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REV. F

–16–

AD7712

ANALOG INPUT FUNCTIONS

Analog Input Ranges

The analog inputs on the AD7712 provide the user with consid-

erable flexibility in terms of analog input voltage ranges. One of

the inputs is a differential, programmable gain, input channel

that can handle either unipolar or bipolar input signals. The

common-mode range of this input is from VSS to AVDD provided

that the absolute value of the analog input voltage lies between

VSS – 30 mV and AVDD + 30 mV. The second analog input is a

single-ended, programmable gain, high level input that accepts

analog input ranges of 0 to +4

VREF/GAIN or

±4 V

REF/GAIN.

The dc input leakage current on the AIN1 input is 10 pA maxi-

mum at 25

°C (±1 nA over temperature). This results in a dc

offset voltage developed across the source impedance. However,

this dc offset effect can be compensated for by a combination of

the differential input capability of the part and its system cali-

bration mode. The dc input current on the AIN2 input depends

on the input voltage. For the nominal input voltage range of

±10 V, the input current is ±225 µA typ.

Burnout Current

The AIN1(+) input of the AD7712 contains a 4.5

µA current

source that can be turned on/off via the control register. This

current source can be used in checking that a transducer has not

burned out or gone open circuit before attempting to take mea-

surements on that channel. If the current is turned on and is

allowed to flow into the transducer and a measurement of the

input voltage on the AIN1 input is taken, it can indicate that the

transducer is not functioning correctly. For normal operation,

this burnout current is turned off by writing a 0 to the BO bit in

the control register.

Bipolar/Unipolar Inputs

The two analog inputs on the AD7712 can accept either unipo-

lar or bipolar input voltage ranges. Bipolar or unipolar options

are chosen by programming the B/U bit of the control register.

This programs both channels for either unipolar or bipolar

operation. Programming the part for either unipolar or bipolar

operation does not change any of the input signal conditioning;

it simply changes the data output coding. The data coding is

binary for unipolar inputs and offset binary for bipolar inputs.

The AIN1 input channel is differential and, as a result, the

voltage to which the unipolar and bipolar signals are referenced

is the voltage on the AIN1(–) input. For example, if AIN1(–) is

1.25 V and the AD7712 is configured for unipolar operation

with a gain of 1 and a VREF of 2.5 V, the input voltage range on

the AIN1(+) input is 1.25 V to 3.75 V. If AIN1(–) is 1.25 V and

the AD7712 is configured for bipolar mode with a gain of 1 and

a VREF of 2.5 V, the analog input range on the AIN1(+) input is

–1.25 V to +3.75 V. For the AIN2 input, the input signals are

referenced to AGND.

REFERENCE INPUT/OUTPUT

The AD7712 contains a temperature compensated 2.5 V refer-

ence, which has an initial tolerance of

±1%. This reference

voltage is provided at the REF OUT, pin and can be used as the

reference voltage for the part by connecting the REF OUT pin

to the REF IN(+) pin. This REF OUT pin is a single-ended

output, referenced to AGND, which is capable of providing up

to 1 mA to an external load. In applications where REF OUT

is connected directly to REF IN(+), REF IN(–) should be tied

to AGND to provide the nominal 2.5 V reference for the

AD7712.

The reference inputs of the AD7712, REF IN(+) and

REF IN(–), provide a differential reference input capability.

The common-mode range for these differential inputs is from

VSS to AVDD. The nominal differential voltage, VREF (REF

IN(+) – REF IN(–)), is 2.5 V for specified operation, but the

reference voltage can go to 5 V with no degradation in perfor-

mance provided that the absolute value of REF IN(+) and REF

IN(–) does not exceed its AVDD and VSS limits and the VBIAS

input voltage range limits are obeyed. The part is also functional

with VREF voltages down to 1 V but with degraded performance

as the output noise will, in terms of LSB size, be larger. REF

IN(+) must always be greater than REF IN(–) for correct opera-

tion of the AD7712.

Both reference inputs provide a high impedance, dynamic load

similar to the AIN1 analog inputs. The maximum dc input

leakage current is 10 pA (

±1 nA over temperature), and source

resistance may result in gain errors on the part. The reference

inputs look like the AIN1 analog input (see Figure 7). In this

case, RINT is 5 k

Ω typ and C

INT varies with gain. The input

sample rate is fCLK IN/256 and does not vary with gain. For gains

of 1 to 8, CINT is 20 pF; for a gain of 16, it is 10 pF; for a gain

of 32, it is 5 pF; for a gain of 64, it is 2.5 pF; and for a gain of

128, it is 1.25 pF.

The digital filter of the AD7712 removes noise from the refer-

ence input just as it does with the analog input, and the same

limitations apply regarding lack of noise rejection at integer

multiples of the sampling frequency. The output noise perfor-

mance outlined in Tables I and II assumes a clean reference. If

the reference noise in the bandwidth of interest is excessive, it

can degrade the performance of the AD7712. Using the on-chip

reference as the reference source for the part (i.e., connecting

REF OUT to REF IN) results in somewhat degraded output

noise performance from the AD7712 for portions of the noise

table that are dominated by the device noise. The on-chip refer-

ence noise effect is eliminated in ratiometric applications where

the reference is used to provide its excitation voltage for the analog

front end. The connection scheme shown in Figure 9 between

the REF OUT and REF IN pins of the AD7712 is recommended

when using the on-chip reference. Recommended reference

voltage sources for the AD7712 include the AD780 and AD680

2.5 V references.

REF OUT

REF IN(+)

AD7712

REF IN(–)

Figure 9. REF OUT/REF IN Connection

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