EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: FrankBuss on July 27, 2018, 12:46:51 pm
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I have a DAC with an internal 2.44 V reference, with an output range from 0 V to 2.44 V, and I want to convert it to the range -10 V to 10 V. I found 4 circuits for it:
- one opamp and 3 resistors: http://www.ti.com/lit/ug/slau525/slau525.pdf (http://www.ti.com/lit/ug/slau525/slau525.pdf)
- one opamp and 2 resistors: https://www.maximintegrated.com/en/app-notes/index.mvp/id/5581 (https://www.maximintegrated.com/en/app-notes/index.mvp/id/5581)
- one opamp and 4 resistors: https://masteringelectronicsdesign.com/design-a-unipolar-to-bipolar-converter-for-a-unipolar-voltage-output-dac/ (https://masteringelectronicsdesign.com/design-a-unipolar-to-bipolar-converter-for-a-unipolar-voltage-output-dac/)
- two opamps and 2 resistors (application note of the DAC I'm using, page 18) : https://datasheets.maximintegrated.com/en/ds/MAX5134-MAX5137.pdf (https://datasheets.maximintegrated.com/en/ds/MAX5134-MAX5137.pdf)
What are the advantages and disadvantages of the 4 circuits and which one should I use? Would be nice if I wouldn't need exact matched resistors, like R and 4 x R, but it doesn't need to be exact, could be like +/-10.5 V, or even shifted zero point, and I can calibrate it then in software (if it is stable). But I wouldn't like it, if there are different gain factors for the positive and negative voltages, which I guess could happen with the solutions with more than 2 resistors?
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The second, third and fourth links are effectively the same solution (two resistors) and only useful when you want the output range to be +-Vref. No good for you, so use the TI app-note in the first link.
In your third link the author starts with a conventional diff-amp (4 resistors) and derives the same two resistor solution from it.
In your fourth link the extra op-amp is used to buffer the Vref output to prevent loading by the resistors in the amplifier/level shifting stage. The reference on that device has a typical output impedance is only 1 ohm, so providing you keep the resistor values reasonably high you can probably get away without the buffer amp.
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Thanks. I simulated it in LTSpice and looks like the gain is always constant, the resistors just move the zero and change the whole gain:
(https://i.imgur.com/a17nPhz.png)
The datasheet of the DAC says that the REF output is stable up to 100 uA, so I changed the resistors to higher values to be in the safe region. I built the full circuit diagram on a breadboard, with a OPA4188 with very low offset voltage and "zero drift":
(https://i.imgur.com/8Uw2BZd.png)
Looks like it works. I guess C2 is to compensate for higher frequencies and I have to try it out for best compensation, but what is the purpose of C7 and R4? Somewhere I've read you shouldn't connect a capacitor to an op-amp output.