end of the power-on reset (POR) cycle. The POR cycle is concluded approximately 0.5ms after VCC exceeds 2.2V. An internal

), the internal pull-up is activated.

EOCtest

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.

SDO

SCK

(INTERNAL)

2.7V TO 5.5V

1µF

214

REF

IN GND

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

–0.5V

TO 0.5V

REF

1, 7, 8, 9, 10, 15, 16

BIT 31

EOC

SLEEP

MSBSIG

LTC2410

–

DATA OUTPUT CONVERSIONCONVERSION

SCK

SDO

= 50Hz REJECTION = EXTERNAL OSCILLATOR = 60Hz REJECTION

3-WIRE SPI INTERFACE

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

LSB

2410 F10

Figure 10. Internal Serial Clock, Continuous Operation

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APPLICATIO S I FOR ATIO

Internal Serial Clock, Autostart Conversion

This timing mode is identical to the internal serial clock, 2-wire I/O described above with one additional feature. Instead of grounding CS, an external timing capacitor is tied to CS.

While the conversion is in progress, the CS pin is held HIGH by an internal weak pull-up. Once the conversion is complete, the device enters the low power sleep state and an internal 25nA current source begins discharging the capacitor tied to CS, see Figure 11. The time the converter spends in the sleep state is determined by the value of the external timing capacitor, see Figures 12 and 13. Once the voltage at CS falls below an internal threshold (≈1.4V), the device automatically begins outputting data. The data output cycle begins on the first rising edge of SCK and ends on 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. After the 32nd rising edge, CS is pulled HIGH and a new conversion is immediately started. This is useful in applications requiring periodic monitoring and ultralow power. Figure 14 shows the average supply current as a function of capacitance on CS.

It should be noticed that the external capacitor discharge current is kept very small in order to decrease the converter power dissipation in the sleep state. In the autostart mode the analog voltage on the CS pin cannot be observed without disturbing the converter operation using a regular oscilloscope probe. When using this configuration, it is important to minimize the external leakage current at the CS pin by using a low leakage external capacitor and properly cleaning the PCB surface.

The internal serial clock mode is selected every time the voltage on the CS pin crosses an internal threshold voltage. An internal weak pull-up at the SCK pin is active while

GND

SDO

SCK

(INTERNAL)

2.7V TO 5.5V

1µF

214

REF

IN GND

BIT 31

EOC

REFERENCE

VOLTAGE

0.1V TO V

ANALOG INPUT RANGE

–0.5V

REF

1, 7, 8, 9, 10, 15, 16

SLEEP

TO 0.5V

LTC2410

–

SCK

SDO

DATA OUTPUT CONVERSIONCONVERSION

BIT 29BIT 30

SIG

= 50Hz REJECTION = EXTERNAL OSCILLATOR = 60Hz REJECTION

3-WIRE SPI INTERFACE

EXT

BIT 0

Hi-ZHi-Z

2410 F11

Figure 11. Internal Serial Clock, Autostart Operation

LTC2410

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APPLICATIO S I FOR ATIO

(SEC)

SAMPLE

10 100 100000

CAPACITANCE ON CS (pF)

Figure 12. CS Capacitance vs t

SAMPLE RATE (Hz)

VCC = 5V

10 100 10000

CAPACITANCE ON CS (pF)

Figure 13. CS Capacitance vs Output Rate

300

)

RMS

250

200

150

VCC = 5V

VCC = 3V

VCC = 5V

1000 10000

= 3V

1000

= 3V

2400 F12

SAMPLE

100000

2400 F13

CS is discharging; therefore, the internal serial clock timing mode is automatically selected if SCK is floating. It is important to ensure there are no external drivers pulling SCK LOW while CS is discharging.

PRESERVING THE CONVERTER ACCURACY

The LTC2410 is 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 LTC2410’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 LTC2410 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)].

100

SUPPLY CURRENT (µA

10 100 1000 10000

CAPACITANCE ON CS (pF)

100000

2400 F14

Figure 14. CS Capacitance vs Supply Current

During the conversion period, the undershoot and/or overshoot of a fast digital signal connected to the LTC2410 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 LTC2410. For reference, on a regular FR-4 board, signal propagation

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APPLICATIO S I FOR ATIO

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 LTC2410 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 LTC2410 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 multiple ground pins used in this package configuration, as well as the differential input and reference architecture, reduce substantially the converter’s sensitivity to ground currents.

Particular attention must be given to the connection of the FO signal when the LTC2410 is 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 LTC2410 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 15.

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␣ 15), 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 LTC2410’s front-end switched-capacitor network is clocked at 76800Hz corresponding to a 13µs sampling period. Thus, for settling errors of less than 1ppm, the driving source impedance should be chosen such that τ ≤ 13µs/14 = 920ns. When an external oscillator of frequency f used, the sampling period is 2/f error of less than 1ppm, τ ≤ 0.14/f

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 15 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).

and, for a settling

EOSC

LTC2410

SOURCE

(Ω)

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

+FS ERROR (ppm OF V

REF

)

2410 F18

–10

–20

–30

–40

–50

VCC = 5V REF

= 5V

REF

–

= GND

–

= 2.5V

= GND

= 25°C

CIN = 0.01µF

CIN = 0.001µF

CIN = 100pF

CIN = 0pF

WUU

APPLICATIO S I FOR ATIO

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

2410 F15

Figure 15. LTC2410 Equivalent Analog Input Circuit

18pF (TYP)

VV V

+−

IN INCM REFCM

IIN

()

AVG

−

IIN

()

AVG

IREF

()

AVG

−

IREF

()

