ADC resolution is low

mehmet.karakaya · ‎2010-09-30

Posted on September 30, 2010 at 17:48

#adc #adc-problem

John F. · ‎2011-05-17

Posted on May 17, 2011 at 14:09

Noise and source impedance are significant.

Try using ADC_SampleTime_239Cycles5: Sample time equal to 239.5 cycles which will tolerate higher impedance sources.

Otherwise, take multiple readings and average them to remove noise effects.

mrost9 · ‎2011-05-17

Posted on May 17, 2011 at 14:09

It's essential have low noise not only on the line to be measured, but also on the Reference supply.

On many STM32 the ADC reference is tied to VCC, so your resolution depends heavily on the blocking capacitors.

As I understood, the internal voltage reference can only be measured, not be used as reference for the ADC. But you can measure the internal reference, and any deviations seen there results from noisy VCC lines. We had to improve our DC blocking capacitors and filters to get cleaner measurements.

mehmet.karakaya · ‎2011-05-17

Posted on May 17, 2011 at 14:09

I am measuring directly a battery which is the most low impedance and most noise free thing on the world

second - I am power supplying the STM32 with LM2575-3.3 and it is the only thing on this line which consumes power

I dont see a deviation on measured results , I see a jitter

the board I use can be seen on this link

http://www.futurlec.com/ET-STM32_Stamp.shtml

trevor23 · ‎2011-05-17

Posted on May 17, 2011 at 14:09

I can't see a schematic for your board when I follow the link but....

If you are using VCC as reference it will not be very precise especially if you are using a switch mode power supply (LM2575). Your going to have noise with this setup -- so you need to live with it. However some _random_ noise can be good when you over sample and filter and can effectivly get you more resolution e.g. 16 bits. Have a google for ''over sampling'' and maybe ''software ADC filter'' etc.

mehmet.karakaya · ‎2011-05-17

Posted on May 17, 2011 at 14:09

I checked the schematic of the board of futurlec which I use

first - there is no Vref pins on 64 pin package microcontroller TM32f103ret6

second - the futurlec board has ferrite bead and 10 uF and 100 nF capacitors in front of VDDA

I wonder if I have to power on the internal reference with some bit in the ADC registers - maybe thats why so much jitter ?

parag · ‎2011-05-17

Posted on May 17, 2011 at 14:09

HI All,

Could you please tell how to calibrate the ADC. I am giving input of 3.3v and when i write a code to read PC4 pin, i am getting 4095 so i want to convert this value 3.3v. Please tell me is there any specific formula to calibrate ADC?