Signal Input Protection through clamping diodes

[osterchrisi]

Hi!

I have a question concerning input protection: Say, I have a circuit that is running on a 9V battery and so are the opamps used in this circuit of course. The input signal is is ac coupled and shifted to float around 4.5V.

If I want to protect the chips from being damaged I was thinking about putting clamping diodes before the first signal processing opamp, so as to limit any incoming signal to max. 9Vpp. But what to do with the two diode drops on each side of the power rails? Because obviously the signal is then going between -0.6V and +9.6V which I think is out of most common mode voltage ranges of opamps, no?

Can you give a hint at how to protect the input of a circuit safe?
I attached a simulation for better understanding...
Thanks a lot for any advice!!!

 

[(*steve*)]

You need to check the specs for whatever op amp you intend to use.

A simple alternative is to use the diodes to clip the input to a diode drop either side of 0V if you are expecting a very small signal, or clip to the supply rails than use a potential divider to reduce the voltage swing to that permitted by the op-amp.

 

[osterchrisi]

Mhm, another question that pops up: When I made an inverting amplifier with say 1M gain so that the input signal clips for sure, am I not exceeding the input voltage range as well through the feedback path? Why isn't that bad for an opamp? Isn't that basically the same thing?

 

[(*steve*)]

You will find that the voltage at the inputs of the op-amp will be almost identical. It is unlikely that you will exceed the input voltage limits in this case.

Imagine that the supply rail is +/- 10V, the input signal 1mV, and the gain 1,000,000.

The output should go to -1000V, but it is obviously clipped at -10V. Assuming the ratio of the feedback resistors is 1,000,000:1, you have -10V at one end, 10mV at the other, and the input (at the junction of the resistors) is around 9.99 mV. If the amplifier was not clipping, it would be 0V.