Practical use of ADC in small pin count package, with no Vref pins

gp · ‎2011-10-09

Posted on October 09, 2011 at 13:49

We are currently evaluating STM32F103 series as replacement in a current product, and it was quite a surprise to discover no ADC Vref pin is present in the smaller packages. It is internally connected to Vdda ( supply ), which in our case is not very accurate ( 3.0..3.6 V ).

Is there a clever way to overcome this without using a larger package ?

#adc

trevor23 · ‎2011-10-09

Posted on October 09, 2011 at 17:53

Connect one channel of ADC to a precision reference (e.g. 2.5V) and use the measurement from this to calibrate your other channels in software. I did this on our last project and it worked nicely.

gp · ‎2011-10-09

Posted on October 10, 2011 at 07:00

This is not a bad solution. I also noticed the internal 1.2V ref is readable by the ADC. I think we will continue with this.

Thanks Trevor

trevor23 · ‎2011-10-10

Posted on October 10, 2011 at 11:12

I remember the internal ref not being accurate/stable enough for what we needed at the time. Can't remember figures but worth checking before you commit to using it.

donald2 · ‎2011-10-11

Posted on October 12, 2011 at 05:05

The 1.2V reference is spec'ed at 1.16-1.24V over the typical temperature range. Which isn't especially accurate.

My guess is that spec is pessimistic. Bandgap references are usually very accurate, especially with the precise process geometry needed to make a chip this complex. But if you are doing a production design, you can't rely on a batch being at the limits of the spec.

An external reference will be speced to be much more accurate, and the calculated sample time will be much less than the internal reference. The drawback is that it will consume more power.