My caller was looking for some suggestions to improve his design. He had a quad op amp in the circuit but only needed three of the functions. What should he do with unused inputs?
There are several options but what is best and, even more interesting, why?
We started off by listing the obvious possible combinations. I have drawn these circuits here.
We can assume these op amps are powered by dual bi-polar supplies and make the necessary adjustments later for single supply operation.
The circuit in (A) is the worst possible selection. The high impedance inputs are left open and are susceptible to stray voltages that could damage the IC. Once an Electrical Over Stress (EOS) voltage is conducted into the chip other stages can suffer damage.
Circuit (B) is almost as bad. The op amp is running open loop and therefore the input voltage offset will be magnified by the open loop gain of the amplifier, which will drive the output to one of the supply rails. This can be especially bad if the output stage is composed of bipolar transistors in a rail-to-rail output op amp. If the output voltage approaches the supply voltage a path develops where the quiescent current becomes quite large. It is possible for the quiescent current of an op amp to increase by a factor of five or even ten when the output approaches the supply voltage. This higher than normal quiescent current does not cause damage to the device, however, the battery draw can be significant to the power budget.
If space is at a premium then circuit (C) can be a good choice. Here the amplifier is in a stable gain of one and will not latch at one supply. There is one better choice if the space is available and that is shown in circuit (D).
It seems that many times during final testing a discovery is made where the system could be improved with the addition of one more op amp. If the PC board is prepared with circuit D the modification could be minimum. In the first build just do not populate the resistors labeled NP and short out those pads marked SC with PCB trace in the design. If this op amp is need then a knife cut could allow the SC component positions to be used along with the NP component positions.
If the op amp under consideration is operated from a single supply then these circuits need one modification. The input ground connection should be tied to a bias point that is near mid-supply voltage. This point could be the output from another op amp or a reference voltage associated with a data converter. In this configuration the output will be able to drive to a voltage that satisfies the loop requirements.
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