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AD7710

REV. G

–15–

Antialias Considerations

The digital filter does not provide any rejection at integer mul-

tiples of the modulator sample frequency (n

× 19.5 kHz, where

n = 1, 2, 3 . . . ). This means that there are frequency bands

±f

3 dB wide (f3 dB is cutoff frequency selected by FS0 to FS11),

where noise passes unattenuated to the output. However, due to

the AD7710’s high oversampling ratio, these bands occupy only

a small fraction of the spectrum, and most broadband noise is

filtered. In any case, because of the high oversampling ratio a

simple RC, single-pole filter is generally sufficient to attenuate

the signals in these bands on the analog input and thus provide

adequate antialiasing filtering.

If passive components are placed in front of the AD7710, ensure

that the source impedance is low enough to keep from intro-

ducing gain errors in the system. The dc input impedance for

the AD7710 is over 1 G

Ω. The input appears as a dynamic

load that varies with the clock frequency and with the selected

gain (see Figure 7). The input sample rate, as shown in Table III,

determines the time allowed for the analog input capacitor CIN

to be charged. External impedances result in a longer charge

time for this capacitor, which may result in gain errors being

introduced on the analog inputs. Table IV shows the allowable

external resistance/capacitance values that do not introduce gain

error to the 16-bit level, while Table V shows the allowable

external resistance/capacitance values that do not introduce gain

error to the 20-bit level. Both inputs of the differential input

channels look into similar input circuitry.

RINT

TYP

CINT

11.5pF TYP

VBIAS

AIN

SWITCHING FREQUENCY DEPENDS ON

fCLKIN AND SELECTED GAIN

HIGH

IMPEDANCE

>1G

AD7710

Figure 7. Analog Input Impedance

Table IV. External Series Resistance That Do Not Introduce

16-Bit Gain Error

External Capacitance (pF)

Gain

100

500

1000

5000

184 k

Ω 45.3 kΩ 27.1 kΩ 7.3 kΩ 4.1 kΩ 1.1 kΩ

88.6 k

Ω 22.1 kΩ 13.2 kΩ 3.6 kΩ 2.0 kΩ 560 Ω

41.4 k

Ω 10.6 kΩ 6.3 kΩ

1.7 k

Ω 970 Ω

270

Ω

8–128

17.6 k

Ω 4.8 kΩ

2.9 k

Ω

790

Ω

440

Ω

120

Ω

Table V. External Series Resistance That Do Not Introduce

20-Bit Gain Error

External Capacitance (pF)

Gain

100

500

1000

5000

145 k

Ω 34.5 kΩ 20.4 kΩ 5.2 kΩ 2.8 kΩ 700 Ω

70.5 k

Ω 16.9 kΩ 10 kΩ

2.5 k

Ω 1.4 kΩ 350 Ω

31.8 k

Ω 8.0 kΩ

4.8 k

Ω

1.2 k

Ω 670 Ω

170

Ω

8–128

13.4 k

Ω 3.6 kΩ

2.2 k

Ω

550

Ω

300

Ω

The numbers in Tables IV and V assume a full-scale change on

the analog input. In any case, an error introduced due to longer

charging times is a gain error that can be removed using the

system calibration capabilities of the AD7710, provided that the

resultant span is within the span limits of the system calibration

techniques.

ANALOG INPUT FUNCTIONS

Analog Input Ranges

Both analog 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 VSS to AVDD,

provided that the absolute value of the analog input voltage lies

between VSS –30 mV and AVDD +30 mV.

The dc input leakage current 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 differen-

tial input capability of the part and its system calibration mode.

Burnout Current

The AIN1(+) input of the AD7710 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

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 has burned out or gone open circuit. For normal

operation, this burnout current is turned off by writing a 0 to

the BO bit in the control register.

Output Compensation Current

The AD7710 also contains a feature that allows the user to

implement cold junction compensation in thermocouple appli-

cations. This can be achieved using the output compensation

current from the IOUT pin of the device. Once again, this current

can be turned on/off via the control register. Writing a 1 to the

IO bit of the control register enables this compensation current.

The compensation current provides a 20

µA constant current

source that can be used in association with a thermistor or a

diode to provide cold junction compensation. A common

method of generating cold junction compensation is to use a

temperature dependent current flowing through a fixed resistor

to provide a voltage that is equal to the voltage developed across

the cold junction at any temperature in the expected ambient

range. In this case, the temperature coefficient of the compensa-

tion current is so low compared with the temperature coefficient

of the thermistor that it can be considered constant with tem-

perature. The temperature variation is then provided by the

variation of the thermistor’s resistance with temperature.

Normally, the cold junction compensation will be implemented

by applying the compensation voltage to the second input chan-

nel of the AD7710. Periodic conversion of this channel gives the

user a voltage that corresponds to the cold junction compensa-

tion voltage. This can be used to implement cold junction com-

pensation in software with the result from the thermocouple

input being adjusted according to the result in the compensation

channel. Alternatively, the voltage can be subtracted from the

input voltage in an analog fashion, thereby using only one chan-

nel of the AD7710.

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