My mistake was that I reduced the resistor the DAC outputs through from 100k to 22k, thinking that it had to be less than 60k when in reality it should be MORE THAN 60k.
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| I need to replace R5 with a 60k resistor |
Fixing this means desoldering at least one resistor, but after that I have some options.
The "correct fix"
The correct fix would be desoldering R3 and R5 and replace them with new 60k resistors. It is a bit finicky to do this, especially removing R3 will be hard. (UPDATE: Even this would not make the output perfect, see Update 3 below)
Easier fixes (?)
The DAC feeds into an inverting op amp with unity gain (Rf = Rin = 22k).
By replacing only Rin with a 60k resistor, I would get an inverting amplifier that attenuates the signal to -Rf/Rin = -22k/60k = -0.37.
Unfortunately, attenuating inverting amps are known to self oscillate. The solution, as described here and here, is to put a resistor of the same value as Rf from the negative input to ground.
There are a few ways to add this resistor.
First, I may add it directly between leg 5 and 6 on IC3. It may be possible to use a 0603 resistor, or at least a 0402 directly, but it may be a bit hard.
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| IC3 pins 5 and 6 are in the lower right corner |
Second, I may desolder R5 and put a tiny wire from the top pad to pin U1 (utility 1). Doing things that way means I could add a replacement for R5, as well as a resistor to ground, on the board the DCO plugs into.
Charge current
Since the output from the inverting op amp now is much smaller, I also need to place a resistor in parallel with R1 to increase the capacitor charging current. I need to do this anyway, as the DAC output in my original design was 5V, but running the DCO from 3.3V means the DAC output now can never reach above 3.3V.
Another option would be to simply amplify the DCO output. I do this right now on my prototype, I need to do some calculations and experimentation here to see what is the best solution.
Change nothing
The last option I have is to do a bit of measuring on the DAC output. When do we run into problems? Is the max 60kOhm load rule for the whole range or just for the higher values? Is it related to the output current?
If it is indeed current related, we may have a few options. First of all, maybe reducing the reference voltage, which in turn reduces the output current at max, is enough?
Similarly, if the higher values are the issue, could I get away with using only 15 bits, leaving the MSB at 0? Testing is needed!
Update 1:
Measurements from the DCO
This is weird. I did a lot of measurements and calculations. The DAC voltages follows the pitch, no errors as we go up the scale (apart from some tiny errors at the bottom that may as well be with my probes).
However, I have connected 3.3V as Vref. According to my calculations this means the output voltages are off by 25%. BUT - when I replace Vref with 2.5V in my spreadsheet, I get exactly the output I see on the dac! The error is around 0.5%, and that's to be expected since I haven't included the slope part of the lookup. This is SO weird. I need to look at this again tomorrow.
Update 2:
Changing to a 2.5v reference made the max value drop from 2.17 to 1.66V (Expected is 2.84 and 2..16 respectively). The diff is still 23% and it is still consistent across the DAC range.
Next up: breadboard a new circuit and check if the output changes (and at what rate) when changing the resistor.
PS: the DAC output impedance is approximately 6.25 kΩ
Update 3:
I did some measuring during my lunch break, and the results are extremely uplifting:
I replaced the input resistor Rin and measured the DAC output for various values. Then my hunch told me to compare it to what would happen if we measured the mid point of a resistor divider between Vref and GND, where the top resistor is equal to the DAC output impedance (6.25k) and the bottom resistor is equal to Rin.
The two voltage columns speak for themselves - it is clear that this is what is going on. This means that
- The output will probably be exactly the same between the various DACs I'm using
- Even if I changed to a 60k resistor, I would not get a perfect result
In other words, I can probably just leave the 22k Rin as it is, and compensate in the charging resistor part instead! This is just amazing!
Even better, I added a buffer after the DAC, to see if tapping the output for my wave player idea affects the output, and it absolutely doesn't :)
Update 4:
I tested connecting a resistor in parallel with R1 to increase the charging current. Originally I intended the max charging current to be 5V / 56kΩ = 89.3uA. Now the max DAC output is 1.94V if we use a 2.5V reference, so we need R1 to conform to 1.94V / R1 = 89.3uA, meaning R1 has to be 21725Ω. By putting a resistor in parallel with the current R1 we can lower the combined resistance - to get a combined resistance of 21.7kΩ the second resistor needs to be 35.5kΩ. If we choose a 36kΩ resistor, the combined resistance is 21.9kΩ and the current 88.6uA, fairly close to the ideal.
I tested this, and the wave amplitude instantly jumped to 9.4V (With a 3.3V reference, it passes 10V. The tops are cut off but I assume that is because the calibration output (connected to the microcontroller) is > 3.3V, I've seen this happen before).
So - all I have to do to make the DCOs work properly - with a 2.5V reference instead of 3.3V btw - is connecting a 36kΩ resistor in parallel with R1, and 75kΩ and 3.3kΩ resistors (in series, so 78.3k) in parallel with R7 to get the correct value at the MCU calibration pin - see https://atosynth.blogspot.com/2022/01/big-bug-weekend.html for calculations of R7
Increasing max frequency
Right now, the DCO has a range of 8Hz to 8kHz. I want to move this to 16Hz-16kHz (or 20Hz-20kHz).
This can be done either by decreasing C3 from 1nF to 500pF (560pF may be the closest option), or by changing the value of the resistor in parallel with R1:
We now need the max charging current to be 178.6uA, meaning R1 should be 10.86kΩ. The necessary parallel resistor would be 13.4kΩ. It is better to choose a slightly lower value than a too high one, so 12k may be the best standard value.
Update 5:
By swapping R1 with a combined resistance of 14.5kΩ and using a Vref of 2.5V I'm able to cover the full midi range (8Hz to 13.3kHz) with only 3 cent errors on the highest pitches. This does not leave room for pitch bend at top/bottom, but each note is exactly 512 steps up from the previous which is a nice number to work with in the voice controller.
Reducing to 9.1kΩ lets us add another 12 semitones, giving us a range of 5Hz to 21kHz, though I'm not sure it's worth it.
All in all, I think I should consider making a bootloader for the DCOs, there are so many things that could be changed after install, and having to manually change 16 (or 32!) of them would be a real pain.
