AD7738 datasheet(26/28 Pages) AD

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AD7738

Voltage Reference Inputs

The AD7738’s reference inputs, REFIN(+) and REFIN(–),

provide a differential reference input capability. The common-

mode range for these differential inputs is from AGND to AVDD.

The nominal reference voltage for specified operation is 2.5 V.

Both reference inputs feature a high impedance, dynamic load.

Because the input impedance on each reference input is dynamic,

external resistance/capacitance combinations may result in gain

errors on the part.

The output noise performance outlined in Tables I to VI is for

an analog input of 0 V and is unaffected by noise on the reference.

To obtain the same noise performance as shown in the noise tables

over the full input range requires a low noise reference source for

the AD7738. If the reference noise in the bandwidth of interest

is excessive, it will degrade the performance of the AD7738.

Recommended reference voltage sources for the AD7738 include

the ADR421, AD780, REF43, and REF192. It is generally

recommended to decouple the output of these references to

further reduce the noise level.

Reference Detect

The AD7738 includes on-chip circuitry to detect if the part has

a valid reference for conversions.

If the voltage between the REFIN(+) and REFIN(–) pins goes

below the NOREF Trigger Voltage (0.5 V typ) and the AD7738 is

performing conversion, the NOREF bit in the Channel Status

I/O Port

The AD7738 Pin

SYNC/P1 can be used as a general-purpose

digital I/O pin or to synchronize the AD7738 with other devices

in the system. When the SYNC bit in the I/O Port register is set

and the

SYNC pin is low, the AD7738 doesn’t process any

conversion. If it is put into single conversion mode, Continuous

Conversion mode, or any Calibration mode, the AD7738 waits

until the

SYNC pin goes high and then starts operation. This

allows the user to start conversion from a known point in time,

i.e., the rising edge of the

SYNC pin.

0.1 F

10 F

ADR421

VOUT

GND

VIN

AVDD

10 F

0.1 F

AVDD

CLOCK

GENERATOR

MCLKIN

MCLKOUT

33pF

6.144MHz

DVDD

10 F

0.1 F

DVDD

ADCIN

24-BIT

ADC

BUFFER

MUXOUT -

AD7738

AIN7

MUX

AINCOM

AIN0

SERIAL

INTERFACE

AND

CONTROL

LOGIC

SCLK

DIN

DOUT

RDY

RESET

REFIN(–)

REFIN(+)

DGND

AGND

HOST

SYSTEM

ANALOG

INPUTS

DVDD

0.1 F

Figure 17. Typical Connection for the AD7738 Application

AD7738 Datasheet(PDF) 26 Page - Analog Devices

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