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In many applications, when a PWM signal is converted to an analog signal, they use a second order low pass active filter cascaded with a low pass passive filter, like the one shown below, (from microchip https://www.microchip.com/developmenttools/ProductDetails/MCP1630RD-DDBK3) page 16/30,

In the attached circuit, I did not understand the usefulness of R28 and R35. Why do we need to create a voltage divider of V=0.18, is this voltage needed as an offset or it has other role in the circuit ?

enter image description here

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    \$\begingroup\$ It is just a design optimization and potential safe-guard for an upper Iset limit. If you look at the full schematic you are generating a reference current limit for a 50 mOhm current shunt. There is no need to try and run 5 V across 50 mOhms (100A), so they scale the DAC reference to a usable range. You also benefit in output DAC scale V/LSB by the same division factor. \$\endgroup\$ – sstobbe Oct 30 '19 at 3:06
  • \$\begingroup\$ What about R24, it is in the range of mega,.? and also C12 and R18, are they for filtering ? how do they do it ? \$\endgroup\$ – luxina pado Nov 4 '19 at 23:41
  • \$\begingroup\$ Please, could you explain more concerning "There is no need to try and run 5 V across 50 mOhms (100A), so they scale the DAC reference to a usable range. You also benefit in output DAC scale V/LSB by the same division factor", I understood from your comment that the reference current is generated through a DAC and not a PWM signal \$\endgroup\$ – luxina pado Dec 5 '19 at 6:24
  • \$\begingroup\$ @sstobbe, please could you explain more your first comment? I find it very interesting concerning the scaling of the DAC. \$\endgroup\$ – luxina pado Dec 15 '19 at 21:15
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It's hard to say without looking at the response of the whole integrator.

R28 and R35 are in a resistor divider configuration, and provide a shift at DC, because this is fed into an integrator it also shifts the gain of the amplifier down 20dB from DC to ~1kHz. Below is a sim of the AC response, blue has R28 and R35 removed.

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  • \$\begingroup\$ Please try to put the reference at the inverting input of the amplifier and the perturbation at the non-inverting input, this is the way it is designed in the circuit, and that's the point, "a non-inverting compensator" why is it designed that way in the full circuit ? \$\endgroup\$ – luxina pado Dec 5 '19 at 6:14
  • \$\begingroup\$ I did exactly as shown above the the OP's circuit diagram. I left out the DC reference (probably a DAC) because the OP did not care to include what the DAC was. Regardless, A DAC (which I am assuming is at DC) will not have much effect on an AC analisys. \$\endgroup\$ – Voltage Spike Dec 5 '19 at 16:02

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