Input voltage range inverting OpAmp

Question

Considering a OpAmp in inverting configuration with a gain <1 what maximum input rating do I need to respect?

• single 3.3V supply, rail-to-rail OpAmp
• gain of 0.66 via negative feedback resistor ratio of 330k/500k Ohms
• bias voltage on non-inverting input of approx 1.9V via resistor divider
• input signal swing of GND to 5V

The OpAmp is rated for 0.3V above/below rails which should be fine when I can assume that the device will keep the inverting input equal to the bias voltage of 1.9V when in operation. Is this reasoning correct, or does the 5V input violate the maximum rating.

I am designing a anti-aliasing filter with cut-off around 150Hz. I am not currently referring to a specific device but think that the MCP6001/2/4 could be suitable.

Answer 1

Since the inverting pin will always be at your 1.9V bias, the 5V at the inverter input (i.e., the side of the input resistor not at \$V_{in-}\$ does not violate the input maximum rating.)

You might want to pay attention to absolute maximum values, though. If for some reason feedback were to be broken (if that's even possible in your scenario) you'd like your amplifier to resume working correctly if feedback were restored, and if you exceed the absolute maximums that's not guaranteed.