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I'm interesting in putting together an inverting op amp to take a signal voltage from 10 V to 0.01 V (Gain = -60 dB). By signal voltage I mean the current input and output will be low (e.g., <1 mA).

Searching suppliers, I haven't found an op amp where their gain plots reach -60 dB.

This makes me wonder, can I simply interpret the gain plots to mean +X dB for non-inverting configurations and -X dB for inverting configurations?

Gain plot examples,

Gain plot #1

Or

Gain plot #2

Here is the second plot's spec sheet for a little more detail: https://cdn.sparkfun.com/assets/8/4/9/e/1/DS-15946.pdf

Null
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biscuits-and-jamie
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    Perhaps I don't understand, but it seems you want to _attenuate_ a signal by 60 dB as well as invert it? Is that correct? Then simply using a unity gain inverting amp followed by a 1000:1 voltage divider. (Or put some of the voltage division before the amp) – Math Keeps Me Busy Jan 15 '22 at 00:32
  • @MathKeepsMeBusy. . opamps are often used as attenuators. Low output impedance. Same gain equation. Rf / Rin . – Marla Jan 15 '22 at 00:44
  • Yes, @MathKeepsMeBusy, looking to attenuate. I intended to use an opamp to avoid fluctuations due to connecting the signal to different sinks. Is that an advantage of using a unity gain amp with a voltage divider? – biscuits-and-jamie Jan 15 '22 at 03:19
  • @biscuits-and-jamie Changes in the output voltage of a circuit caused by changes in the load applied to that output depend upon the ratio between the output impedance of that circuit to the load impedance. If you have an idea of the load impedance and the output stability you require, you can estimate the maximum output impedance for your circuit that will give you the stability you want. The output impedance of an op-amp with a feedback resistor is the resistance of the feedback resistor in parallel with the internal resistance of the output stage. Zout = Rf || Ro. (cont.) – Math Keeps Me Busy Jan 15 '22 at 04:30
  • @biscuits-and-jamie the output impedance of a voltage divider composed of R1 and R2, where R1 is about 1000 times R2, is approximately R2. If you use a 1K Ri and 1 Ohm Rf your output impedance will be somewhere around 1 Ohm. If you use a voltage divider with a 1K R1 and a 1 Ohm R2, your output impedance will again be approximately 1 Ohm. No clear winner in that case. However, voltage sag can also arise on the input side. The input impedance of an inverting amp is Rin. This can be made quite large if the output impedance is free to be large, as it would be in unit gain followed by v - divider. – Math Keeps Me Busy Jan 15 '22 at 04:47
  • @biscuits-and-jamie However, with the attenuating amp config, the larger Rin, the larger Rf needs to be, so the larger the output impedance (up to a point -- if Rf >> Ro then Ro will dominate in Rf || Ro). So, in some cases the unity gain inverting amp followed by a voltage divider will be the winner because it can give a sufficiently large input impedance. Whether this is needed depends upon the impedance what is driving the input to your op-amp. Since the unity gain inverting amp plus voltage divider uses 2 extra resistors, we need to know more ... – Math Keeps Me Busy Jan 15 '22 at 04:54
  • @biscuits-and-jamie ...about the circuit around your circuit in order to determine whether those 2 extra resistors add value, or are superfluous. – Math Keeps Me Busy Jan 15 '22 at 04:54
  • Doubtful this will be answered, curious to know - why downvoted? – biscuits-and-jamie Jan 15 '22 at 19:01
  • @MathKeepsMeBusy, appreciate the information about impedance. I suspect the load impedance to be 50 ohm, and no idea about input impedance. (I'm working on this problem remotely atm.) I offered the solution of a voltage divider, but I was told that the signal seemed unstable (cannot remember why). And as a result, started to look down the path of opamps for attenuation. Long way to answer your question, I'll have to get more information about impedances. – biscuits-and-jamie Jan 18 '22 at 00:06
  • @MathKeepsMeBusy, I accepted at Barry's answer because it answered the question I actually submitted. Which wasn't exactly the question I wanted to ask. But your answer leads me to another result, impedance matching with a unity gain op amp. Thanks friend! – biscuits-and-jamie Jan 18 '22 at 00:12

1 Answers1

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You are confusing open loop gain and closed loop gain. The gain in the data sheet plots is open loop gain and is very high as this is how op amps are designed. It is independent of how you are going to use the op amp (that is, inverting or non-inverting).Feedback is used in the actual circuit using the op amp to set the desired gain. The configuration of the feedback resistors determines whether it is an inverting or non-inverting amplifier. When the open loop gain is much higher than the closed loop gain the closed loop gain is primarily set by the feedback resistors and is independent of open loop gain. In your case, you actually want attenuation (-60 dB) rather than gain so you don't even need much open loop gain. A simple inverting amplifier for which the resistor ratio is 1/1000 will provide what you want. You didn't mention the frequency range of your signal. That will also help determine which op amp you should use.

Barry
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  • The signal's frequency range is in the MHz range, but for my purpose I am only concerned with a few kHz. I understand the resistors set the opamp gain, and that I am interested in attenuation. Do you have any insight in @MathKeepsMeBusy's suggestion: unity gain amp with voltage divider? – biscuits-and-jamie Jan 17 '22 at 23:52
  • For future readers, here is useful post: https://electronics.stackexchange.com/questions/91230/op-amp-gain-less-than-1 – biscuits-and-jamie Jan 18 '22 at 00:14