AD7631BSTZ datasheet(20/32 Pages) AD

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AD7631

Rev. A | Page 20 of 32

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 of the AD7631 is a true differential

structure allowing 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.

120

10000

FREQUENCY (kHz)

100

1000

0V TO 10V

±5V

0V TO 5V

±10V

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.

Because the input impedance of the AD7631 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

AD7631 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 significantly affect the ac performance, especially

the 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, as shown in Figure 30.

–130

–110

–90

–70

100

15Ω

33Ω

100Ω

200Ω

FREQUENCY (kHz)

Figure 30. THD vs. Analog Input Frequency and Source Resistance

DRIVER AMPLIFIER CHOICE

Although the AD7631 is easy to drive, the driver amplifier must

meet the following requirements:

•

For multichannel, multiplexed applications, the driver

amplifier and the AD7631 analog input circuit must be

able to settle for a full-scale step of the capacitor array

at a 18-bit level (0.0004%). For the amplifier, settling at

0.1% to 0.01% is more commonly specified. This differs

significantly from the settling time at a 18-bit level and

should be verified prior to driver selection. The AD8021 op

amp combines ultralow noise with high gain bandwidth and

meets this settling time requirement even when used with

gains of up to 13.

•

The noise generated by the driver amplifier needs to be

kept as low as possible to preserve the SNR and transition

noise performance of the AD7631. The noise coming from

the driver is filtered by the external, 1-pole, low-pass filter,

as shown in Figure 27. The SNR degradation due to the

amplifier is

⎟

⎠

⎞

⎜

⎝

⎛

−

3dB

)

(

)

(

log

NADC

LOSS

SNR

where:

VNADC is the noise of the ADC, which is

SNR

INp-p

NADC

f–3dB is the cutoff frequency of the input filter (3.9 MHz).

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