active-filterbattery-chargingpwm
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 ?
Take your 24V pulses and strongly differentiate them with an RC high pass filter: -
Make sure they are properly differentiated like example (a) below: -
The resistor shown above needs to be connected to -2.5 volts - maybe use two equal value resistors - one to 0V and one to -5V
Reduce the amplitude to 5Vp-p (resistors and/or diode clamps can do this)
Set up a schmitt trigger running from a -5V rail and feed the clamped signal to its input. The output from the schmitt trigger will be the PWM signal you want. It needs to be a schmitt trigger so that it only changes state when a spike occurs: -
There doesn't seem to be anything wrong with your circuit. I would check that it has been assembled correctly. Do a check on the Gerbers to make sure the PCB does what you intended it to and also that there is not any significant contaminants on the PCB (flux remnants for example).
Best Answer
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.