AVG

where

=−

V REF REF

REF

REFCM

+−

=−

VININ

INCM

R M INTERNAL OSCILLATOR Hz Notch F LOW

361 60

EQ O

R M INTERNAL OSCILLATOR Hz Notch F HIGH

432 50

EQ O

R f EXTERNAL OSCILLATOR

=•

0 555 10

()

EQ EOSC

•

−+ −

+−



REF REF





+−



IN IN





Ω

VV V

IN INCM REFCM

•

•− +

VV V

REF INCM REFCM

•

VV V

−• − +

REF INCM REFCM

•

+−









− 



−

•

REF EQ

•

REF EQ

()

The effect of this input dynamic current can be analyzed using the test circuit of Figure 16. The C includes the LTC2410 pin capacitance (5pF typical) plus the capacitance of the test fixture used to obtain the results shown in Figures 17 and 18. A careful implementation can bring the total input capacitance (CIN + C thus achieving better performance than the one predicted by Figures 17 and 18. For simplicity two distinct situations can be considered.

)

REF

+FS ERROR (ppm OF V

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

CIN = 0.001µF

VCC = 5V

= 5V

REF

–

= GND

REF

= 5V

–

= 2.5V

= GND

= 25°C

CIN = 0.01µF

CIN = 100pF

CIN = 0pF

(Ω)

SOURCE

at IN+ or IN– (Small CIN)

SOURCE

capacitor

PAR

) closer to 5pF

PAR

2410 F17

SOURCE

+ 0.5V

INCM

– 0.5V

SOURCE

Figure 16. An RC Network at IN+ and IN

Figure 18. –FS Error vs R

SOURCE

PAR

≅20pF

C ≅20pF

PAR

LTC2410

–

2410 F16

–

at IN+ or IN– (Small CIN)Figure 17. +FS Error vs R

LTC2410

WUU

APPLICATIO S I FOR ATIO

For 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 LTC2410 can maintain its exceptional accuracy while operating with relative large values of source resistance as shown in Figures 17 and 18. 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 C 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.

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. When FO = LOW (internal oscillator and 60Hz notch), the typical differential input resistance is 1.8MΩ which will generate a gain error of approximately 0.28ppm for each ohm of source resistance driving IN+ or IN–. When FO = HIGH (internal oscillator and 50Hz notch), the typical differential input resistance is 2.16MΩ which will generate a gain error of approximately 0.23ppm for each ohm of source resistance driving IN+ or IN–. When FO is driven by an external oscillator with a frequency f conversion clock operation), the typical differential input resistance is 0.28 • 1012/f source resistance driving IN+ or IN– will result in

1.78 • 10–6 • f 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 Figures 19 and 20.

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

ppm gain error. The effect of the source

EOSC

Ω and each ohm of

EOSC

(external

IN+ and IN– and with the difference between the input and 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. 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

0.28ppm. 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 0.23ppm. When FO is driven by an external oscillator with a frequency f every 1Ω mismatch in source impedance transforms a full-scale common mode input signal into a differential mode input signal of 1.78 • 10–6 • f shows the 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.

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 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 typical better than 0.5%. 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.

ppm. Figure 21

EOSC

LTC2410

WUU

APPLICATIO S I FOR ATIO

300

VCC = 5V

= 5V

REF

–

= GND

REF

240

)

REF

+FS ERROR (ppm OF V

= 3.75V

–

= 1.25V

IN F

= GND

180

= 25°C

120

100 200 300 400 500 600 700 800 900 1000

Figure 19. +FS Error vs R

–60

)

REF

–120

VCC = 5V

–180

–FS ERROR (ppm OF V

–240

–300

= 5V

REF

–

= GND

REF

= 1.25V

–

= 3.75V

= GND

= 25°C

100 200 300 400 500 600 700 800 900 1000

Figure 20. –FS Error vs R

120 100

)

REF

–20 –40 –60

–FS ERROR (ppm OF V

–80 –100 –120

80 60

E F

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

A: ∆RIN = +400Ω

= +200Ω

B: ∆R

= +100Ω

C: ∆R

= 0Ω

D: ∆R

Figure 21. Offset Error vs Common Mode Voltage (V

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

INCM

(∆R

= R

SOURCEIN

+ – R

SOURCEIN

CIN = 1µF, 10µF

CIN = 0.1µF

CIN = 0.01µF

(Ω)

SOURCE

at IN+ or IN– (Large CIN)

SOURCE

CIN = 0.01µF

CIN = 0.1µF

CIN = 1µF, 10µF

(Ω)

SOURCE

at IN+ or IN– (Large CIN)

SOURCE

VCC = 5V

= 5V

REF

–

= GND

REF

= IN– = V

FO = GND T

R C

= 25°C

SOURCEIN

= 10µF

INCM

– = 500Ω

(V)

E: ∆RIN = –100Ω F: ∆R G: ∆R

= –200Ω

= –400Ω

2410 F19

2410 F20

2410 F21

–) for Large CIN Values (C

≥ 1µF)

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 LTC2410 samples 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) may

REF

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. When FO = LOW (internal oscillator and 60Hz notch), the typical differential reference resistance is 1.3MΩ which will generate a gain error of approximately 0.38ppm for each ohm of source resistance driving REF+ or REF–. When FO = HIGH (internal oscillator and 50Hz notch), the typical differential reference resistance is 1.56MΩ which will generate a gain error of approximately 0.32ppm for each ohm of source resistance driving REF+ or REF–. When FO is driven by an external oscillator with a frequency f

(external conver-

EOSC

sion clock operation), the typical differential reference resistance is 0.20 • 1012/f

