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It is my understanding that a square wave can be converted to a smooth analog signal via passive low pass filter. However, this low pass filter acts like an integrator which ideally achieves the same result.

For the application of a DAC using PWM in the neighborhood of 1MHz, would an integrator or a low pass filter give me better results in terms of the quality of the analog output? Are there advantages/disadvantages to each method?

Would an integrator cause issues by relying on the capabilities of the OP amp used?

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    \$\begingroup\$ "low pass filter acts like an integrator which ideally achieves the same result" This statement is not correct. If the PWM is outputting only zero or positive voltage, the output of the (ideal) integrator will keep increasing and will not decrease. \$\endgroup\$
    – AJN
    Commented Jun 18, 2020 at 16:45
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    \$\begingroup\$ the low pass limits the current available to maintain the voltage level, but at 1MHz, you won't need much resistance, especially with a 2-stage. You can use a unity gain amp after the lowpass to beef up the "DAC" current, or a series pass transistor(s) if you can live with the junction drop... \$\endgroup\$
    – dandavis
    Commented Jun 18, 2020 at 16:47
  • \$\begingroup\$ Got it. Thank you guys \$\endgroup\$
    – Andrew
    Commented Jun 18, 2020 at 17:05
  • \$\begingroup\$ read up on sigma-delta ADC's and Sinc3 fiters, especially how they are done in analogue \$\endgroup\$
    – user16222
    Commented Jun 19, 2020 at 0:01

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You can do it the following way: you have PWM output that goes into a low-pass filter, which goes into buffer. The higher the frequency of pwm, the better the same RC-filter will smoothen out the line.

You can also increase R or C, but doing so will slow down voltage change response time. Find the sweet spot where you can change DAC output quickly enough while filtering good enough. And your op-amp better be rail-to-rail. Make sure to check out op amp's offset voltage to see the minimum output voltage it can give, it can be uV or even some mV even for rail-to-rail.

As a downside - that need to filter out square wave into constant voltage. Check RC-filter for 1MHz, see the rejection ratio you want, probably around 40db, but if 60db is not a problem in terms of parts, it's up to you, of course.

Anyone add/modify?

enter image description here

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    \$\begingroup\$ Using a delta-sigma modulated output instead of simple PWM gives a much better noise performance. And it is quite easy to do in FPGA-logic! \$\endgroup\$
    – michi7x7
    Commented Jul 25, 2022 at 19:35

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