[sdiy] ADc & DAC voltage dividers.. fixed or trimmer adjustables ??

Brian Willoughby brianw at audiobanshee.com
Mon Feb 15 09:52:46 CET 2021



On Feb 15, 2021, at 00:01, Roman Sowa wrote:
> W dniu 2021-02-14 o 18:31, Mike Beauchamp pisze:
>> On 2/14/21 6:59 AM, Roman Sowa wrote:
>>> . Not to mention curious user turning it for fun.
>> You mean circuit bending? :)
> you wouldn't believe what people can do for fun with an equipment. I'm telling that from occasional synth-tech perspective

Yeah, some people have been known to replace every capacitor, even when nothing is broken!


>> Would it be advisable to run an auto calibration with the output fed back into the input and iterate on every single bit and then store the ideal DAC value into a 16bit look up table?
> 
> not if this is to be used with microcontroller having just 4k words of memory

A lot of modern MCU chips have way more Flash memory than RAM. It's easiest to store fresh calibration values in RAM, but if there's not enough room then perhaps the chip can write to its own Flash memory. This would probably require that calibrate doesn't happen on every power-up, but instead happens seldom, or just once.


>> strange drop in the output that happens exactly at the halfway point - so with my 14bit dac, the output is fantastic from 0-8191 and then is offset below the expected voltage from 8192 to 16313.
>> 
>> This is probably just an issue with the specific DAC i happened to have, but my point is that you're going to have to test your system and see what needs correcting.
> 
> aparently your DAC was R-2R ladder so it shows the biggest jump in midscale.

There are other DAC types besides R-2R that are also binary weighted. All of the binary-weighted types could have an MSB that's not well matched to the sum of the remaining bits.

> Many modern DACs are resistor string, not sensitive to jumps of binary code where all bits flip (like from 0x3FFF to 0x4000) but also because such architecture their linearity is bent all over the place without any plan.

"Because of the physical size, pure string DACs are primarily limited to resolutions of 8 to 10 bits."

Mike designed with a 14-bit DAC, so I rather doubt it's available as a string of resistors type DAC. I've worked with a binary-weighted current output DAC that is 14-bit, but never checked for this sort of offset error.

https://www.analog.com/media/en/training-seminars/tutorials/MT-014.pdf

Brian





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