Ω and each ohm of source

EOSC

resistance drving REF+ or REF– will result in

2.47 • 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 and –FS errors for various combinations of source resistance seen by the

LTC2410

SOURCE

(Ω)

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

–FS ERROR (ppm OF V

REF

)

2410 F23

VCC = 5V REF

= 5V

REF

–

= GND

–

= 2.5V

= GND

= 25°C

REF

= 0.01µF

REF

= 0.001µF

REF

= 100pF

REF

= 0pF

SOURCE

(Ω)

100 200 300 400 500 600 700 800 900 1000

+FS ERROR (ppm OF V

REF

)

2410 F25

450

360

270

180

VCC = 5V REF

= 5V

REF

–

= GND

= 1.25V

–

= 3.75V

= GND

= 25°C

REF

= 0.01µF

REF

= 0.1µF

REF

= 1µF, 10µF

WUU

APPLICATIO S I FOR ATIO

REF+ and REF– pins and external capacitance C nected to these pins are shown in Figures 22, 23, 24 and␣ 25.

In addition to this gain error, the converter INL performance is degraded by the reference source impedance. When FO = LOW (internal oscillator and 60Hz notch), every 100Ω of source resistance driving REF+ or REF– translates into about 1.34ppm additional INL error. When FO = HIGH (internal oscillator and 50Hz notch), every 100Ω of source resistance driving REF+ or REF– translates into about

1.1ppm additional INL error. When FO is driven by an

VCC = 5V

= 5V

REF

–

= GND

REF

–10

)

REF

–20

IN IN F

= 5V

–

= 2.5V

= GND = 25°C

REF

con-

external oscillator with a frequency f

, every 100Ω of

EOSC

source resistance driving REF+ or REF– translates into about 8.73 • 10–6 • f

ppm additional INL error.

EOSC

Figure␣ 26 shows the typical INL error due to the source resistance driving the REF+ or REF– pins when large C

REF

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

–30

+FS ERROR (ppm OF V

–40

–50

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

REF

Figure 22. +FS Error vs R

–90

)

REF

–180

VCC = 5V

= 5V

REF

–

= GND

REF

= 3.75V

–

= 1.25V

= GND

= 25°C

100 200 300 400 500 600 700 800 900 1000

+FS ERROR (ppm OF V

–270

–360

–450

Figure 24. +FS Error vs R

= 0.01µF

= 0.001µF

= 100pF

REF

= 0pF

REF

(Ω)

SOURCE

at REF+ or REF– (Small CIN)

SOURCE

= 0.01µF

REF

= 0.1µF

REF

= 1µF, 10µF

REF

(Ω)

SOURCE

at REF+ and REF– (Large C

SOURCE

2410 F22

2410 F24

Figure 23. –FS Error vs R

) Figure 25. – FS Error vs R

REF

at REF+ or REF– (Small CIN)

SOURCE

at REF+ and REF– (Large C

SOURCE

REF

)

LTC2410

WUU

APPLICATIO S I FOR ATIO

)

REF

3 0

–3

= 100Ω

INL (ppm OF V

–12 –15

Figure 26. INL vs Differential Input Voltage (VIN = IN+ – IN–) and Reference Source Resistance (R Large C

Values (C

REF

SOURCE

–6

–9

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

VCC = 5V REF+ = 5V REF– = GND

= 0.5 • (IN+ + IN–) = 2.5V

INCM

≥ 1µF)

REF

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

0.5%. 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 LTC2410 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). The actual output

SOURCE

INDIF/VREFDIF

= 1000Ω

= 500Ω

SOURCE

FO = GND

= 10µF

REF

= 25°C

2410 F26

at REF+ and REF– for

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 (FO connected to an external oscillator), the LTC2410 output data rate can be increased as desired. The duration of the conversion phase is 20510/ f

EOSC

. If f

= 153600Hz, the converter behaves as if the

EOSC

internal oscillator is used and the notch is set at 60Hz. There is no significant difference in the LTC2410 performance between these two operation modes.

An increase in f

over the nominal 153600Hz will

EOSC

translate into a proportional increase in the maximum output data rate. This substantial advantage is nevertheless accompanied by three potential effects, which must be carefully considered.

First, a change in f

will result in a proportional change

EOSC

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 LTC2410’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

) are used, the

REF

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

. If small external input and/

EOSC

) are used, the effect of

REF

the external source resistance upon the LTC2410 typical performance can be inferred from Figures 17, 18, 22 and 23 in which the horizontal axis is scaled by 153600/f

EOSC

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 linear-

LTC2410

OUTPUT DATA RATE (READINGS/SEC)

0 102030405060708090100

OFFSET ERROR (ppm OF V

REF

)

2410 F27

500 450 400 350 300 250 200 150 100

TA = 85°C

VCC = 5V REF

= 5V

REF

–

= GND

INCM

= 2.5V

= 0V

= EXTERNAL OSCILLATOR

TA = 25°C

APPLICATIO S I FOR ATIO

ity. Typical measured performance curves for output data rates up to 100 readings per second are shown in Figures␣ 27, 28, 29, 30, 31, 32, 33 and 34. 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

The combined effect of the internal Sinc4 digital filter and of the analog and digital autocalibration circuits determines the LTC2410 input bandwidth. When the internal oscillator is used with the notch set at 60Hz (FO = LOW), the 3dB input bandwidth is 3.63Hz. When the internal oscillator is used with the notch set at 50Hz (FO = HIGH), the 3dB input bandwidth is 3.02Hz. If an external conversion clock generator of frequency f FO pin, the 3dB input bandwidth is 0.236 • 10–6 • f

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 LTC2410 input bandwidth is shown in Figure␣ 35 for FO = LOW and FO = HIGH. When an external oscillator of frequency f LTC2410 input bandwidth can be derived from Figure␣ 35, FO = LOW curve in which the horizontal axis is scaled by f

/153600.

