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Ive been hurting my brain a lot on this one and reading up on opamps, clamps, and level shifters, but still can't figure it out.

I need to take a 0v to 5v signal (TTL) and convert it to 0v to (negative) 300mV.

So far, I only could find this circuit which shifts this in reverse and meant for AC: https://www.maximintegrated.com/en/design/technical-documents/app-notes/4/4836.html

Id be super grateful for any help! :)

Edit: The reason I was adding the 2nd opamp was I wanted to use it as a simple NOT gate, since I have it to spare. Any tips how to do that with the split supply?

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    \$\begingroup\$ Use an opamp with a gain of -0.0600. You will need a negative rail if you want a negative voltage. \$\endgroup\$
    – user1850479
    Commented Apr 5, 2022 at 3:18
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    \$\begingroup\$ That’s an unusual requirement. Can you explain a bit more what it is for? \$\endgroup\$
    – Kevin White
    Commented Apr 5, 2022 at 3:28
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    \$\begingroup\$ Refer to the old Texas Instruments "TTL Data Book" for the details on TTL levels. \$\endgroup\$
    – John R. Strohm
    Commented Apr 5, 2022 at 3:56
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    \$\begingroup\$ Your question is missing important information. 1. What power supply voltages are available for this? 2. What is the longest possible time the signal will be in the -300 mV state? 3. Which input level (0 V or +5 V) is being translated to -300 mV? \$\endgroup\$
    – AnalogKid
    Commented Apr 5, 2022 at 4:23
  • \$\begingroup\$ you can invert to -5 then scale down with R's . How much current? Use a suitable V+/V- supply. \$\endgroup\$
    – D.A.S.
    Commented Apr 5, 2022 at 4:24

2 Answers 2

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You don't need two op-amps. Your circuit has a double inversion so it will give +300mV out for 5V in, not -300mV.

schematic

simulate this circuit – Schematic created using CircuitLab

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Thanks @user1850479, I needed that nudge to know I was going in the right direction. It helped me have the confidence to keep going (which I often lack and self-doubt) and make this :D

edit: removed 2nd opamp (thanks @Spehro Pefhany)

Simulated the the opamp in Circuit Lab and it worked perfectly. Ran out of my free trial before simulating the opamp beginning inverter, lol, but I think it should work fine. Really happy with this! enter image description here

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