AD7796 datasheet(20/24 Pages) AD

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AD7796/AD7797

Data Sheet

Rev. B | Page 20 of 24

CIRCUIT DESCRIPTION

ANALOG INPUT CHANNEL

The AD7796/AD7797 have one differential analog input

channel. The input channel feeds into a high impedance input

stage of the amplifier. Therefore, the input can tolerate signifi-

cant source impedances and is tailored for direct connection to

external resistive-type sensors such as strain gages.

The absolute input voltage range is restricted to a range between

GND + 300 mV and AVDD − 1.1 V. Care must be taken in setting

up the common-mode voltage to avoid exceeding these limits.

Otherwise, there is degradation in linearity and noise

performance.

This low noise in-amp means that signals of small amplitude

can be gained within the AD7796/AD7797 while still maintain-

ing excellent noise performance. The amplifier is configured to

have a gain of 128. Therefore, with an external 2.5 V reference,

the unipolar range is 0 mV to 20 mV while the bipolar range is

±20 mV. The common-mode voltage ((AIN(+) + AIN(–))/2

must be ≥ 0.5 V.

BIPOLAR/UNIPOLAR CONFIGURATION

The analog input to the AD7796/AD7797 can accept either

unipolar or bipolar input voltage ranges. A bipolar input range

does not imply that the device can tolerate negative voltages

with respect to system GND. Unipolar and bipolar signals on

the AIN(+) input are referenced to the voltage on the AIN(–)

input. For example, if AIN(−) is 2.5 V and the ADC is

configured for unipolar mode, the input voltage range on the

AIN(+) pin is 2.5 V to 2.02 V.

If the ADC is configured for bipolar mode, the analog input

range on the AIN(+) input is 2.48 V to 2.52 V. The bipolar/

unipolar option is chosen by programming the U/B bit in the

configuration register.

DATA OUTPUT CODING

When the ADC is configured for unipolar operation, the output

code is natural (straight) binary with a zero differential input

voltage resulting in a code of 00...00, a midscale voltage resulting in

a code of 100...000, and a full-scale input voltage resulting in a

code of 111...111. The output code for any analog input voltage

can be represented as

Code = (2N × AIN × 128)/VREF

When the ADC is configured for bipolar operation, the output

code is offset binary with a negative full-scale voltage resulting

in a code of 000...000, a zero differential input voltage resulting

in a code of 100...000, and a positive full-scale input voltage

resulting in a code of 111...111. The output code for any analog

input voltage can be represented as

Code = 2N – 1 × [(AIN × 128 /VREF) + 1]

where:

AIN is the analog input voltage

N = 16/24 for the AD7796/AD7797.

REFERENCE

The AD7796/AD7797 have a fully differential input capability

for the channel. The common-mode range for these differential

inputs is GND to AVDD. The reference input is unbuffered;

therefore, excessive R-C source impedances introduce gain

errors. The reference voltage REFIN (REFIN(+) − REFIN(−)) is

2.5 V nominal, but the AD7796/AD7797 are functional with

reference voltages 0.1 V to AVDD. In applications where the

excitation (voltage or current) for the transducer on the analog

input also drives the reference voltage for the device, the effect

of the low frequency noise in the excitation source is removed

because the application is ratiometric. If the AD7796/AD7797

are used in a nonratiometric application, a low noise reference

should be used.

Recommended 2.5 V reference voltage sources for the AD7796/

AD7797 include the ADR381 and ADR391, which are low noise,

low power references. Also note that the reference inputs provide

a high impedance, dynamic load. Because the input impedance

of each reference input is dynamic, resistor/ capacitor combinations

on these inputs can cause dc gain errors, depending on the

output impedance of the source that is driving the reference

inputs.

Reference voltage sources such as those recommended above

(the ADR391, for example) typically have low output impedances

and are, therefore, tolerant to decoupling capacitors on REFIN(+)

without introducing gain errors in the system. Deriving the

reference input voltage across an external resistor means that

the reference input sees a significant external source impedance.

External decoupling on the REFIN pins is not recommended in

this type of circuit configuration.

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