Crap. I really messed up when I ordered the DCOs. I misread the datasheet, it says the DAC can drive loads of 60k unbuffered. I read this as "max 60k resistors" on the output, when in fact it is the opposite, minimum 60k.
I even changed the 100k input resistor to 22k to prevent issues - 100k would have worked just fine :-(
I have tested the circuit and the effect is that the DAC is not able to get the output all the way up to 5V, it tops out around 4V and more importantly, at an unknown voltage. For charging the cap this may not be a big deal as the charge time is calibrated anyway, but for using the DAC as a wavetable DCO this really messes up things.
So, what can be done about it? Well, we have a few options
- 1: Replace R5 and R3 with 120k resistors (this lets us do what was originally planned, having a second external buffer to use as wavetable output.
- 2: Remove R3, R5 and R1 and R5. Replace R3 with a solder blob, effectively making an op amp buffer connected to ground, the same way we normally disable opamps. Then use an external opamp instead of IC3B, tapping the CHARGE_VOLTAGE pin and re-injecting the result at the CAP output pin. We may even be able to leave R3 in place, though I'll need to read up on this first.
- 3: remove R5 only, tapping the CHARGE_VOLTAGE, buffer it and then reinjecting at the other end of R5, however, that point is not exposed so we need a wire.
- 4: Desolder R5, cut the power lines and both cap inputs to the IC3 TL072 and use a whole separate one for both A and B.
Seeing as I can't solder anymore, option 2 or 4 is the simplest work for someone else desoldering the resistors, or me cutting the resistors in two. Option 2 will use a little bit more power as the opamp will still be in place, option 4 requires more opamps on the "main" board.
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