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I'm building a board that converts a DAC 0 5 V output to -10 0 10 V output.

As a reference for my design, I'm using TI paper SLAA869.

I'm also using AD5663 as a 16-bit DAC, ADUM1300 for isolation, and a generic LM358 for the unipolar to bipolar conversion.

Find below the schematic:

enter image description here

The problem is that the micro takes milliseconds to set the DAC output to the middle value of 2.5 V, the initial state of the DAC output is 0 V as the non-inverting input of the LM358. So if the op amp has supply the output will spike to -10 V until the DAC output is set to 2.5 V by the microcontroller logic.

One option would be to use an op amp with a SHUTDOWN pin, but I want to use a common op amp, so my idea is to use an optocoupler (OK2 on the schematic) controlled by the micro to power the op amp on.

Do you think this would work? Do you see any issues or have suggestions?

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  • \$\begingroup\$ So if the opamp has supply the output will spike to -10V until the DAC output is set to 2.5V by the microcontroller logic. but the -12V supply is still there. So if you want to cut the supply, you should cut both i.e. one optocoupler for each rail. Do you think this would work? As long as you saturate the output transistor you're fine. \$\endgroup\$ Commented Jan 5, 2022 at 11:51
  • \$\begingroup\$ Thank you @RohatKılıç, I must admin I'ven't try on protoboard, I should I know. My doubt was exactly what you point out. I will try on proto and let you know. \$\endgroup\$
    – LK Davide
    Commented Jan 5, 2022 at 12:00
  • \$\begingroup\$ This version of your schematic might be easier to read on a small screen. You might choose to replace the one in your question with this one. \$\endgroup\$
    – Andy aka
    Commented Jan 5, 2022 at 12:51
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    \$\begingroup\$ @Andyaka image changed \$\endgroup\$
    – LK Davide
    Commented Jan 5, 2022 at 13:08
  • \$\begingroup\$ As @RohatKılıç suggested, I've to cut the -12V line too. But I think I've to add a P-channel MOSFET on the +12V line, and a N-Channel on the -12V line, cause the op amp can draw more than the optocoupler give due to his low CTR. \$\endgroup\$
    – LK Davide
    Commented Jan 13, 2022 at 16:54

1 Answer 1

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Solution 1: Use the AD5663BRMZ-1 which powers up to midscale (same datasheet)

Solution 2: Bias the inputs of the OPAMPs to midscale and use an analog switch like ADG714 to connect them to the output of the DAC when ready.

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