
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
, 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
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
Offset may be nulled by feeding a correcting voltage at the V
REF
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
+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
+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
V
OUT
11k
11k
V+
V–
Figure 10. Current Injection Offsetting Is Not
Recommended