AD7713 datasheet(16/29 Pages) AD

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REV. D

AD7713

–15–

In any case, the error introduced due to longer charging times is

a gain error that can be removed using the system calibration

capabilities of the AD7713 provided that the resultant span is

within the span limits of the system calibration techniques for

the AD7713.

The AIN3 input contains a resistive attenuation network as

outlined in Figure 8. The typical input impedance on this input

is 44 k

Ω. As a result, the AIN3 input should be driven from a

low impedance source.

AIN3

33k

11k

VBIAS

MODULATOR

CIRCUIT

Figure 8. AIN3 Input Impedance

ANALOG INPUT FUNCTIONS

Analog Input Ranges

The analog inputs on the AD7713 provide the user with consid-

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

the inputs are differential, programmable-gain, input channels

that can handle either unipolar or bipolar input signals. The

common-mode range of these inputs is from AGND to AVDD,

provided that the absolute value of the analog input voltage lies

between AGND – 30 mV and AVDD + 30 mV. The third analog

input is a single-ended, programmable gain high-level input that

accepts analog input ranges of 0 to 4

VREF/GAIN.

The dc input leakage current on the AIN1 and AIN2 inputs

is 10 pA maximum 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 calibration mode. The dc input current on

the AIN3 input depends on the input voltage. For the nominal

input voltage range of 10 V, the input current is 225

µA typ.

Burn Out Current

The AIN1(+) input of the AD7713 contains a 1

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 burnt out

or gone open circuit before attempting to take measurements on

that channel. If the current is turned on and is allowed 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 func-

tioning correctly. For normal operation, this burn out current is

turned off by writing a 0 to the BO bit in the control register.

RTD Excitation Currents

The AD7713 also contains two matched 200

µA constant cur-

rent sources which are provided at the RTD1 and RTD2 pins of

the device. These currents can be turned on/off via the control

these excitation currents.

For 4-wire RTD applications, one of these excitation currents is

used to provide the excitation current for the RTD; the second

current source can be left unconnected. For 3-wire RTD con-

figurations, the second on-chip current source can be used

to eliminate errors due to voltage drops across lead resistances.

Figures 19 and 20 in the Application section show some RTD

configurations with the AD7713.

The temperature coefficient of the RTD current sources is

typically 20 ppm/

°C with a typical matching between the

temperature coefficients of both current sources of 3 ppm/

°C.

For applications where the absolute value of the temperature

coefficient is too large, the following schemes can be used to

remove the drift error.

The conversion result from the AD7713 is ratiometric to the

VREF voltage. Therefore, if the VREF voltage varies with the RTD

temperature coefficient, the temperature drift from the current

source will be removed. For 4-wire RTD applications, the refer-

ence voltage can be made ratiometric to the RTD current source

by using the second current with a low TC resistor to generate the

reference voltage for the part. In this case, if a 12.5 k

Ω resistor is

used, the 200

µA current source generates 2.5 V across the resistor.

This 2.5 V can be applied to the REF IN(+) input of the AD7713

and the REF IN(–) input at ground will supply a VREF of 2.5 V for

the part. For 3-wire RTD configurations, the reference voltage for

the part is generated by placing a low TC resistor (12.5 k

Ω for

2.5 V reference) in series with one of the constant current sources.

The RTD current sources can be driven to within 2 V of AVDD.

The reference input of the AD7713 is differential so the REF IN(+)

and REF IN(–) of the AD7713 are driven from either side of the

resistor. Both schemes ensure that the reference voltage for the part

tracks the RTD current sources over temperature and, thereby,

removes the temperature drift error.

Bipolar/Unipolar Inputs

Two analog inputs on the AD7713 can accept either unipolar or

bipolar input voltage ranges while the third channel accepts only

unipolar signals. Bipolar or unipolar options for AIN1 and AIN2

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

This programs both channels for either unipolar or bipolar opera-

tion. 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 and AIN2 input channels are differential, and as a

result, the voltage to which the unipolar and bipolar signals are

referenced is the voltage on the AIN1(–) and AIN2(–) inputs. For

example, if AIN1(–) is 1.25 V and the AD7713 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. For the

AIN3 input, the input signals are referenced to AGND.

REFERENCE INPUT

The reference inputs of the AD7713, 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 performance, provided that the

absolute value of REF IN(+) and REF IN(–) does not exceed

its AVDD and AGND limits. 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 operation of

the AD7713.

Both reference inputs provide a high impedance, dynamic load

similar to the analog inputs. The maximum dc input leakage cur-

rent is 10 pA (

±1 nA over temperature), and source resistance

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

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