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Inverting a signal up to 100mV using op amps

  1. Jun 22, 2016 #1

    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.

    Thanks for any advice.
  2. jcsd
  3. Jun 22, 2016 #2


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    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?
  4. Jun 22, 2016 #3


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    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).
  5. Jun 22, 2016 #4


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    Many photodetectors output a current, can you double check that the output variable really is voltage?
  6. Jun 22, 2016 #5

    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!
  7. Jun 22, 2016 #6


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    Actually that makes perfect sense. All op-amps have a minimum required supply voltage to operate. This is because there are several transistors, diodes, etc inside the op-amp that are connected in series. Each one of those devices has a minimum voltage below which it will not function. If there is not enough voltage across each one they will act as either a resistor or an open circuit, rather than a diode or transistor.
  8. Jun 23, 2016 #7


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    One thing to watch out for is that the output pin of an OP Amp can't always get close to the supply rails. So with low voltage supplies the output might not be able to swing as much as you want. For example the output of the LM324 can only go upto about Vcc-1.5V. Might be an issue if you were trying to run it from say Vcc =3V or use it to generate logic level outputs.
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