Datasheet AMP04 (Analog Devices) - 9

AMP04 Offset Nulling in Single Supply 1mV 2s
Nulling the offset in single supply systems is difficult because
100
the adjustment is made to try to attain zero volts. At zero volts
90
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.
10
First, the potentiometer should be adjusted to cause the output
0%
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. Figure 7c. 10 Hz Low-Pass Filtered Output
RG 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
AMP04 1 8
current may flow in the input leads if the positive supply is
2 7 5V
applied before the negative supply during power-up. A similar
INPUT 3 6 OUTPUT
condition may also result upon a loss of the negative supply. If
4 5
these conditions could be present in you system, it is recom-
OP113
mended that a series resistor up to 1 kΩ 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
5V 100 50k
return path. Figure 9. Offset Adjust for Single Supply Applications
Offset Nulling in Dual Supply
Offset may be nulled by feeding a correcting voltage at the V
Alternative Nulling Method
REF pin (Pin 5). However, it is important that the pin be driven with An alternative null correction technique is to inject an offset a low impedance source. Any measurable resistance will degrade current into the summing node of the output amplifier as in the amplifier’s common-mode rejection performance as well as Figure 10. This method does not require an external op amp. its gain accuracy. An op amp may be used to buffer the offset However, the drawback is that the amplifier will move off its null circuit as in Figure 8. null as the input common-mode voltage changes. It is a less desirable nulling circuit than the previous method.
RG V+ V– 100k AMP04 1 8 RGAIN – IN(–) 2 V+ 7 5V INPUT INPUT BUFFERS VOUT + 3 6 OUTPUT IN(+) 4 V– REF 5 +5V +5V 11k –5V 50k * 100 11k 5mV *OP90 FOR LOW POWER ADJ OP113 FOR LOW DRIFT –5V 50k 100k RANGE –5V REF
Figure 8. Offset Adjust for Dual Supply Applications Figure 10. Current Injection Offsetting Is Not Recommended REV. C –9–