# Inverting a signal up to 100mV using op amps

## Main Question or Discussion Point

Greetings,

I'd like to invert a signal I get from a photodetector (the signals are up to 100 mV). I decided to use an inverting op amp circuit for that, however, when looking online through the op amp catalogs - the minimum input voltage in the descriptions is a few volts while my input would be in the range of mV. Would that be a problem? Is that only a recommended range given there? I do realize I will get some noise in that case, however, I'll make sure to pick op amps with the optimal slew rate, offset voltage and bandwidth for my needs.

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berkeman
Mentor
Greetings,

I'd like to invert a signal I get from a photodetector (the signals are up to 100 mV). I decided to use an inverting op amp circuit for that, however, when looking online through the op amp catalogs - the minimum input voltage in the descriptions is a few volts while my input would be in the range of mV. Would that be a problem? Is that only a recommended range given there? I do realize I will get some noise in that case, however, I'll make sure to pick op amps with the optimal slew rate, offset voltage and bandwidth for my needs.

Welcome to the PF.

You certainly can use opamps for your application. Where have you read that they have some minimum input signal amplitude requirement?

donpacino
Gold Member
Greetings,

I'd like to invert a signal I get from a photodetector (the signals are up to 100 mV). I decided to use an inverting op amp circuit for that, however, when looking online through the op amp catalogs - the minimum input voltage in the descriptions is a few volts while my input would be in the range of mV. Would that be a problem? Is that only a recommended range given there? I do realize I will get some noise in that case, however, I'll make sure to pick op amps with the optimal slew rate, offset voltage and bandwidth for my needs.

Op amps don't really have a minimum voltage. In many typologies with negative feedback the voltage difference between the positive and negative terminals can be in the uV range.

Remember an op amp is just one component of the circuit that will invert your signal. It is really resistor selection that will determine minimum viable voltage (in conjunction with other things).

analogdesign
Many photodetectors output a current, can you double check that the output variable really is voltage?

Hello,

Thank you for your answers. I got confused due to going through RS Components I noticed that most descriptions provide a supply range (this for example and this from amazon). It did not make sense having "minimum supply" (of course, besides being limited by the offset voltage) so I decided to double check here (running low on confidence there :D)

Moreover, as far as I can recall, the variable is voltage in this case. I guess I should provide some background information - I have a set up of two lasers going through different photodetectors and after I plug in the wires going from them to a box (eeh, excuse my syllabus, still getting to know the lab so not really all that caught up on technical stuff yet aka total newbie) it gives me an option of the voltage difference between voltages generated in each photodetector. However, I'd also would like to know the sum of voltages (there is no option for that), thus I thought I could invert one of the signals with the op amp to achieve that which would be quick and easy.

Thanks again for all the replies. I'll try to buy the op amp after confirming the parameters I would need. Hopefully the set up will succeed!

Tom.G