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AD7787

Data Sheet

Rev. A | Page 18 of 20

CIRCUIT DESCRIPTION

ANALOG INPUT CHANNEL

The AD7787 has two analog input channels that are connected

to the on-chip buffer amplifier when the device is operated in

buffered mode and directly to the modulator when the device is

operated in unbuffered mode. In buffered mode (the BUF bit in

the mode register is set to 1), the input channel feeds into a high

impedance input stage of the buffer amplifier. Therefore, the

input can tolerate significant source impedances and is tailored

for direct connection to external resistive-type sensors, such as

strain gauges or resistance temperature detectors (RTDs).

When BUF = 0, the part is operated in unbuffered mode.

This results in a higher analog input current. 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 15

shows the allowable external resistance/capacitance values for

the unbuffered mode such that no gain error at the 20-bit level

is introduced.

Table 15. External R-C Combination for No 20-Bit Gain

Error

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 range in buffered mode is restricted

to a range between GND + 100 mV and VDD − 100 mV. Care

must be taken in setting up the common-mode voltage so that

these limits are not exceeded. Otherwise, there is degradation in

linearity and noise performance.

The absolute input voltage in unbuffered mode includes the

range between GND − 100 mV and VDD + 30 mV resulting

from being unbuffered. 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 AD7787 can accept either unipolar or

bipolar input voltage ranges. Unipolar and bipolar signals on

the AIN1(+) input are referenced to the voltage on the AIN(−)

input. For example, if AIN1(−) is 2.5 V and the ADC is

configured for unipolar mode, the input voltage range on the

AIN1(+) pin is 2.5 V to 5 V when REFIN = 2.5 V. If the ADC is

configured for bipolar mode, the analog input range on the

AIN1(+) input is 0 V to 5 V.

The voltage on AIN2 is referenced to GND. Therefore, when

bipolar mode is selected and the part is operated in unbuffered

mode, the voltage on AIN2 can vary from GND − 100 mV to

+2.5 V. In unipolar mode, the voltage on AIN2 can vary from

0 V to 2.5 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 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





REF

AIN

Code





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







1





 

REF

AIN

Code

where AIN is the analog input voltage and N = 24.

REFERENCE INPUT

The AD7787 has a single-ended reference that is 2.5 V nominal,

but the AD7787 is 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 part, the effect of the low frequency noise in the

excitation source is removed because the application is

ratiometric. If the AD7787 is used in a nonratiometric

application, a low noise reference should be used.

Recommended 2.5 V reference voltage sources for the AD7787

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

power references. In a system that operates from 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, can be used,

requiring only 300 mV of headroom. Also note that the

reference input provides a high impedance, dynamic load.

Because the input impedance of the reference input is dynamic,

resistor/capacitor combinations on this input can cause dc gain

errors, depending on the output impedance of the source

driving the reference inputs.

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