AD7612_15 datasheet(20/32 Pages) AD

Part #	AD7612

Description	16-Bit, 750 kSPS, Unipolar/Bipolar Programmable Input PulSAR

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Manufacturer	AD [Analog Devices]

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AD7612

Data Sheet

Rev. A | Page 20 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 register. Note

that when using the configuration register, the 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 AD7612.

D1

RIN

CIN

D2

IN+ OR IN–

VEE

VCC

CPIN

AGND

D3

D4

AVDD

0TO 5V

RANGE ONLY

Figure 28. AD7612 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

0

1

10000

FREQUENCY (kHz)

90

80

70

60

50

40

30

20

10

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 capac-

itance. RIN is typically 70 Ω and is a lumped component comprised

of serial resistors and the on resistance of the switches. CIN is pri-

marily 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 AD7612 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 AD7612 analog

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

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

in Figure 27. However, large source impedances significantly

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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