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While testing a MCP6H01 I came across some weird behavior when the CM voltage exceeds the operating range specified in the datasheet (VDD-2.3V Max).

This is with the op-amp configured as a unity gain follower and rails of ±3V, Both channels are 1V/div. Channel 1 (orange) is the output, and Channel 2 (purple) is the input, as you can see when the input (and common mode signal) exceeds the specified maximum, it "latches" to VDD until the CM signal drops below the spec.

O-Scope Plot

What causes this to happen? Why does it show this peculiar behavior? Looking at analogous CMOS op-amps like the ADA4665-2, they have CM ranges that go to or slightly exceed the rails as you would expect.

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  • \$\begingroup\$ electronics.stackexchange.com/questions/432775/… To answer why we would have to study the internal circuit. But we don't have it, this the only answer you can get is to does not exceed the recommended CM voltage for a given op-amp. \$\endgroup\$ – G36 Apr 30 at 5:08
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When you go outside the common mode range, beyond a bit of margin that is designed in for unit-to-unit and temperature variations, it's bound to do something undesirable. The op-amp you compared it to is a rail-to-rail input type, which has its own weirdness, though perhaps slightly more subtle.

Unless the op-amp creates internal supply rails that are higher than the external, it's not practical to make a rail-to-rail front end that works with a single differential pair, so they end up doing something like making two front ends, one for high and one for low input common mode range.

enter image description here

As you can see the yellow sample had an offset voltage that changed from almost +3mV to about 0 over about 500mV of common mode range. Think about what that means to CMRR. 55dB is guaranteed but that's the average over the entire 0-5V range. That compares to 82dB guaranteed for the Microchip part. If your CM noise happens to be in the 3.5-4V range you might get 20dB worse again than the average, more like 45 or even 35dB.

As you can see, the MCP6H01 offset voltage is very well behaved if you stay within the proper CM range, typically not changing more than tens of microvolts:

enter image description here

So, use a rail-to-rail input op-amp if you must, but there are real disadvantages and it's often not necessary.

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Some opamps implement rail-rail input performance using both Nchannnel and Pchannel input differential pairs. As result, there are glitches/ooppsies as the internal circuitry changes roles. This opamp MCH6H01 does not exhibit such anomalies, but also does not provide rail-to-rail input behavior.

This MCP6H01 is likely implemented with a single Pchannel input diffpair, perhaps using a high-voltage process option. The high noise floor (30 nanoVolts/rtHz) likely is result of the 0.13 mA Iddq.

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It is similar to the Opamp Phase Inversion problem in the tL081 and other older jfet-inut opamps. When an input voltage exceeds the negative CM range then the output suddenly goes as high as it can.

Phase Inversion

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