AMP04

REV. B

–9–

10

90

100

0%

1mV

2s

Figure 7c. 10 Hz Low-Pass Filtered Output

Power Supply Considerations

In dual supply applications (for example

±15 V) if the input is

connected to a low resistance source less than 100

Ω, a large

current may flow in the input leads if the positive supply is

applied before the negative supply during power-up. A similar

condition may also result upon a loss of the negative supply. If

these conditions could be present in you system, it is recom-

mended that a series resistor up to 1 k

Ω be added to the input

leads to limit the input current.

This condition can not occur in a single supply environment

as losing the negative supply effectively removes any current

return path.

Offset Nulling in Dual Supply

pin (Pin 5). However, it is important that the pin be driven with

a low impedance source. Any measurable resistance will degrade

the amplifier’s common-mode rejection performance as well as

its gain accuracy. An op amp may be used to buffer the offset

null circuit as in Figure 8.

8

7

6

5

1

2

3

4

AMP04

REF

V+

V–

5V

–

+

INPUT

+5V

–5V

50k

100

50k

5mV

ADJ

RANGE

*

*OP90 FOR LOW POWER

OP113 FOR LOW DRIFT

–5V

OUTPUT

+5V

–5V

Figure 8. Offset Adjust for Dual Supply Applications

Offset Nulling in Single Supply

Nulling the offset in single supply systems is difficult because

the adjustment is made to try to attain zero volts. At zero volts

out, the output is in saturation (to the negative rail) and the

output voltage is indistinguishable from the normal offset error.

Consequently the offset nulling circuit in Figure 9 must be used

with caution.

First, the potentiometer should be adjusted to cause the output

to swing in the positive direction; then adjust it in the reverse

direction, causing the output to swing toward ground, until

the output just stops changing. At that point the output is at

the saturation limit.

8

7

6

5

1

2

3

4

AMP04

5V

INPUT

100

OUTPUT

OP113

50k

5V

Figure 9. Offset Adjust for Single Supply Applications

Alternative Nulling Method

An alternative null correction technique is to inject an offset

current into the summing node of the output amplifier as in

Figure 10. This method does not require an external op amp.

However, the drawback is that the amplifier will move off its

null as the input common-mode voltage changes. It is a less

desirable nulling circuit than the previous method.

IN(–)

IN(+)

INPUT BUFFERS

REF

100k

11k

11k

100k

V–

V+

Figure 10. Current Injection Offsetting Is Not

Recommended