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AD7798/AD7799

Data Sheet

Rev. B | Page 22 of 28

CIRCUIT DESCRIPTION

ANALOG INPUT CHANNEL

The AD7798/AD7799 each have three differential analog input

channels. These are connected to the on-chip buffer amplifier

when the devices are operated in buffered mode, and directly to

the modulator when the devices are 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 gages or resistance

temperature detectors (RTDs).

When BUF = 0, the parts are 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 gain errors, depending on the output impedance

of the source that is driving the ADC input. Table 17 shows the

allowable external resistance/capacitance values for unbuffered

mode such that no gain error at the 20-bit level is introduced.

Table 17. External Resistance/Capacitance Combination for

Unbuffered Mode (Without 20-Bit Gain Error)

Capacitance (pF)

Resistance (Ω)

9 k

100

6 k

500

1.5 k

1000

900

5000

200

The AD7798/AD7799 can be operated in unbuffered mode only

when the gain equals 1 or 2. At higher gains, the buffer is auto-

matically enabled. The absolute input voltage range in buffered

mode is restricted to a range between GND + 100 mV and

AVDD – 100 mV. When the gain is set to 4 or higher, the in-amp

is enabled. The absolute input voltage range when the in-amp is

active 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 so that these limits are not exceeded; otherwise,

linearity and noise performance degrade.

The absolute input voltage in unbuffered mode includes the

range between GND − 30 mV and AVDD + 30 mV as a result of

being unbuffered. The negative absolute input voltage limit

allows the possibility of monitoring small true bipolar signals

with respect to GND.

INSTRUMENTATION AMPLIFIER

When the gain equals 4 or higher, the output from the buffer is

applied to the input of the on-chip instrumentation amplifier.

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

can be gained within the AD7798/AD7799 while still maintaining

excellent noise performance. For example, when the gain is set

to 64 and the update rate equals 4.17 Hz, the rms noise is 27 nV

typically for the AD7799, which is equivalent to 25.5 bits effective

resolution, or 20 bits peak-to-peak resolution when VREF = 5 V.

The AD7798/AD7799 can be programmed to have a gain of 1, 2,

4, 8, 16, 32, 64, or 128 using Bit G2 to Bit G0 in the configuration

from (0 mV to 19.53 mV) to (0 V to 2.5 V), and the bipolar

ranges are from ±19.53 mV to ±2.5 V. When the in-amp is active

(gain ≥ 4), the common-mode voltage (AIN(+) + AIN(−))/2 must

be greater than or equal to 0.5 V.

If the AD7798/AD7799 operate with a reference that has a value

equal to AVDD, the analog input signal must be limited to 90% of

VREF/gain when the in-amp is active for correct operation.

BIPOLAR/UNIPOLAR CONFIGURATION

The analog input to the AD7798/AD7799 can accept either

unipolar or bipolar input voltage ranges. A bipolar input range

does not imply that the parts 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 and a gain of 1, 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 configuration register.

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