REV. B

–16–

AD7722

Even though the voltage on the input sampling capacitors may not

have enough time to settle to the accuracy indicated by the resolu-

tion 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 15.

R

100

C

2.7nF

AD7722

C

2.7nF

C

2.7nF

R

100

Figure 15. 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

antialias filter is given by Equation 1, and the attenuation of the

filter is given by Equation 2.

f

RC

dB

3

1

6

=

(1)

Attenuation

log

f

=+

























20

1 / 1

3

2

(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

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 16. The inter-

nal reference voltage is connected to REF1 through a 3 k

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.

24

3k

AD7722

REFERENCE

BUFFER

22

1V

2.5V

REFERENCE

SWITCHED-CAP

DAC REF

REF1

REF2

COMPARATOR

100nF

Figure 16. 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 the 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 17).

24

4pF

CLKIN

REF2

AD7722

4pF

SWITCHED-CAP

DAC REF

100nF

Figure 17. REF2 Equivalent Input Circuit

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

AD7722. Figure 18 shows a suggested connection diagram.

AD780

1

2

3

4

8

7

6

5

NC

TEMP

GND

O/P

SELECT

NC

TRIM

22nF

1 F

24

REF2

AD7722

22 F

100nF

22

REF1

5V

Figure 18. External Reference Circuit Connection