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I've connected a DAC to a Raspberry Pi. The DAC gives a voltage between 0V and 3.3V.

I would like to convert this to an eurorack voltage between -5V and +5V.

Using the op amp calculator at masteringelectronicsdesign.com and circuitjs I found that this should work:

schematic

As an editable schematic: (circuitjs)

In reality though, this doesn't work. I put it on a breadboard and measured with an oscilloscope and the output is between -2.5V and +2.5V. Note that I power the op amp with -5V and +5V. The op amp is an MCP6001.

What is wrong with this schematic? Am I right that I can't rely on circuitjs and masteringelectronicsdesign.com for this, or is my breadboard setup more likely to be incorrect?

*** UPDATE ***

Conclusion:

  1. I misunderstood op amps: they have a maximum difference between their Vss and Vcc (NOT neccessarily between 0V and Vcc) which can be fairly low like 6V

  2. I misunderstood my function generator: 1.65Vpp means from -0.825 to +0.825 (NOT -1.65...+1.65V)

  3. After replacing the op amp by a TL072, I indeed get a wave between -5V and +5V (almost, slightly less - suggestions for a better DIP op amp are welcome!)

  4. I should use the -12/+12V from the eurorack power supply to feed the op amp

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  • \$\begingroup\$ How are you powering your opamp? \$\endgroup\$
    – Wesley Lee
    Jul 26 at 9:37
  • \$\begingroup\$ Circuit seems correct. The op-amp you're using has a rail-to-rail input and output, so +/-5V supply should be fine. \$\endgroup\$ Jul 26 at 9:44
  • \$\begingroup\$ @WesleyLee I power the op amp with -5V and +5V \$\endgroup\$ Jul 26 at 9:44
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    \$\begingroup\$ Wait, hang on, that op-amp uses a unipolar supply with max 6V rating. If you are feeding it with a -5V power rail connected to the \$V_{ss}\$ pin, with +5V connected to \$V_{dd}\$, then you are also exceeding its supply capabilities of max 6V \$V_{dd}-V_{ss}\$. \$\endgroup\$ Jul 26 at 9:47
  • \$\begingroup\$ @TomCarpenter ah I did not know that. Somehow I was thinking that was relative to 0V. \$\endgroup\$ Jul 26 at 10:19

1 Answer 1

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Date sheet page 1: -

enter image description here

Also note that to truly get +/- 5 volts from a rail-to-rail op-amp that can legally be powered with +/- 5 volts, you will fall short by a few tens of mV. So, choose an op-amp supply that is slightly higher than +/- 5 volts and choose an op-amp that is R2R and won't be damaged by this voltage.

Other than that, I believe the calculator you used to be correct. Modified version for output between -4.95 volts and +4.95 volts when you only have a +/- 5 volt supply: -

enter image description here

Also note that the input offset voltage specified in the data sheet is +/- 4.95 mV and this will add an error to your output signal of up to +/- 15 mV. I mention this because if you change the op-amp to one that can operate on +/- 5 volts, you should take into account the errors that may be produced. Given that a Δ Vin of 3.3 volts produces a Δ Vout of 10 volts, there is an inherent circuit gain of 3 and this magnifies input offset voltages also.

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