Connecting an op-amp input without power

[Oznog]

I distinctly recall hearing that an op-amp's input should not have a voltage on it if the op-amp is not powered- many, many years ago. IIRC, they said it would damage it.

I have a situation where I may need to power down an op-amp, and the output must go to 0v. However, one input I cannot switch off.

The circuit is a simple differential shunt regulator. When Vin>Vref, it turns on an output transistor. That is all it does, really.

Why would the input be a problem? Is there a protection diode that shunts the pin to the Vdd rail, thus powering the amp from the input itself?

 

[crutschow]

Often there are input protection diodes to the plus and minus power pins. You should be able to protect the input by adding a series resistor at each input. The Absolute Maximum ratings in the op amp data sheet should tell you what the maximum allowed input current is. Use a resistor large enough to keep the current below that value.

 

[MikeMl]

Put a resistor in series with the input. There is usually a parasitic diode inside the opamp (anode to input, cathode to V+). If you connect a stiff voltage source to the input, the v+ rail gets powered backwards through that parasitic diode. This will power other things tied to the V+ line, too. It can fuse the input, or latch-up the opamp, too. Putting a ~10K to 100k resistor between the source and the input pin will usually prevent any problems.

 

[Oznog]

Battery power, and sensitivity to offset error due to high input resistance, and more important the output may not be predictable with the op-amp having a small voltage on Vdd coming through the input through a resistor.
Ain't gonna happen. Not a biggie, I'll reevaluate the problem.
If something won't work, do something else!

 

[Nigel Goodwin]

Try posting the circuit, so we can see what's happening.

 

[indulis]

In general, it is “BAD” to allow either input to exceed the potential of either rail, as what were (ideally) infinite impedance inputs will now start to draw current… as much as they can.