REV. A

AD7724

–11–

An alternative circuit configuration for driving the differential

inputs to the AD7724 is shown in Figure 24.

R

100

C

2.7nF

VIN(+)

VIN(–)

C

2.7nF

C

2.7nF

R

100

Figure 24. Differential Input with Antialiasing

A capacitor between the two input pins sources or sinks charge

to allow most of the charge needed by one input to be effectively

supplied by the other input. This minimizes undesirable 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 of the antialias filter is

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

Equation 2.

π R

(1)









(2)

The choice of the filter cutoff frequency will depend on the

amount of roll-off that is acceptable in the passband of the digi-

tal filter and the required 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 or polycarbonate are suitable. If ceramic

capacitors are used, they must have NPO dielectric.

Applying the Reference

The reference circuitry used in the AD7724 includes an on-chip

2.5 V bandgap reference and a reference buffer circuit. The

block diagram of the reference circuit is shown in Figure 25.

The internal reference voltage is connected to REF1 via a 3 k

Ω

resistor and is internally buffered to drive the analog modulator’s

switched capacitor DAC (REF2). When using the internal refer-

ence, connect 110 nF between REF1 and AGND. If the internal

reference is required to bias external circuits, use an external

precision op amp to buffer REF1.

4k

REFERENCE

BUFFER

1V

2.5V

REFERENCE

SWITCHED-CAP

DAC REF

REF1

REF2

COMPARATOR

110nF

Figure 25. Reference Circuit Block Diagram

The AD7724 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, an error will be 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 con-

nected to REF2.

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

lator, a 110 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 26).

A

B

B

4pF

A

B

A

B

MCLK

REF2

A

4pF

SWITCHED-CAP

DAC REF

110nF

Figure 26. REF2 Equivalent Circuit

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

AD7724. Figure 27 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

REF2

22 F

110nF

REF1

5V

NC = NO CONNECT

Figure 27. External Reference Circuit Connection