EOSC

The conversion noise (800nV can be modeled by a white noise source connected to a noise free converter. The noise spectral density is

62.75nV√Hz for an infinite bandwidth source and

76.8nV√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 solution is a high gain, low bandwidth amplifier stage followed by a high bandwidth unity-gain buffer.

is used, the shape of the

EOSC

RMS

WUU

is connected to the

EOSC

typical for V

REF

EOSC

= 5V)

Figure 27. Offset Error vs Output Data Rate and Temperature

7000

VCC = 5V

6000

)

REF

5000

4000

3000

2000

+FS ERROR (ppm OF V

1000

= 5V

REF

–

= GND

REF

= 3.75V

–

= 1.25V

= EXTERNAL OSCILLATOR

TA = 85°C

TA = 25°C

0 102030405060708090100

OUTPUT DATA RATE (READINGS/SEC)

2410 F28

Figure 28. +FS Error vs Output Data Rate and Temperature

–1000

)

REF

–2000

–3000

–4000

–5000

–FS ERROR (ppm OF V

–6000

–7000

VCC = 5V

= 5V

REF

–

= GND

REF

= 1.25V

–

= 3.75V

= EXTERNAL OSCILLATOR

0 102030405060708090100

OUTPUT DATA RATE (READINGS/SEC)

TA = 85°C

TA = 25°C

2410 F29

Figure 29. –FS Error vs Output Data Rate and Temperature

LTC2410

OUTPUT DATA RATE (READINGS/SEC)

0 102030405060708090100

RESOLUTION (BITS)

2410 F31

TA = 85°C

VCC = 5V REF

= 5V

REF

–

= GND

INCM

= 2.5V

–2.5V < V

< 2.5V

= EXTERNAL OSCILLATOR

RESOLUTION = LOG2(V

REF

/INL

MAX

)

TA = 25°C

OUTPUT DATA RATE (READINGS/SEC)

0 102030405060708090100

RESOLUTION (BITS)

2410 F33

24 23 22 21 20 19 18 17 16 15 14 13 12

REF

= 5V

VCC = 5V REF

–

= GND

INCM

= 2.5V

= 0V

= EXTERNAL OSCILLATOR

= 25°C

RESOLUTION = LOG

2(VREF

/NOISE

RMS

)

REF

= 2.5V

WUU

APPLICATIO S I FOR ATIO

24 23 22 21 20 19 18

VCC = 5V

17 16

RESOLUTION (BITS)

15 14 13 12

= 5V

REF

–

= GND

REF

= 2.5V

INCM

= 0V

= EXTERNAL OSCILLATOR

RESOLUTION = LOG

0 102030405060708090100

OUTPUT DATA RATE (READINGS/SEC)

Figure 30. Resolution (Noise vs Output Data Rate and Temperature

250

VCC = 5V

225 )

200

REF

175

150

125

100

75 50

OFFSET ERROR (ppm OF V

= GND

REF

= 2.5V

INCM

= 0V

= EXTERNAL OSCILLATOR

= 25°C

0 102030405060708090100

OUTPUT DATA RATE (READINGS/SEC)

TA = 85°C

2(VREF

= 2.5V

REF

TA = 25°C

/NOISE

RMS

= 5V

REF

)

RMS

2410 F30

≤ 1LSB)

2410 F32

Figure 31. Resolution (INL

RMS

≤ 1LSB)

vs Output Data Rate and Temperature

Figure 32. Offset Error vs Output Data Rate and Reference Voltage

Figure 34. Resolution (INL

RESOLUTION =

/INL

= 2.5V V

RESOLUTION (BITS)

REF

TA = 25°C

= 5V

–

= GND

REF

= 0.5 • REF

INCM

–0.5V • V

REF

FO = EXTERNAL OSCILLATOR

0 102030405060708090100

OUTPUT DATA RATE (READINGS/SEC)

≤ 1LSB) vs Output Data Rate and Reference Voltage

MAX

LOG

REF

< VIN < 0.5 • V

2(VREF

= 5V

REF

Figure 33. Resolution (Noise

≤ 1LSB) vs

RMS

Output Data Rate and Reference Voltage

)

MAX

2410 F34

LTC2410

WUU

APPLICATIO S I FOR ATIO

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 LTC2410 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 LTC2410’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

the maximum output data rate. In the internal oscillator mode with a 50Hz notch setting, fS = 12800Hz and with a 60Hz notch setting fS = 15360Hz. In the external oscillator mode, fS = f

The combined normal mode rejection performance is shown in Figure␣ 36 for the internal oscillator with 50Hz notch setting (FO = HIGH) and in Figure␣ 37 for the internal oscillator with 60Hz notch setting (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 Figure␣ 38 (rejection near DC) and Figure␣ 39 (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 40 and 41. Typical measured values of the normal mode rejection of the LTC2410 operating with an internal oscillator and a 60Hz notch setting are shown in Figure 40 superimposed over the theoretical calculated curve. Similarly, typical measured values of the normal mode rejection of the LTC2410 operating with an internal oscillator and a 50Hz notch setting are shown in Figure 41 superimposed over the theoretical calculated curve.

As a result of these remarkable normal mode specifications, minimal (if any) antialias filtering is required in front of the LTC2410. If passive RC components are placed in

where fN in the notch frequency and f

/10.

