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This DAC circuit drives a piezo buzzer used as a scanning tunneling microscope tip. The X, Y and Z are the final outputs that drive the piezo.

Why does it use two opamps per channel output? From my little knowledge it looks like the first one is just a buffer to not load down the DAC output, but I may be wrong.

What does the second opamp do? What type of configuration is used with the arrangements (2x3 kΩ and 1x1.5 kΩ) of the resistors?

Schematic showing analog outputs

Image source: Dan Berard - Analog PCB schematics (pdf) linked from Home Built STM, Electronics

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  • \$\begingroup\$ @CGeeeee for your part of question with resistors please see allaboutcircuits.com/textbook/semiconductors/chpt-8/… \$\endgroup\$ May 21, 2023 at 12:57
  • \$\begingroup\$ See fig 63 in DAC8814 datasheet for this exact circuit. The extra cap in the 2nd opamp adds a "free" low pass. \$\endgroup\$
    – td127
    May 21, 2023 at 19:08
  • \$\begingroup\$ Please, learn to crop an image and post an image of the detail of the schematic you are interested in. It's annoying to click an external link to see a schematic that should be easily discernible in the post itself. \$\endgroup\$ May 21, 2023 at 21:52
  • \$\begingroup\$ @LorenzoDonatisupportUkraine - Hi, I hope I haven't caused a problem. I'm not completely sure what improvement you are suggesting, but there was originally no link to the schematic source in the question. A comment linked to this page but the schematic isn't displayed on that page. The schematic is on a pdf which is linked (among several other files) on that page. So I added links into the question directly to the schematic pdf and the "main page"; it was the best I could think to do, and better than nothing. Do you want something else? \$\endgroup\$
    – SamGibson
    May 21, 2023 at 22:19
  • \$\begingroup\$ @SamGibson Sorry, I didn't examine the editing history. I saw the schematic in the PDF, but I think that the question itself should contain images that can be viewed comfortably without resorting to external links that may rot. In particular, since the question was about the output stage, it would have been better to have a cropped and magnified image of that part of the schematic, together with the whole schematic (otherwise one has to zoom the browser window to the point where everything is blurred). OFC this was OP's responsibility, I didn't realize you had already improved the question. \$\endgroup\$ May 22, 2023 at 10:07

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The DAC8814 is a current-output converter, so the first opamp in each channel is performing a current-to-voltage conversion, using a feedback resistor that's built into the chip. The second opamp is a reconstruction (low-pass) filter that eliminates unwanted frequency components.

The second opamp is also removing the DC offset (VREF) from the signal.

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  • \$\begingroup\$ From the article about your second opamp: "This essentially means doing PWM between steps of the DAC and filtering the output with a low pass filter." \$\endgroup\$
    – pipe
    May 21, 2023 at 12:43
  • \$\begingroup\$ Dave, perhaps another reason for two stages is to restore DAC's polarity, so that when the DAC goes +ve, output also goes +ve. Let's not quibble over what a +ve current means - that can be a can-of-worms. My point is that both stages have their "+" input GNDed, so they're both inverting stages. \$\endgroup\$
    – glen_geek
    May 21, 2023 at 13:18

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