[sdiy] Odp: Single supply op-amp filter biasing question

[Roman]

The bias to reach your desired 2-7V range is pure DC, so you can calculate it just like DC opamp circuit with assumption PWM is at mid level, that is 50%.  You want 4.5V mean value at the output, while PWM mean value is 2.5V. So there is 2V across R2+R3  which means that the R2,R3 node and opamp inverting input is higher by 1V than PWM =  3.5V. So 3.5V should also be set at non-inverting input with PR1.   Wouldn't it be easier just to use Rail-to-Rail opamp and skip the level shifting?   Roman  Dnia 27 kwietnia 2017 20:41 Tom Wiltshire <tom at electricdruid.net> napisał(a):  Hi All,   I have the following situation:   A PIC produces a PWM output representing an LFO waveform. This is then sent to an op-amp MFB filter for smoothing. The circuit looks like this:   www.electricdruid.net www.electricdruid.net   If I'm running the PIC on 0-5V, and running the op-amp on 0-9V, the op-amp can't handle the raw PWM output, since it clips below about 1.6V (for the TL07x - other typical op-amps are not terribly dissimiliar). So the PWM signal needs biasing to move it to the correct range.   I found the following useful document discussing biasing, both AC and DC, for the various op-amp configurations:   ocw.mit.edu ocw.mit.edu   I've used this guide with success on other circuits. However, in this case I can't make theory and practice match up. For a start, the MFB filter isn't a typical DC-coupled inverting op-amp application, although that's the closest. The filter is inverting and has no overall gain, so gain = -1.   How would I calculate the required bias to get the LFO output signal (0-5V) in the centre of the op-amps range (2-7V)?   I know the answer to this question from practical experiments, but I'd like to know how I'm supposed to derive it from the theory.   Thanks,  Tom     ______________________________  Synth-diy mailing list   Synth-diy at synth-diy.org  synth-diy.org synth-diy.org
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