EOSC

OUTMAX

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 FO = LOW

2410 F35

Figure 35. Input Signal Bandwidth Using the Internal Oscillator

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

2410 F36

Figure 36. Input Normal Mode Rejection, Internal Oscillator and 50Hz Notch

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,

= 10 • f

EOSC

2410 F37

Figure 37. Input Normal Mode Rejection, Internal Oscillator and 60Hz Notch or External Oscillator

LTC2410

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APPLICATIO S I FOR ATIO

0 –10 –20 –30 –40 –50 –60 –70 –80 –90

–100

INPUT NORMAL MODE REJECTION (dB)

–110 –120

Figure 38. Input Normal Mode Rejection Figure 39. Input Normal Mode Rejection

fN0 2fN3fN4fN5fN6fN7fN8f

INPUT SIGNAL FREQUENCY (Hz)

2410 F38

–20

–40

–60

0 –10 –20 –30 –40 –50 –60 –70 –80 –90

–100

INPUT NORMAL MODE REJECTION (dB)

–110 –120

250f

252fN254fN256fN258fN260fN262f

MEASURED DATA CALCULATED DATA

INPUT SIGNAL FREQUENCY (Hz)

VCC = 5V

REF

= 5V

–

REF

= GND

= 2.5V

INCM

= 5V

IN(P-P)

= GND

= 25°C

2410 F39

–80

NORMAL MODE REJECTION (dB)

–100

–120

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

INPUT FREQUENCY (Hz)

Figure 40. Input Normal Mode Rejection vs Input Frequency

–20

–40

–60

–80

NORMAL MODE REJECTION (dB)

–100

–120

12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200 INPUT FREQUENCY (Hz)

MEASURED DATA CALCULATED DATA

VCC = 5V

REF

= 5V

–

REF

= GND

INCM

IN(P-P)

= 5V

= 25°C

2410 F40

= 2.5V

= 5V

2410 F41

Figure 41. Input Normal Mode Rejection vs Input Frequency

LTC2410

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front of the LTC2410, 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 LTC2410 third order modulator resolves this problem and guarantees a predictable stable behavior at input signal levels of 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 LTC2410 is eminently suited for such tasks. When the perturbation is differential, the specification of interest is the normal mode rejection

–20

–40

–60

for large input signal levels. With a reference voltage V

␣ =␣ 5V, the LTC2410 has a full-scale differential input

REF

range of 5V peak-to-peak. Figures 42 and 43 show measurement results for the LTC2410 normal mode rejection ratio with a 7.5V peak-to-peak (150% of full scale) input signal superimposed over the more traditional normal mode rejection ratio results obtained with a 5V peak-to-peak (full scale) input signal. In Figure 42, the LTC2410 uses the internal oscillator with the notch set at 60Hz (FO = LOW) and in Figure 43 it uses the internal oscillator with the notch set at 50Hz (FO = HIGH). It is clear that the LTC2410 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.

= 5V

IN(P-P)

= 7.5V

IN(P-P)

(150% OF FULL SCALE)

VCC = 5V

REF

= 5V

–

REF

= GND

INCM

= GND

= 25°C

= 2.5V

–80

NORMAL MODE REJECTION (dB)

–100

–120

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

INPUT FREQUENCY (Hz)

2410 F3a

Figure 42. Measured Input Normal Mode Rejection vs Input Frequency

–20

–40

–60

–80

NORMAL MODE REJECTION (dB)

–100

–120

12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200

= 5V

IN(P-P)

= 7.5V

IN(P-P)

(150% OF FULL SCALE)

INPUT FREQUENCY (Hz)

VCC = 5V

REF

= 5V

–

REF

= GND

INCM

= 5V

= 25°C

= 2.5V

2410 F4a

Figure 43. Measured Input Normal Mode Rejection vs Input Frequency

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SYNCHRONIZATION OF MULTIPLE LTC2410s

Since the LTC2410’s absolute accuracy (total unadjusted error) is 5ppm, applications utilizing multiple synchronized ADCs are possible.

Simultaneous Sampling with Two LTC2410s

One such application is synchronizing multiple LTC2410s, see Figure 44. The start of conversion is synchronized to the rising edge of CS. In order to synchronize multiple LTC2410s, CS is a common input to all the ADCs. To prevent the converters from autostarting a new conversion at the end of data output read, 31 or fewer SCK clock signals are applied to the LTC2410 instead of 32 (the 32nd falling edge would start a conversion). The exact timing and frequency for the SCK signal is not critical since it is only shifting out the data. In this case, two LTC2410’s simultaneously start and end their conversion cycles under the external control of CS.

Increasing the Output Rate Using Mulitple LTC2410s

A second application uses multiple LTC2410s to increase the effective output rate by 4×, see Figure 45. In this case, four LTC2410s are interleaved under the control of separate CS signals. This increases the effective output rate from 7.5Hz to 30Hz (up to a maximum of 60Hz). Additionally, the one-shot output spectrum is unfolded allowing further digital signal processing of the conversion results. SCK and SDO may be common to all four LTC2410s. The four CS rising edges equally divide one LTC2410 conversion cycle (7.5Hz for 60Hz notch frequency). In order to synchronize the start of conversion to CS, 31 or less SCK clock pulses must be applied to each ADC.

Both the synchronous and 4× output rate applications use the external serial clock and single cycle operation with reduced data output length (see Serial Interface Timing Modes section and Figure 6). An external oscillator clock is applied commonly to the FO pin of each LTC2410 in order to synchronize the sampling times. Both circuits may be extended to include more LTC2410s.

