External VREF+ and VREF- for ADC

[rakib2023]

Hello,

I am using NUCLEO-F756ZG. My application is ADC related. I am measuring an analog signal at high sampling rate. Recently, I had a discussion with my team and had an interesting proposal. Our target signal is pretty much guranteed to be within 0.5V to 1V range. So, I thought of supplying external reference voltage for a custom range. On nucleo boards, the VDDA and VREF+ are tied to VDD by default and makes the ADC range 0V~3.3V. But from the MB1137 schematic I found here, it seems that I can decouple VREF+ from VDD by removing SB12. Problem is that VREF+ is tied to VDDA on the board and can't be decoupled. So, if I put 1V or less in AVDD pin, it's probably not going to be enough to power the ADC. Or is it? Additionally, VREF- seems to be connected to ground through SB162 and by removing it, seems like I can supply an external VREF-. Would it work if I supply custom voltages from an external DAC to both VREF+ and VREF-. The planned reference voltages are less than 1V for VREF+ and around 0.5V for VREF-.

[TDK · Accepted solution]

This won't work.

Consult the datasheet for limits on these pins.

VSSA/VREF- must be tied to VSS/GND.

VDDA must be tied to VDD.

If it weren't tied to VDDA, VREF+ must be at least 1.7 and no more than 1.2V from VDD.

[rakib2023]

@TDK, Thanks for clearing that up. From what I gathered from scouring internet it seems that most SRA ADC measures from 0 to VREF+, custom VREF- seems rare. Furthermore, these ADCs doesn't seem to work with very small swings like 50mV. What I wanted to do is pretty much impossible. One workaround seems to be to boost the input voltage with op-amp. I'll try that for now.

[TDK]

Yes, exactly.