Convert 0-3.3v PWM signal to +/-12v Please Help

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To be honest I'm sure it's something I did but with a 40KHz square wave on V1 at 50-99% duty cycle It will randomly drop from 12v to -12v and then back up every millisecond or so.
Could you explain how it's supposed to work?
 
milolouis said:
Could you explain how it's supposed to work?
Click to expand...
U2 is a low pass filter. The frequency sweep says that at 100hz it will pass the signal and give it gain. At 40khz it will only pass -60db of the signal. So it should remove the PWM signal but pass the "average" of the signal.

U1 is an example of a simple RC filter. (with a buffer) How is it doing?
milolouis said:
It will randomly drop from 12v to -12v and then back up every millisecond or so.
Click to expand...
You need to set V4 to 3.3 volts. (supply for your PWM source) "Set to 3.3 for DC. Set to 0 for AC study "
 
You are right! It is oscillating. I have never pushed the gain beyond 2 or maybe 4. OK let me see why.
 
Thanks Ron this, is fantastic thanks for the explanation as well. I get about 10mV noise with just one op-amp. Cut-off is a bit low however so just fiddling around with it a bit. Hoping to get 1000Hz pass through rail to rail. Or at least more than 500hz.
 
Still alec_t that's exactly what I was looking for originally. Spot-on Thanks so much.

The target goals are:
- 40-50KHz 0-3.3v PWM input
-Smooth Analog Voltage output very low noise with effectively zero-drop off at 1KHz (PWM duty modulation)
-Lowest part-count/cost
-Must be able to reach less than 0.1v

As there will be 8 of these on one board.

So I think now with all the advice that probably the best way is two op-amps with RC filter before the voltage translator and then second op-amp as Sallen Key Filter.
So 3-poles and translation with two-opamps.

If you or anyone disagrees or thinks there is a better way better way then I'd love to hear it. I'll post a picture of the schematic when it's finalised
 
Alec_t,
The shoot-through comes from two places.
The first two transistors were really slow at turn off. So I made them "grounded base" and keep them out of saturation.
The last two transistors need a "turn off" resistor.
I have not gone back and checked each resistor. This is just a first try. Shoot through 10 to 15 mA.
 

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