SCK1

SCK2

SDO1

SDO2

SCK2 SCK1

LTC2410

REF

SCK

–

SDO

µCONTROLLER

CS SDO1 SDO2

31 OR LESS CLOCK CYCLES

REF

GND

–

31 OR LESS CLOCK CYCLES

REF

GND

LTC2410

–

SCK

SDO

EXTERNAL OSCILLATOR (153,600HZ)

REF

–

REF

2410 F44

Figure 44. Synchronous Conversion—Extendable

LTC2410

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LTC2410

REF

SCK

–

SDO

REF

–

µCONTROLLER

SCK

SDO

CS1 CS2 CS3 CS4

CS1

CS2

CS3

GND

LTC2410

REF

–

GND

REF

–

REF

EXTERNAL OSCILLATOR (153,600HZ)

REF

GND

LTC2410

–

SCK

SDO

SCK

–

SDO

REF

GND

–

LTC2410

–

SCK

SDO

CS4

31 OR LESS

SCK

SDO

CLOCK PULSES

Figure 45. Actual Frequency Rate LTC2410 System

BRIDGE APPLICATIONS

Typical strain gauge based bridges deliver only 2mV/Volt of excitation. As the maximum reference voltage of the LTC2410 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 10000 without averaging. For many solid state sensors, this is still better than the sensor. For example, averaging 64 samples however reduces the noise level by a factor of eight, bringing the resolving power to 1 part in 80000, 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

2410 F45

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 LTC2410 alone, compensating for error in external amplification can be done effectively due to the “no latency” feature of the LTC2410. 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

LTC2410

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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 LTC2410 and developing excitation via fixed gain, or LTC1043 based voltage multiplication, along with remote feedback in the excitation amplifiers, as shown in Figures 34 and 35.

Figure 46 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 LTC2410’s provides the benefit of a root square reduction in noise. The low power consumption of the LTC2410 makes 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 the sensor largely eliminates the need for protection

LT1019

3 4350Ω

BRIDGE

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

Figure 46. Simple Bridge Connection

REF REF

–

REF

–

LTC2410

GND

1, 7, 8, 9, 10, 15, 16

SDO

SCK

12 13

2410 F46

devices, RFI suppression and wiring. The LTC2410 exhibits 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 47 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 LTC2410 has 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 15 before its input referred noise dominates the LTC2410 noise. This example shows a gain of 34, that is determined by a feedback network built using a resistor array containing 8 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 LTC2410 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.

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

LTC2410

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APPLICATIO S I FOR ATIO

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 0.1µV can also be configured to provide gain of up to 50 with high gain stability and linearity.

Figure 48 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 AV = 1+ R2/(R1+175Ω). 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

REF

2-amplifier topology above.

. The buffer stages

RMS

REF

in the

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 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 49. The LTC2410 can accept inputs up to 1/2 V

. Hence, the reference resistor R1 must be at

REF

least 2x 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 49 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

Input referred noise for AV = 34 for approximately 0.05µV

RMS

0.048µV

, whereas at a gain of 50, it would be

RMS

350Ω

BRIDGE

0.1µF

U1A

–

RN1

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

611

–

U1B

710

512

0.1µF

U2A

–

U2B

–

Figure 47. Using Autozero Amplifiers to Reduce Input Referred Noise

REF

0.1µF

312

2410 F47

REF REF

–

LTC2410

–

GND

SDO SCK

1, 7, 8, 9, 10, 15, 16

LTC2410

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APPLICATIO S I FOR ATIO

reference inputs do not have the same rejection. If 60Hz or other noise is present on the reference input, a low pass filter is recommended as shown in Figure 50. 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 acceptalby low. If instead the single 25k resistor is replaced with a 10k 5% and a 10k 0.1% negative 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 50 shows a more rigorous example of Figure 49, with increased noise suppression and more protection for remote applications.

Figure 51 shows an example of gain in the excitation circuit and remote feedback from the bridge. The LTC1043’s 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 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.

350Ω

BRIDGE

0.1µV

LTC1050S8

–

1µF

AV = 9.98 1 +

4.98k

()

4.99k + 175Ω

Figure 48. Bridge Amplification Using a Single Amplifier

46.4k

175Ω

1µF

20k

10µF

3 4

REF REF

–

LTC2410

–

GND

0.1µF

1, 7, 8, 9, 10, 15, 16

2410 F48

LTC2410

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APPLICATIO S I FOR ATIO

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 52 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 LTC2410 of 2.5V, maximizing the AC input range for applications where induced 60Hz could reach amplitudes up to 2V

The last two example circuits could be used where multiple bridge circuits are involved and bridge output can be multiplexed onto a single LTC2410, via an inexpensive multiplexer such as the 74HC4052.

RMS

25.5k

0.1%

Figure 53 shows the use of an LTC2410 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.

