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I'm trying to use a TL081, for which I used a DC power supply regulated by MPL1303, to feed the TL081 and a Vin from a DAQ assistant giving an analog signal of 0 to 5 V. I tested the signal from the DAQ and the supply from the MPL and it is working well, but I'm not being able to read anything from the output of the opamp.

My questions are:

1) Should I set the power supply on the TL081 with +12 V on Vcc+ and -12 V on Vcc-, or can I set 0 V on Vcc-?

2) Can I use any value for the two resistors, because I want gain of 2, so R1/R2 = 1?

3) When measuring the output with a voltimeter, can I use any ground, or should it be the virtual ground?

Thanks,

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![Thanks for the explanations, new circuit that I'll test]4

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  • \$\begingroup\$ As far as I know, the TL081 needs a symmetric power supply. \$\endgroup\$
    – Bart
    Dec 5, 2018 at 11:31
  • \$\begingroup\$ @Bart That is not true, the TL081 can work on a single 5 V supply, however the usable range of the input voltages is quite limited (maybe less than 2 V) when using a 5 V supply. The TL081 is designed to work with a supply of 30 V (+/- 15 V). For a 5 V supply, use an MCP601 for example. \$\endgroup\$ Dec 5, 2018 at 12:53
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    \$\begingroup\$ @Bart - no normal opamp needs a symmetric power supply because no normal opamp has a ground/0V connection. Even the old 741 would be perfectly happy with something like +7/-16 because all it 'sees' is the 23V difference. The only consideration is that your inputs and outputs stay within the limits. \$\endgroup\$
    – brhans
    Dec 5, 2018 at 13:14

2 Answers 2

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The datasheet will tell you what the common mode input voltage can be. From memory, you can't go closer than 4V above the negative supply and I think you need at least 10 V between the two supplies. You can use a wide range of resistors. Very low values may overload the op-amp output and very high values may lead to excessive phase shift at high frequencies, offsets due to input bias current, and high noise.

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For your question 1: You say you are giving it a signal from 0-5V, so while theoretically you can give it 0V on VCC-, be aware that it is not rail to rail, this op-amp needs a bit of headroom so your output won't be what you are expecting. With this in mind, it would be a good idea to give it ±12V if you can.

For your question 2: Again, theoretically you could choose any resistances you want, but a good bet is just stick to a pair of 10k resistors. Using too high or too low values can cause different issues (too low could overload the op-amp, as in it cannot drive that sort of load, and too high can cause noise or other undesired effects at different frequencies) so stick with the trusty 10k, as these are also widely available.

For your question 3: Always use proper ground, not virtual ground. Also, if possible, use an oscilloscope. Depending on the frequency of your input, a standard multimeter may not give you the exact reading you are after as it will average out the signal. It might be ok for a very low frequency signal, but really you should use an oscilloscope.

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