# 0-5V to -24-24V using an opamp?

Discussion in 'General Electronics Discussion' started by JawadWalker, Jul 29, 2017.

13
0
Feb 6, 2016
Hi, I'm trying to scale a 0-5 volt signal to a -24-24 volt signal (0V to -24V, 2V to -4.8V, 4V to 14.4V, 5V to 24V, etc.). I made this circuit with the equations from http://www.stefanv.com/calculators/hp67_offset_gain.html and it works, but will it work in real life?

It says on that page that "Theoretically, the formulae presented here work perfectly well for gains between -1 and 1 (i.e. |m| < 1). However, many real-world op-amps are unstable in such cases. Instead, it will usually be necessary to design for a higher gain (|m| ≥ 1), with an attenuator on the input side.".

How would I make an attenuator if I need one? Thank you.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
Aside from the fact that this is a very high voltage power supply voltage for an op-amp, your calculations are essentially correct.

I guess you could make a high voltage op-amp from discrete components, or add a high voltage output stage to a normal op-amp.

The next issue is that you have chosen some quite bizarre value resistors . Consider that for normal resistors you can't get exact values like the ones you have chosen. Would 120 ohms, 2700 ohms, and maybe 270 ohms be close enough?

13
0
Feb 6, 2016
OPA445 is a high enough voltage op amp right? And yeah I've fixed the resistor values to use ones in parallel. So the output will be pretty linear to the input right?

Last edited: Jul 29, 2017
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
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Jan 21, 2010
R3 and R4 are unnecessary. Just calculate R1 and R2 for a gain of 24/5.

13
0
Feb 6, 2016
How will I offset it then?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
You're right. The second time I looked at this I assumed the voltage source on the left was your input, sorry.