2.7V TO 5.5V

REF

–

REF

LTC2410

PLATINUM

100Ω

RTD

PLATINUM

100Ω

RTD

–

GND

Figure 49. Remote Half Bridge Interface

LTC1050

–

0.1%

10k, 5%

10k

1µF

10k

1, 7, 8, 9, 10, 15, 16

2410 F49

560Ω

10k

REF

–

REF

LTC2410

–

GND

1, 7, 8, 9, 10, 15, 16

2410 F50

Figure 50. Remote Half Bridge Sensing with Noise Suppression on Reference

LTC2410

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15V15V

LTC1150

–15V

15V

LTC1150

–15V

–

350Ω

BRIDGE

2N3904

33Ω

–15V

20Ω

33Ω

Q2 2N3904

20Ω

0.1µF

15V

U1 LTC1043

200Ω

10V 5V

1µF

U2 LTC1043

LT1236-5

+ +

47µF 0.1µF

10µF

0.1µF

3 4

5 6

REF REF

IN IN

–

LTC2410

–

GND

1, 7, 8, 9, 10, 15, 16

10V

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

Figure 51. LTC1043 Provides Precise 3X Reference for Excitation Voltages

2410 F51

LTC2410

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APPLICATIO S I FOR ATIO

15V

–5V

20Ω

RN1

10k

RN1

10k

20Ω

350Ω BRIDGE

TWO ELEMENTS

VARYING

2N3904

22Ω

10V

33Ω

×2

Q2, Q3

2N3904

×2

–15V

0.1µF

1/2

LT1112

15V

1/2

LT1112

4 –15V

–

RN1 10k

–

LT1236-5

C3 47µF

LTC2410

REF

–

REF

–

RN1 10k

RN1 IS CADDOCK T914 10K-010-02

GND

1, 7, 8, 9, 10, 15, 16

0.1µF

2410 F52

Figure 52. Use Resistor Arrays to Provide Precise Matching in Excitation Amplifier

TO OTHER

DEVICES

12 14 15 11

74HC4052

1 5

2 4

47µF

13 3

REF REF

IN IN

–

LTC2410

GND

1, 7, 8

2410 F53

A0 A1

Figure 53. Use a Differential Multiplexer to Expand Channel Capability

TYPICAL APPLICATIO S

LTC2410

Sample Driver for LTC2410 SPI Interface

The LTC2410 has a very simple serial interface that makes interfacing to microprocessors and microcontrollers very easy.

The listing in Figure 55 is a simple assembler routine for the 68HC11 microcontroller. It uses PORT D, configuring it for SPI data transfer between the controller and the LTC2410. Figure 54 shows the simple 3-wire SPI connection.

The code begins by declaring variables and allocating four memory locations to store the 32-bit conversion result. This is followed by initializing PORT D’s SPI configuration. The program then enters the main sequence. It activates the LTC2410’s serial interface by setting the SS output low, sending a logic low to CS. It next waits in a loop for a logic low on the data line, signifying end-of-conversion. After the loop is satisfied, four SPI transfers are completed, retrieving the conversion. The main sequence ends by setting SS high. This places the LTC2410’s serial interface in a high impedance state and initiates another conversion.

The performance of the LTC2410 can be verified using the demonstration board DC291A, see Figure 56 for the schematic. This circuit uses the computer’s serial port to generate power and the SPI digital signals necessary for starting a conversion and reading the result. It includes a Labview application software program (see Figure 57) which graphically captures the conversion results. It can be used to determine noise performance, stability and with an external source, linearity. As exemplified in the schematic, the LTC2410 is extremely easy to use. This demonstration board and associated software is available by contacting Linear Technology.

SCK

LTC2410

Figure 54. Connecting the LTC2410 to a 68HC11 MCU Using the SPI Serial Interface

SDO

12 11

68HC11 SCK (PD4) MISO (PD2) SS (PD5)

2410 F54

LTC2410

TYPICAL APPLICATIO S

***************************************************** * This example program transfers the LTC2410's 32-bit output * * conversion result into four consecutive 8-bit memory locations. * ***************************************************** *68HC11 register definition PORTD EQU $1008 Port D data register * " – , – , SS* ,CSK ;MOSI,MISO,TxD ,RxD" DDRD EQU $1009 Port D data direction register SPSR EQU $1028 SPI control register * "SPIE,SPE ,DWOM,MSTR;SPOL,CPHA,SPR1,SPR0" SPSR EQU $1029 SPI status register * "SPIF,WCOL, – ,MODF; – , – , – , – " SPDR EQU $102A SPI data register; Read-Buffer; Write-Shifter * * RAM variables to hold the LTC2410's 32 conversion result * DIN1 EQU $00 This memory location holds the LTC2410's bits 31 - 24 DIN2 EQU $01 This memory location holds the LTC2410's bits 23 - 16 DIN3 EQU $02 This memory location holds the LTC2410's bits 15 - 08 DIN4 EQU $03 This memory location holds the LTC2410's bits 07 - 00 * ********************** * Start GETDATA Routine * ********************** *

ORG $C000 Program start location

INIT1 LDS #$CFFF Top of C page RAM, beginning location of stack

LDAA #$2F –,–,1,0;1,1,1,1

* –, –, SS*-Hi, SCK-Lo, MOSI-Hi, MISO-Hi, X, X

STAA PORTD Keeps SS* a logic high when DDRD, bit 5 is set LDAA #$38 –,–,1,1;1,0,0,0

STAA DDRD SS*, SCK, MOSI are configured as Outputs * MISO, TxD, RxD are configured as Inputs *DDRD's bit 5 is a 1 so that port D's SS* pin is a general output

LDAA #$50

STAA SPCR The SPI is configured as Master, CPHA = 0, CPOL = 0 * and the clock rate is E/2 * (This assumes an E-Clock frequency of 4MHz. For higher E* Clock frequencies, change the above value of $50 to a value * that ensures the SCK frequency is 2MHz or less.) GETDATA PSHX

PSHY

PSHA

LDX #$0 The X register is used as a pointer to the memory locations * that hold the conversion data

LDY #$1000

BCLR PORTD, Y %00100000 This sets the SS* output bit to a logic * low, selecting the LTC2410 *

TYPICAL APPLICATIO S

********************************** * The next short loop waits for the * * LTC2410's conversion to finish before * * starting the SPI data transfer * ********************************** * CONVEND LDAA PORTD Retrieve the contents of port D

ANDA #%00000100 Look at bit 2 * Bit 2 = Hi; the LTC2410's conversion is not * complete * Bit 2 = Lo; the LTC2410's conversion is complete

BNE CONVEND Branch to the loop's beginning while bit 2 remains

high * * ******************** * The SPI data transfer * ******************** * TRFLP1 LDAA #$0 Load accumulator A with a null byte for SPI transfer

STAA SPDR This writes the byte in the SPI data register and starts * the transfer WAIT1 LDAA SPSR This loop waits for the SPI to complete a serial

transfer/exchange by reading the SPI Status Register

BPL WAIT1 The SPIF (SPI transfer complete flag) bit is the SPSR's MSB * and is set to one at the end of an SPI transfer. The branch * will occur while SPIF is a zero.

