AD7722AS datasheet(15/24 Pages) AD

15 / 24 page

AD7722

–15–

REV. 0

Even though the voltage on the input sampling capacitors may

not have enough time to settle to the accuracy indicated by the

resolution of the AD7722, as long as the sampling capacitor

charging follows the exponential curve of RC circuits, only the

gain accuracy suffers if the input capacitor is switched away

too early.

An alternative circuit configuration for driving the differential

inputs to the AD7722 is shown in Figure 30.

100

Ω

2.7nF

AD7722

VIN(+)

VIN(–)

2.7nF

100

Ω

Figure 30. Differential Input with Antialiasing

A capacitor between the two input pins sources or sinks charge

to allow most of the charge that is needed by one input to be

effectively supplied by the other input. This minimizes undesir-

able charge transfer from the analog inputs to and from ground.

The series resistor isolates the operational amplifier from the

current spikes created during the sampling process and provides

a pole for antialiasing. The –3 dB cutoff frequency

(f3 dB) of the antialias filter is given by Equation 1, and the

attenuation of the filter is given by Equation 2.

f 3dB =

π RC

(1)

Attenuation

= 20 log 1/ 1+

f 3dB





















(2)

The choice of the filter cutoff frequency will depend on the

amount of roll-off that is acceptable in the pass band of the

digital filter and the required attenuation at the first image

frequency. For example, when operating the AD7722 with a

12.5 MHz clock; with the typical values of R and C of 100

Ω and

2.7 nF shown in Figure 30, the –3 dB cutoff frequency (f3 dB)

creates less than 1 dB of in band (90.625 kHz) roll-off and

provides about 36 dB attenuation at the first image frequency.

The capacitors used for the input antialiasing circuit must have

low dielectric absorption to avoid distortion. Film capacitors

such as Polypropylene, Polystyrene or Polycarbonate are

suitable. If ceramic capacitors are used, they must have NP0

dielectric.

Applying the Reference

The reference circuitry used in the AD7722 includes an on-chip

2.5 V band gap reference and a reference buffer circuit. The

block diagram of the reference circuit is shown in Figure 31.

The internal reference voltage is connected to REF1 through a

Ω resistor and is internally buffered to drive the analog

modulator’s switched cap DAC (REF2). When using the

internal reference, connect 100 nF between REF1 and AGND.

If the internal reference is required to bias external circuits, use

an external precision op amp to buffer REF1.

Ω

AD7722

REFERENCE

BUFFER

2.5V

REFERENCE

SWITCHED-CAP

DAC REF

REF1

REF2

COMPARATOR

100nF

Figure 31. Reference Circuit Block Diagram

The AD7722 can operate with its internal reference or an

external reference can be applied in two ways. An external

reference can be connected to REF1, overdriving the internal

reference. However, there will be an error introduced due to

the offset of the internal buffer amplifier. For lowest system

gain errors when using an external reference, REF1 is grounded

(disabling the internal buffer) and the external reference is

connected to REF2.

In all cases, since the REF2 voltage connects to the analog

modulator, a 100 nF capacitor must connect directly from

REF2 to AGND. The external capacitor provides the charge

required for the dynamic load presented at the REF2 pin

(Figure 32).

4pF

CLKIN

REF2

AD7722

4pF

SWITCHED-CAP

DAC REF

100nF

Figure 32. REF2 Equivalent Input Circuit

The AD780 is ideal to use as an external reference with the

AD7722. Figure 33 shows a suggested connection diagram.

AD780

+VIN

TEMP

GND

O/P

SELECT

VOUT

TRIM

22nF

1µF

REF2

AD7722

22µF

100nF

REF1

+5V

Figure 33. External Reference Circuit Connection

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