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AD7791

Data Sheet

Rev. A | Page 18 of 20

CIRCUIT DESCRIPTION

ANALOG INPUT CHANNEL

The AD7791 has one differential analog input channel. This is

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 detec-

tors (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

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)

16.7K

100

9.6K

500

2.2K

1000

1.1K

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 will be degrada-

tion in linearity and noise performance.

The absolute input voltage in unbuffered mode includes the

range between GND – 30 mV and VDD + 30 mV as a result of

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 AD7791 can accept either unipolar or

bipolar input voltage ranges. A bipolar input range does not

imply that the part 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 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 B/U bit in the mode

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/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 and N = 24.

REFERENCE INPUT

The AD7791 has 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 will introduce gain

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

2.5 V nominal, but the AD7791 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 will be removed

because the application is ratiometric. If the AD7791 is used

in a nonratiometric application, a low noise reference should

be used.

Recommended 2.5 V reference voltage sources for the AD7791

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 will require some head-

room. 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 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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