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AD7790

Rev. 0 | Page 17 of 20

CIRCUIT DESCRIPTION

ANALOG INPUT CHANNEL

The AD7790 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 16

shows the allowable external resistance/capacitance values for

unbuffered mode such that no gain error at the 16-bit level is

introduced.

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

C (pF)

R (Ω)

22.8K

100

13.1K

500

3.3K

1000

1.8K

5000

360

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.

PROGRAMMABLE GAIN AMPLIFIER

The output from the buffer on the ADC is applied to the input

of the on-chip programmable gain amplifier (PGA). The PGA

gain range is programmed via the gain bits G1 and G0 in the

mode register. With an external 2.5 V reference applied, the

PGA can be programmed to have a bipolar range of ±2.5 V,

±1.25 V, ±625 mV, or ±312.5 mV. These are the ranges that

should appear at the input to the on-chip PGA.

BIPOLAR CONFIGURATION

The analog input to the AD7790 accepts a bipolar input voltage

range. A bipolar input range does not imply that the part can

tolerate negative voltages with respect to system GND. 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 a gain of 1, the analog input range on the AIN(+)

input is 0 V to 5 V.

DATA OUTPUT CODING

The output code is offset binary with a negative full-scale volt-

age 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 × GAIN/VREF) + 1]

where AIN is the analog input voltage, GAIN is the PGA gain,

and N = 16.

REFERENCE INPUT

The AD7790 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 for specified operation, but the AD7790 is func-

tional 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

AD7790 is used in a nonratiometric application, a low noise

reference should be used.

Recommended 2.5 V reference voltage sources for the AD7790

include the ADR381 and ADR391 because these are low noise,

low power references. If the complete analog section is driven

from a 2.5 V power supply, the reference voltage source will

require some headroom. In this case, a 2.048 V reference such as

the ADR390 or ADR380 is recommended, again 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 combina-

tions 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 like those recommended

above (e.g., ADR391) will typically have low output impedances

and are, therefore, tolerant to having decoupling capacitors on

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