AD630ADZ datasheet(15/20 Pages) AD

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Data Sheet

AD630

Rev. G | Page 15 of 20

OFFSET VOLTAGE NULLING

The offset voltages of both input stages and the comparator

have been pretrimmed so that external trimming is only required

in the most demanding applications. The offset adjustment of

the two input channels is accomplished by means of a differential

and common-mode scheme. This facilitates fine adjustment of

system errors in switched gain applications. With the system

input tied to 0 V, and a switching or carrier waveform applied

to the comparator, a low level square wave appears at the output.

The differential offset adjustment potentiometers can be used

to null the amplitude of this square wave (Pin 3 and Pin 4).

The common-mode offset adjustment can be used to zero the

residual dc output voltage (Pin 5 and Pin 6). Implement these

functions using 10 kΩ trim potentiometers with wipers

connected directly to Pin 8 as shown in Figure 28 and

Figure 29.

CHANNEL STATUS OUTPUT

The channel status output, Pin 7, is an open collector output

referenced to −VS that can be used to indicate which of the two

input channels is active. The output is active (pulled low) when

Channel A is selected. This output can also be used to supply

positive feedback around the comparator. This produces

hysteresis which serves to increase noise immunity. Figure 26

shows an example of how hysteresis may be implemented. Note

that the feedback signal is applied to the inverting (−) terminal

of the comparator to achieve positive feedback. This is because

the open collector channel status output inverts the output

sense of the internal comparator.

Figure 26. Comparator Hysteresis

The channel status output may be interfaced with TTL inputs

as shown in Figure 27. This circuit provides appropriate level

shifting from the open-collector AD630 channel status output

to TTL inputs.

Figure 27. Channel Status—TTL Interface

1MΩ

100kΩ

–15V

+5V

100Ω

–15V

+5V

TTL INPUT

AD630

+15V

IN914s

6.8kΩ

22kΩ

100kΩ

2N2222

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