Fluctuation in ADC VREF samples

vaibhavi_padwal — Fri, 21 Aug 2020 16:08:10 GMT

Hi,

I am working with 12Bit ADC on custom board with MK22FX512AVLQ12 controller. I am trying to sample VREFH channel (channel no. 29) but I am obeserving considerable fluction of 7 counts (Please see the attached Sheet).

The supply voltage to the controller is stable 3.3V with no ripples. I have connected VDDA to VREFH and VSSA to VREFL. There are capacitors of value 0.01uF, 0.1uF (as mentioned in the reference manual) and 1 uF connected between the pins VREFH and VREFL.( Please see the attached snippet)

ADC Configuration:

12 bit resolution, software trigger mode

ADC module clock: Bus clock with frequency 20MHz and frequency divider of 2

Longsampling disable, continuousConversion disabled, Hardware averaging disabled.

I am not able to understand the reason behind fluctuation in VREFH.

Please help in resolving the issue.

Re: Fluctuation in ADC VREF samples

myke_predko — Fri, 21 Aug 2020 16:23:57 GMT

Hi vaibhavi.padwal@lntebg.com‌,

Why do you consider a fluctuation of 7 units "considerable"? This is a range of 7 units (0.17% variation) or, if you're measuring in a range of 0V to 3.3V, this is a variation of 5.6mV.

Normally, these results would be considered really good for an MCU, you've clearly done a good job with your PCB layout and power suplly. Personally I think a 10mV variation to be reasonable for a well designed PCB and you've almost halved that (actually, when I look at the graph in the spreadsheet the normal variation is between 4090 and 4095 you're doing quite a bit better that the 5.6mV).

If you're looking for less variation, then you should be looking at an external ADC with much higher precision (and much higher cost).

myke

topic Fluctuation in ADC VREF samples in Kinetis Microcontrollers

Fluctuation in ADC VREF samples

Re: Fluctuation in ADC VREF samples