Is there any disadvantage to running an op-amp off asymmetrical voltage rails?

Question

On a PCB I'm working with, I have analog feedbacks running through op-amps into a 3.3V microcontroller. Thus, signals outside 0-3.3V are useless, and potentially dangerous to the microcontroller. The op amps have +-15V rails, because those rails were available and convenient. However, that means it's possible for my op amp to destroy my microcontroller by railing out positive. In perfect operation this should never happen, but I'm considering edge cases. To improve reliability, I'm considering connecting my op amps to +3.3V and -15V.

Is there any reason I should not run an op amp off asymmetrical voltage rails?

Obviously, railing out negative could also destroy the processor. I'm only considering the positive-rail case at the moment.

Answer 1

As long as the input pins or the output pin(s) of a typical rail-to-rail op amp are between the rail voltages, the "ground" reference is not actually locked to midway between the rails. (n.b. IIRC there are some op-amps which actually have a separate ground pin)

To illustrate, let us consider a rail-to-rail input/output (RRIO) op-amp in a voltage follower configuration:

We supply the op-amp rails with +/- 9.15 Volts, and inject an AC signal with a DC bias of 7.5 Volts and a peak-to-peak AC amplitude of 3.3 Volts.

The output would traverse between +9.15 Volts and +5.85 Volts, which is within the operating range of this hypothetical op-amp. The rest of the operating range, from -9.15 Volts through to +5.85 Volts would be unused so long as the signal stayed within parameters.

The mid value, 7.5 Volts, can thus be treated as the "ground" for this signal.

In other words, the op-amp doesn't care about the symmetry of the rails, it doesn't know where "ground" is.

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For a real op-amp rather than an ideal one, there may be some minimum headroom for the output swing, from a few millivolts to a couple of Volts. So, this needs to be accounted for in the strategy laid out in the question.

In addition, the MCU pins may not take kindly to a traversal below the MCU ground rail. Hence, a clipping diode to the ground rail, on the input side, would be a good idea.

Answer 2

Should be no problem. A dual-rail Op-Amp has no clue where GND is so running it from +/- 15V or 3.3V/-15V simply provides a smaller output range. In effect this is similar to the case of single-rail Op-Amps that must run from 0V to 3.3V and be biased around Vcc/2.

One caveat to mention is that using asymetrical voltages you may run into asymetrical clipping of your signal on the output. A rail to rail swing due to saturation (3.3V to -15V) will probably not clip the positive rail (as 3.3V is far away from 15V) but may clip on the -15V. If this is something you don't care about then go for it.

This is an uncommon use of an Op-Amp. I am curious if your MCU can only take 0 - 3.3V why not simply use a rail-to-rail single supply OpAmp such as OP340 (datasheet)

Answer 3

Overvoltage clamping diodes is a common way of dealing with the risk that an analog section with large supply rails can overvoltage the A/D with logic supply rails.

Here’s an example. Suppose that you have a small differential signal, which rides on a common mode, and the latter can vary in a wide range. You need wide power supply rails to accommodate the common mode. Then there’s a concern that the output of the analog stage can overvoltage the A/D.

D11 or D12 will clamp, while R7 will limit the current. In this example, the absolute max voltage range for the input is -0.3V to (VDD+ 0.3V).

This is just one type of clamp. It’s also possible to clamp with a Zener.