LDAA SPDR Load accumulator A with the current byte of LTC2410 data

that was just received STAA 0,X Transfer the LTC2410's data to memory INX Increment the pointer CPX #DIN4+1 Has the last byte been transferred/exchanged? BNE TRFLP1 If the last byte has not been reached, then proceed to the

* next byte for transfer/exchange

BSET PORTD,Y %00100000 This sets the SS* output bit to a logic high,

* de-selecting the LTC2410

PULA Restore the A register PULY Restore the Y register PULX Restore the X register RTS

LTC2410

Figure 55. This is an Example of 68HC11 Code That Captures the LTC2410’s Conversion Results Over the SPI Serial Interface Shown in Figure 54

LTC2410

TYPICAL APPLICATIO S

BANANA JACK

EXT

BANANA JACK

REF

BANANA JACK

–

REF

BANANA JACK

–

BANANA JACK

J10

GND

JP1

JUMPER

R2 3Ω

JP3

JUMPER

GND

JP5 JUMPER

LT1460ACN8-2.5

OUTVIN

GND

C1 10µF 35V

NOTES: INSTALL JUMBER JP1 AT PIN 1 AND PIN 2 INSTALL JUMBER JP2 AT PIN 1 AND PIN 2 INSTALL JUMBER JP3 AT PIN 1 AND PIN 2

0.1µF

C2 22µF 25V

C5 10µF 35V

REF

–

REF

+ SDO

– GND

LTC2410CGN

JP2

JUMPER

GNDGND GND GND

CSV

SCK

GND GND

112

9871

JP4

JUMPER

143 134 125 166 15 10

LT1236ACN8-5

OUTVIN

C3 10µF 35V

GND

U3E

74HC14

U3B

74HC14

U3C

74HC14

1110

R7 22k

R8 51k

BYPASS CAP

C4 100µF 16V

U3F

74HC14

U3A

74HC14

U3D

74HC14

FOR U3

10Ω

1312

BAV74LT1

51k

49.9Ω

0.1µF

J1 V

J2 GND

1 6 2 7 3 8 4 9 5

R6 3k

Q1 MMBT3904LT1

EXT

DB9

2410 F56

Figure 56. 24-Bit A/D Demo Board Schematic

Figure 57. Display Graphic

PACKAGE DESCRIPTIO

0.015

± 0.004

(0.38 ± 0.10)

0.007 – 0.0098 (0.178 – 0.249)

0.016 – 0.050

(0.406 – 1.270)

* DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

0° – 8° TYP

PCB LAYOUT A D FIL

Dimensions in inches (millimeters) unless otherwise noted.

GN Package

16-Lead Plastic SSOP (Narrow 0.150)

(LTC DWG # 05-08-1641)

× 45°

0.053 – 0.068

(1.351 – 1.727)

0.008 – 0.012

(0.203 – 0.305)

0.004 – 0.0098 (0.102 – 0.249)

0.0250 (0.635)

BSC

0.229 – 0.244

(5.817 – 6.198)

0.189 – 0.196* (4.801 – 4.978)

LTC2410

12 11 10

0.150 – 0.157**

678

0.009

(0.229)

REF

(3.810 – 3.988)

GN16 (SSOP) 1098

Silkscreen Top Top Layer

LTC2410

PCB LAYOUT A D FIL

Bottom Layer

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LT1019 Precision Bandgap Reference, 2.5V, 5V 3ppm/°C Drift, 0.05% Max LT1025 Micropower Therocouple Cold Junction Compensator 80µA Supply Current, 0.5°C Initial Accuracy LTC1043 Dual Precision Instrumentation Switched Capacitor Precise Charge, Balanced Switching, Low Power

Building Block

LTC1050 Precision Chopper Stabilized Op Amp No External Components 5µV Offset, 1.6µV LT1236A-5 Precision Bandgap Reference, 5V 0.05% Max, 5ppm/°C Drift LT1460 Micropower Series Reference 0.075% Max, 10ppm/°C Max Drift, 2.5V, 5V and 10V Versions LTC2400 24-Bit, No Latency ∆Σ ADC in SO-8 0.3ppm Nosie, 4ppm INL, 10ppm Total Unadjusted Error, 200µA LTC2401/LTC2402 1-/2-Channel, 24-Bit, No Latency ∆Σ ADC in MSOP 0.6ppm Nosie, 4ppm INL, 10ppm Total Unadjusted Error, 200µA LTC2404/LTC2408 4-/8-Channel, 24-Bit, No Latency ∆Σ ADC 0.3ppm Nosie, 4ppm INL, 10ppm Total Unadjusted Error, 200µA LTC2420 20-Bit, No Latency ∆Σ ADC in SO-8 1.2ppm Noise, 8ppm INL, Pin Compatible with LTC2400

P-P

Noise

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear-tech.com

2410i LT/TP 0400 4K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 2000

Datasheet LTC2410 Datasheet (Linear Technology)

Specifications and Main Features

Frequently Asked Questions

User Manual