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

Roman Sowa modular at go2.pl
Sun Feb 14 12:59:25 CET 2021

I would say get rid of the trimpot and make trimming in firmware. One 
multiply and one addition per sample and you're done with gain and 
offset errors. You can even do it as autocalibration - use a patchcable 
to route input to output, press hidden button and softrware does the job.
Trimmers can change when dropped on the floor, when they get old, they 
can get loose wiper contact and as others said - may add noise. Not to 
mention curious user turning it for fun.

Still if you prefer trimmer anyway, why not make it smaller. Now the 
full travel changes your divider by about 6%. Since you plan 0.1% 
resistors there, then IMHO 100ohm trimmer with R1=R3 and R2=R4 would 
work better and provide more precision in adjusting perfect match. Or 
insert digital pot there paraleled by 100 ohm resistor and make auto 
trimming in firmware. This way there's no need to multiply calibration 
data every sample.

BTW, any spectacular matching will be ruined if you keep R5 outisde the 
feedback loop.


W dniu 2021-02-13 o 21:24, Jean-Pierre Desrochers pisze:
> Hi list,
> I’m in the process of design for 16 bits ADC/DAC circuits.
> This circuit will receive a 0-8vdc CV voltage, drop it down to 0-5vdc, 
> process it through a MAX11100 16 bits ADC (reference voltage  REF02 
> +5.00v)
> Then process the cpu data out to a LTC2641 16 bits DAC (reference 
> voltage  REF02 +5.00v).
> The DAC’s 0-5vdc will then be amplified back to 0-8vdc.
> To keep as much as LSB precision I’m wondering if the voltage dividers
> Should be fixed or adjustable..
> See the attached schematic. The 5v to 8v amplification on the DAC section
> Is made using U19B , R3 & R4 (*_fixed_* gain of 1.6)
> And the ADC attenuation is made using U19A, R1, P1, R2. 
> (*_adjustable_* attenuation of 5/8).
> What would you use for a 16 bits ADC/DAC transfer like this ?
> Fixed resistor values (shown in DAC) or 2 resistors with a center 
> trimmer (shown in ADC)?
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