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AMP04GBC Datasheet(PDF) 9 Page - Analog Devices |
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AMP04GBC Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page 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 Offset may be nulled by feeding a correcting voltage at the VREF 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 VOUT 11k 11k V+ V– Figure 10. Current Injection Offsetting Is Not Recommended |
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