AMP04

REV. A

–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 ap-

plied 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.

1

2

3

4

8

7

6

5

AMP-04

V–

REF

V+

R

G

–

+

INPUT

–5V

–5V

+5V

50k

50k

100

Ω

+5V

–5V

OUTPUT

+5V

±5mV

ADJ

RANGE

* OP-90 FOR LOW POWER

OP-113 FOR LOW DRIFT

*

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 out-

put 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.

1

2

3

4

8

7

6

5

AMP-04

R

G

INPUT

50k

100

Ω

+5V

OUTPUT

+5V

OP-113

Figure 9. Offset Adjust for Single Supply Applications

Alternative Nulling Method

An alternative null correction technique is to inject an off-

set 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 previ-

ous method.

2

3

8

1

6

5

IN(–)

IN(+)

INPUT BUFFERS

R

GAIN

100k

REF

100k

11k

11k

V+

V–

Figure 10. Current Injection Offsetting Is Not

Recommended