AD644

REV. A

–6–

Figure 27a illustrates the 10-bit digital-to-analog converter,

AD7533, connected for bipolar operation. Since the digital in-

analog input, the circuit can perform a 4-quadrant multiplying

function. The photos exhibit the response to a step input at

the small signal response.

The output impedance of a CMOS DAC varies with the digital

word thus changing the noise gain of the amplifier circuit. The

effect will cause a nonlinearity the magnitude of which is depen-

dent on the offset voltage of the amplifier. The AD644K with

trimmed offset will minimize the effect. The Schottky protection

diodes recommended for use with many older CMOS DACs are

not required when using the AD644.

ACTIVE FILTERS

Literature on active filter techniques and characteristics based

on operational amplifiers is readily available. The successful ap-

plication of an active filter however, depends on the component

selection to achieve the desired performance. The AD644 is rec-

ommended for filters in medical, instrumentation, data acquisi-

tion and audio applications, because of its high gain bandwidth

figure, symmetrical slewing, low noise, and low 1 offset voltage.

The state variable filter (Figure 28) is stable, easily tuned and is

independent of circuit Q and gain. The use of the AD644 with

its low input bias current simplifies the resistor (R3, R4) selec-

tion for the passband center frequency, circuit Q and voltage

gain.

Figure 28. Band Pass State Variable Filter

The sample and hold circuit, shown in Figure 29 is suitable for

use with 8-bit A/D converters. The acquisition time using a

3900 pF capacitor and fast CMOS SPST (ADG200) switch is

15

µs.

The droop rate is very low 25

bias currents of the AD644. Care should be taken to minimize

leakage paths. Leakages around the hold capacitor will increase

the droop rate and degrade performance.

Figure 29. Sample and Hold Circuit

The AD644 in the circuit of Figure 30 provides highly accurate

signal conditioning with high frequency input signals. It pro-

vides an offset voltage drift of 10

µV/°C, CMRR of 80 dB over

the range of dc to 10 kHz and a bandwidth of 200 kHz (–3 dB)

at 1 V p-p output. The circuit of Figure 30 can be configured

for a gain range of 2 to 1000 with a typical nonlinearity of

0.01% at a gain of 10.

Figure 30. Wide Bandwidth Instrumentation Amplifier

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).