søndag 21. februar 2016

XM8 32 channel CV works

I've spent some time the last few days getting the sample and hold part of the 32 channel DAC to work properly.

As is usual, I messed some things up in the initial design. The DAC board is connected to the 32ch sample/hold via a connector. This goes in a U-shape but I forgot that this means that pin 1 on connector one becomes pin 2 on the other and vice versa. As power as well as signals are transferred through this connector, nothing worked right. Weirdly enough though, nothing broke, it just got very hot.

From the initial tests I learned a few things:

- The type of opamp used with the DAC matters, at least visibly on the scope. Using a stock TL07x opamp produces slight a slight overshoot when the DAC output changes a lot. Using a AD8672 precision opamp instead removes the overshoot.

The S&H circuit shows a similar overshoot when it is set. As the S&H is all TL074 it could be that it is fixed in a similar manner. I just don't know how much it matters.
The output as seen between the DAC and the sample and hold shift register - the initial rising curve shows the output from the DAC. The drop is caused by the enabling of the shift register, connecting the DAC to the sample and hold which is currently at its most negative, so it takes a short amount of time before it charges to the same level as the DAC. The peak is where the charging of the sample and hold capacitor overshoots. It is of the same kind but more dramatic than then one we see in the DAC output photos above and may be caused by the TL074.
A more detailed photo showing the charging and overshoot.

- I connected the output of the S&H to the pitch and filter inputs of a Moog Little Phatty. It worked very well, but I got an audible clicking noise four times per second. It went away when I removed sampling of all but one input potentiometer from the PIC32, so it may not be a problem in real life where the potmeters will be sampled from a completely separate circuit.

- At very high frequencies there is a slight ringing to the sound, I presume this is some kind of aliasing. I could not hear much of it when I turned off sampling of all potmeters but it should be investigated further.

- Oscilloscopes have a horizontal scale calibration potmeter. I spent hours trying to understand why the results I got did not seem to match the output frequency I was expecting, untill I realised that I had made two crucial mistakes: The oscilloscope scale was way off (fixed by connecting a function generator) and I had not set the internal PLL divider in the PIC32 correctly (it was running five times slower than expected!)

The output of one of the 32 sample and hold cells when running at full speed with 32 operating cells. DAC switches between +/- 5V.

Just for fun: My test rig. From left: Four potmeters that sets the CV (read using the ADC in the PIC32), a solderless breadboard that just reconfigures one of the ribbon cables, the EasyPIC fusion 7 development board from Mikroelektronika, which contains the PIC32, and on top of it: four quarter inch jacks connected to outputs 29,30 ,31 and 32 of the DAC circuit. To the right: My Moog Little Phatty

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