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I would like to know what happens when a dual power supply op amp misses one of the power rails and what kinds of damage I can expect for op amp.

What prompted this question:

I have a analog PID module from SRS (SIM960). It usually works in a mainframe that supplies the power but because of space constraints I had to supply powers myself (+5V for digital and +/-15V for analog). It worked for a while, but yesterday I discovered that -15V connection had been un-plugged by accident, and the PID module was not behaving correctly. I promptly fixed the connection but now the module seems damaged. At least the input pre-amp section is damaged. The (P gain * error) value does not read correctly, and it seems to be at an almost fixed value, not responding to change in polarity, P gain, or error itself.

The online manual for SIM960 does not show the schematic diagram, and I don't think I can post a copy of the diagram from the paper manual, so the input preamp section looks like instrumentation amplifier followed by some op amps for proportional gain. All the op amps are OPA228/OPA2228.

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  • \$\begingroup\$ It's dead, get a new one \$\endgroup\$ – Voltage Spike Nov 13 '18 at 19:59
  • \$\begingroup\$ Well, I was wondering if I could fix it by replacing several op amps, but I think I'll just ship the unit back for RMA. \$\endgroup\$ – wcc Nov 13 '18 at 20:19
  • \$\begingroup\$ You probably could, not worth it if you dont have a schematic \$\endgroup\$ – Voltage Spike Nov 13 '18 at 20:20
  • \$\begingroup\$ @laptop2d, I do have (it's only available with the manual that comes with purchase), but then I am not sure if simply replacing the op amps in the pre-amp stage will do the job. \$\endgroup\$ – wcc Nov 13 '18 at 20:37
  • \$\begingroup\$ if worst comes to worst, you end up replacing everything \$\endgroup\$ – Voltage Spike Nov 13 '18 at 21:00
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It depends a lot on the topology and the specific devices involved, so without a schematic to review this will be a fairly general answer, but generally the damage conditions are similar to those that occur when a device's IO voltage ratings are exceeded. Essentially, the op-amps power rails will be floating, but likely pulled towards ground (or the opposite supply rail) by all of the devices connected across those same power rails. This includes other ICs, as well as voltage dividers, indicator LEDs, bulk/bypass caps, etc.

So as a a starting condition we can assume that the floating rail--which we'll assume is the negative one since that's the case in your situation--is at the same voltage as the positive rail. Thus, any voltage applied to an input or output pin that is less than the positive rail is effectively outside of the op-amps supply rails. On many devices, this will cause current to flow through parasitic junctions or ESD protection diodes from the input pin to the negative supply rail. If the source impedance is low enough, enough current can flow that the floating negative rail can start to be pulled further negative--essentially the negative rail is being supplied through that signal pin. Because these parasitic junctions were never intended to supply current to the entire IC, let alone all of the surrounding circuitry, this can cause them to fail. Likewise, the ESD diodes, if present, were meant to conduct brief surges where, even though the instantaneous current may be substantial, the total energy is limited. Sustained DC current can cause these to fail as well.

It's quite common for semiconductors to fail short, in which case, depending on the exact nature of the internal failure, a device may wind up with an input or output pin shorted to a supply rail, or may even wind up with both supply rails internally shorted together, which will obviously disrupt the entire surrounding circuit. More subtle failures may also occur, that simply compromise the internal function of the device. Total failures are common, but partial failures, including changes to device parameters, are certainly possible.

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  • \$\begingroup\$ Follow-up question: for op amps without internal ESD protection diode, how do parasitic junctions between inputs and power rails physically form? \$\endgroup\$ – wcc Nov 15 '18 at 19:55
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It will completely depend on the connections around the op-amp and if those connections allowed the op-amp to exceed its maximum ratings.

In nearly all IC processes there are both parasitic and intentional reverse-biased diodes from all pins to the power supply rails. If your op-amp experienced a negative voltage without a negative supply being present those diodes became forward biased and attempted to power the rest of the circuit. If enough current circulated, the op-amp was destroyed.

Additionally, transistors have maximum gate-source or reversed base-emitter voltages that are kept in check as long as the bias network is functional. If this fails, individual transistor junctions can be destroyed.

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