I'm building an analog synth controlled by 2 atmega 1284s. Been making slow but good progress - have midi in and out, scanning for a 61 note velocity keyboard, extra eeprom memory to store settings and a touch lcd for all the controls.
Working on getting the connections right for an old CEM3396 wave shaper and vcf chip now. I can get several waveforms out of it, but can't get them to a audio line out without major distortion.
The chip spec sheet just shows connecting it a inverting op amp input with an unspecified resistor feeding back and the non inverting input grounded. Regular current to voltage converter from what I have on op amps.
I use op amps as voltage amps and a voltage differencer (to get my DAC 2V output amplified to 7.5v and then shifted down to -2.5 to +5, the range I needed for all the controls for this chip) but I can't get a current to voltage arrangement to work.
If I hook the current output of the chip (scope shows centered around ground 4v with no load) to an op amp with any feedback (with or w/o resister) the signal just goes to ground - completely dead.
If I try to treat it as a voltage and hook it to the non inverting input and feedback direct to inverting input to make a voltage follower, it over amplifies, goes to 8 volts and badly clips the signal.
Same thing happens if I feed it into non inverting adjustable gain amp. Even at gain one output is too high and clipped.
But if I feed it into the inverting side, the side with any feedback, it is just flat 0.
The closest I've come was to hook the signal to the center of two 100K resistors hooked to +5 and -5. and then ran this into a voltage follower as a buffer and then an inverting gain op amp where I can adjust the line out level.
Some signals worked, but as I changed the waveshape to a saw (non square) the signal just becomes a very narrow pulse wave. Badly distorts the waveforms.
Can anyone help me turn this current out (max +- 500 uA per the spec sheet) into a clean small voltage that I can buffer and amplify through an op amp?
My background is digital (from the 80's) so the lcd interface and sound generation up to making the pitched square wave triggers (via hardware counter chips) for the analog circuits has gone well, but I'm trying to learn more about analog to do the last part.
Having fun with op amps lately, but have been stuck on this problem for the last week.
I'm trying to test the control ranges that actually work with the chip and how to adjust them as frequency changes, but I need to be able to hear them at this point.