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AD7610

Rev. 0 | Page 19 of 32

ANALOG INPUTS

Input Range Selection

In parallel mode and serial hardware mode, the input range is

selected by using the BIPOLAR (bipolar) and TEN (10 Volt range)

inputs. See Table 6 for pin details and the Hardware

Configuration section and Software Configuration section for

programming the mode selection with either pins or configuration

BIPOLAR and TEN inputs are don’t cares and should be tied to

either high or low.

Input Structure

Figure 28 shows an equivalent circuit for the input structure of

the AD7610.

RIN

CIN

IN+ OR IN–

VEE

VCC

CPIN

AGND

AVDD

0 TO 5V

RANGE ONLY

Figure 28. AD7610 Simplified Analog Input

The four diodes, D1 to D4, provide ESD protection for the analog

inputs, IN+ and IN−. Care must be taken to ensure that the analog

input signal never exceeds the supply rails by more than 0.3 V,

because this causes the diodes to become forward-biased and to

start conducting current. These diodes can handle a forward-

biased current of 120 mA maximum. For instance, these conditions

could eventually occur when the input buffer’s U1 supplies are

different from AVDD, VCC, and VEE. In such a case, an input

buffer with a short-circuit current limitation can be used to protect

the part although most op amps’ short circuit current is <100 mA.

Note that D3 and D4 are only used in the 0 V to 5 V range to

allow for additional protection in applications that are switching

from the higher voltage ranges.

This analog input structure allows the sampling of the differential

signal between IN+ and IN−. By using this differential input,

small signals common to both inputs are rejected as shown in

Figure 29, which represents the typical CMRR over frequency.

For instance, by using IN− to sense a remote signal ground,

ground potential differences between the sensor and the local

ADC ground are eliminated.

100

10000

FREQUENCY (kHz)

100

1000

Figure 29. Analog Input CMRR vs. Frequency

During the acquisition phase for ac signals, the impedance of

the analog inputs, IN+ and IN−, can be modeled as a parallel

combination of Capacitor CPIN and the network formed by

the series connection of RIN and CIN. CPIN is primarily the pin

capacitance. RIN is typically 5 kΩ and is a lumped component

comprised of serial resistors and the on resistance of the switches.

CIN is primarily the ADC sampling capacitor and depending on

the input range selected is typically 48 pF in the 0 V to 5 V range,

typically 24 pF in the 0 V to 10 V and ±5 V ranges and typically

12 pF in the ±10 V range. During the conversion phase, when

the switches are opened, the input impedance is limited to CPIN.

Since the input impedance of the AD7610 is very high, it can be

directly driven by a low impedance source without gain error.

To further improve the noise filtering achieved by the AD7610

analog input circuit, an external, one-pole RC filter between the

amplifier’s outputs and the ADC analog inputs can be used, as

shown in Figure 27. However, large source impedances signifi-

antly affect the ac performance, especially total harmonic

distortion (THD). The maximum source impedance depends

on the amount of THD that can be tolerated. The THD degrades

as a function of the source impedance and the maximum input

frequency.

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