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I have a 0-5V signal that I want to quantize using an ADC with a 0-3.3V input range.

I want to lower the signal maximum amplitude with the following arrangement:

enter image description here

Is it better to power the second opamp with 5V or 3.3V?

Advantages I see for 3.3V:

  • The output can never exceed the maximum input of the ADC (act as a protection).

Advantages I see for 5V:

  • Can buy a dual opamp chip and use a single supply for both (both opamps can have better matching if this is useful for anything).
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  • \$\begingroup\$ What ADC specifically? It may be possible to protect it with one diode and one resistor. \$\endgroup\$
    – devnull
    Commented Apr 21, 2022 at 16:02
  • \$\begingroup\$ You won't get quite to either 5V or 0V with that arrangement. If you really need to accurately handle signals all the way 5V and 0V you'll need higher voltage rails (eg. +5.3V and -0.3V). \$\endgroup\$ Commented Apr 21, 2022 at 16:16
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    \$\begingroup\$ Yup. "rail to rail" really means "rail to rail almost, and really phenomenally close compared to 1970's technology". \$\endgroup\$
    – TimWescott
    Commented Apr 21, 2022 at 16:28
  • \$\begingroup\$ "Which is better" is an opinion question, which is discouraged here. What does each one do (leaving it to you to decide what's better for your application) is nice and factual, and matches the ground rules of the group. \$\endgroup\$
    – TimWescott
    Commented Apr 21, 2022 at 16:29
  • \$\begingroup\$ It's the built-in ADC of the nRF52840. \$\endgroup\$
    – Francois
    Commented Apr 21, 2022 at 16:55

1 Answer 1

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Shouldn't need opamps at all. Output impedance of c12880ma is low, 150 ohms, and ADC's is high, 1M. A simple resistor divider with impedances in the 10-100K range will reduce the level to 0-3.3V and provide enough sources resistance to limit current just in case 3.3V is exceeded.

enter image description here

(I see Vladimir suggested same thing as I was prepping this answer, so it must be right :)

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