What Shall We Do With A Unused Op-amp?

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(Sung to the tune of "What shall we do with the drunken sailor?"1)

Operational amplifiers (op-amps) may be bought with one, two, three (for video), four, or even more devices on a chip and packaged in a single IC package. Sometimes the constraints of optimal design leave us with an unused op-amp. The question then is - what do we do with it? Sometimes we can reconfigure to use one or more extra (but smaller) chips with fewer op-amps in total but this may complicate layout, or inventory. So we've got an op-amp we don't need. In an earlier article I have observed that sometimes it gets used as a comparator - and that this is not necessarily a good idea2. But this is not the topic for today. The best thing to do with a surplus op-amp it is to use it. There are lots of places in an analog circuit where a buffer amplifier may improve performance - and a unity gain buffer uses no extra components. Simply apply the signal to the non-inverting input and join the inverting input and the output. Such a buffer reduces loading on the signal source and, having a very low output impedance, allows the design of the next stage to be simplified. (It may be necessary to use a resistor, rather than a short-circuit, between the output and the inverting input of the op-amp if the device is a current-feedback op-amp, or has a minimum stable Acl >1, but there are very few such multiple op-amps. If needed the resistor should be optimised for bandwidth if the device is actually being used as a buffer amplifier, otherwise 100 kΩ will almost always be suitable.)

Figure 1

Leaving all of an unused op-amp's terminals unconnected is a really really bad idea. An open-circuit output terminal does not present any particular problems, but if an op-amp input is allowed to float there

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is no path for the bias current and the inputs may behave as if they are being over-driven. This will cause the output stage to saturate, which increases its current consumption - sometimes dramatically. And it's wasteful if an amplifier we're not even using draws much more power than one that's doing a useful job. Joining the inputs to each other but leaving them otherwise unconnected has almost exactly the same, bad, effects.

Figure 2

Furthermore an op-amp with open inputs may experience electrostatic fields. If such a field should bias an input outside the device rails it may trigger destructive latch-up and destroy the whole chip, not just the unused device.

Figure 3

Or the electrostatic field may be an AC field and induce AC voltages in the input(s) which are amplified and cause large AC current variation and crosstalk to other op-amps on the chip.

Figure 4

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Some users connect one input to the positive and the other to the negative supply. This is not a good idea, either. It is obvious that such a configuration will saturate the amplifier and, as we have seen, this may waste current, but it may also be ill-advised for other reasons. Some op-amps have quite a small absolute maximum differential input voltage (i.e. between the inputs), even though they may have a much larger common-mode range. Exceeding absolute maximum ratings is forbidden as it is likely to destroy the device. But even op-amps with a rail-rail differential input rating may have problems. Consider the venerable OP-07 as an example.

Figure 5

This op-amp was the first high precision monolithic op-amp and was introduced by Precision Monolithics Inc (PMI - now a division of Analog Devices) over a quarter of a century ago. Despite its age it is still widely used - and improved, more modern, devices still use similar input stages. This input stage contains protective circuitry, resistors R3 and R4 and diodes Q21, Q22, Q23 and Q24, intended to protect the base-emitter junctions of its input transistors Q1 and Q2. If a large differential voltage is applied to its input the diodes Q21 and Q22, or Q23 and Q24, conduct and clamp the potential between the bases of Q1 and Q2 to a safe level. Instead of a few nA of input current there are now many mA. Not only does this waste power, but it heats the chip and, with higher power supplies, may even overheat it and cause thermal damage.

Figure 6

Connecting unused inputs to each other and to a potential within the op-amp's common-mode range, will again cause the output stage to saturate, since the offset voltage of an op-amp is never exactly zero and will be amplified by the open-loop gain.

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Figure 7

What we should do with an unused op-amp is connect the device as a follower (output to inverting input as in Fig 1) and connect the non-inverting input to a potential somewhere within the input common-mode range. With a dual supply system the ground is ideal, but connecting to the positive or negative of a single supply system, even with an op-amp with rail-rail inputs, will cause saturation and the resulting power waste if the offset voltage has the wrong polarity. (If amplifying the offset causes the output to try and go outside the rails it will saturate, and possible sink too much power, instead.) The "potential somewhere between the supply rails" may be any point in the circuit with a suitable potential, since the loading caused by the op-amp input is minimal.

Figure 8

The simplest way to provide such an input is to connect to the midpoint of two high value (~½ MΩ) resistors in series between the supply rails. James Bryant Midsomer Norton March 2009

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What shall we do with the unused op-amp? (X3) Early in the morning. CHORUS Wey-hey and up she rises, (X3) Early in the morning. Don't let it lie with its inputs floating, (X3)

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Early in the morning. CHORUS Don't tie the inputs to the power rails, (X3) Early in the morning. CHORUS Don't short the input pins together, (X3) Early in the morning. CHORUS Use it as a buffer with a DC input, (X3) Early in the morning. CHORUS

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