Since the input is audio, then C1 cannot do anything useful to filter the 3.3V input. Just leave it out.

Because the input is audio, you do not need DC response, so you could also use a coupling capacitor to block the DC, bias the AD input halfway between 0V and 3.3V, and put a series resistor in to attenuate +_10V to +-(half of 3.3V).

 

Yeah, but now we might be running into a different problem. What is the sampling frequency of the Teensy ADC?

If we assume perfect audio, then we have 20 kHz input. The Nyquest Sampling Frequency then should be 2*20 kHz=40 kHz, this is minimum sampling frequency! If you have more than 40 kHz sampling frequency, that would be even better. But... What is sampling frequency of Teensy?

Also. While normal audio range is considered to be 2 Hz to 20 kHz, vast majority of people don't actually have this range, if I remember my DSP class discussion right, most people go up to about 15-16 kHz. So if you don't mind loosing high end frequencies (that most people can not hear anyway), you could set your sampling frequency to 30-32 kHz instead of 40 kHz.

 

the signal is not audio. sorry if i led you to believe that. these are dc control voltages coming out. i am using the DAW software just to create graphic curves to change the dc control signal.

when i hit play in the DAW the speed advances along the DC control curve at the rate of the song. that is what i was trying to say the speed was. sorry about that. so the speed of the changing dc would be at whatever the speed of the advancing marker is when i hit play in the DAW.

i know it seems like if I'm using a DAW it would be audio, but not audio.

hope this clears it up a bit.

let me check the rate of this marker advancer. i'll be back in a bit.

 

The only things I could find are that my ES-3 sends DC control voltages out at a sampled 44.1/48kHz and Teensy has a 12 bit usable ADC.

 

The only things I could find are that my ES-3 sends DC control voltages out at a sampled 44.1/48kHz and Teensy has a 12 bit usable ADC.

Page 37, Table 24: http://cache.freescale.com/files/32bit/doc/data_sheet/K20P64M72SF1.pdf

If I am reading it right:
* 13-bit mode, maximum sampling frequency 18 MHz, which means you can use this mode to sample input signals that have frequency from 0 to 9 MHz.
* 16-bit mode, maximum sampling frequency 12 MHz, which means you can use this mode to sample input signals from 0 to 6 MHz.

 

I think Mike's first circuit will work. I had a similar voltage divider circuit before going with Teensy and the sampling rate and dc speed was ok for my purposes. It just didn't have the negative volts like this one and I was unsure of how to make a circuit go all positive. But we solved that. Just need a cap value and I think I'll be good to go.

Thanks so much for the help!

Will you guys be around Monday evening? I have this one last tiny circuit that I need help with to finish my project. With your expert advice and fancy tools!