AD7788ARMZ datasheet(17/20 Pages) AD

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AD7788/AD7789

Rev. B | Page 17 of 20

CIRCUIT DESCRIPTION

ANALOG INPUT CHANNEL

The AD7788/AD7789 have one differential analog input

channel that is connected to the modulator, thus, the input is

unbuffered. Note that this unbuffered input path provides a

dynamic load to the driving source. Therefore,

resistor/capacitor combinations on the input pins can cause dc

gain errors, depending on the output impedance of the source

that is driving the ADC input. Table 13 shows the allowable

external resistance/capacitance values such that no gain error at

the 16-bit level is introduced (AD7788). Table 14 shows the

allowable external resistance/capacitance values such that no

gain error at the 20-bit level is introduced (AD7789).

Table 13. External R-C Combination for No 16-Bit Gain

Error (AD7788)

C (pF)

R (Ω)

22.8 k

100

13.1 k

500

3.3 k

1000

1.8 k

5000

360

Table 14. External R-C Combination for No 20-Bit Gain

Error (AD7789)

C (pF)

R (Ω)

16.7 k

100

9.6 k

500

2.2 k

1000

1.1 k

5000

160

The absolute input voltage includes the range between GND −

30 mV and VDD + 30 mV. The negative absolute input voltage

limit does allow the possibility of monitoring small true bipolar

signals with respect to GND.

BIPOLAR/UNIPOLAR CONFIGURATION

The analog input to the devices can accept either unipolar or

bipolar input voltage ranges. A bipolar input range does not

imply that the parts can tolerate large 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 5 V. If the ADC is configured for bipolar mode, the

analog input range on the AIN(+) input is 0 V to 5 V. The

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

in the mode 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 000...000, 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/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/VREF) + 1]

where:

AIN is the analog input voltage.

N = 16 for the AD7788, 24 for the AD7789.

REFERENCE INPUT

The AD7788/AD7789 have a fully differential input capability

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

inputs is from GND to VDD. The reference input is unbuffered

and, therefore, excessive R-C source impedances introduce gain

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

2.5 V nominal, but the AD7788/AD7789 are functional with

reference voltages from 0.1 V to VDD. In applications where the

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

input also drives the reference voltage for the parts, the effect of

the low frequency noise in the excitation source is removed

because the application is ratiometric. If the AD7788/AD7789

are used in a nonratiometric application, a low noise reference

should be used.

Recommended 2.5 V reference voltage sources for the AD7788/

AD7789 include the ADR381 and ADR391, because they are

low noise, low power references. If the analog circuitry uses a

2.5 V power supply, the reference voltage source requires some

headroom. In this case, a 2.048 V reference such as the ADR380

or ADR390 can be used. Again, these are low power, low noise

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.

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