topic MK20FX512VLQ12 ADC1_SE11 problem in Kinetis Microcontrollers

MK20FX512VLQ12 ADC1_SE11 problem

tejasgawade — Wed, 12 Apr 2017 11:12:27 GMT

I am facing ADC accuracy issue with MK20FX512VLQ12 device. This problem is specific to ADC1 SE11 and SE10 channels. Other channels on same adc are working fine. Tested and confirmed on multiple boards.

Output of opamp is connected to adc so matching should not be issue.

- Processor expert generated code is used with 1.8uS conversion time, 12bit mode, high speed conversion enabled and normal mode. Expected ADC count is 345 where as i am getting 195-200 count.

- I don't expect issue with pcb as same opamp output gives expected count when connected to other adc channels.

- I have tried different sample time setting but same error.

- But when conversion time changed to 7uS or higher, adc works fine. also observed that connecting oscilloscope probe corrects adc count.

Will different port structure on mcu cause this issue?

Re: MK20FX512VLQ12 ADC1_SE11 problem

xiangjun_rong — Mon, 17 Apr 2017 04:16:26 GMT

Hi, Tejas,

I am sorry for the delay.

Regarding your ADC issue "Expected ADC count is 345 where as i am getting 195-200 count.", I suspect that your analog conditional circuit maybe has issue. As a test, if you connect VDDA or VSSA to ADC1_SE11 or ADC1_SE10, what is the ADC samples?

Do you connect the OP AMP output to the ADC1_SE11 directly? if it is the case, what is the OP AMP output impedance? I suspect that the OP AMP output impedance is too high, which leads to the fact that the ADC Sample/Hold can not be charged enough, so the ADC samples is less than expected. How about using a analog buffer to reduce the OP amp output impedance.

Hope it can help you

Xiangjun Rong

Re: MK20FX512VLQ12 ADC1_SE11 problem

tejasgawade — Thu, 20 Apr 2017 12:44:29 GMT

Dear Xiangjun Rong,

Thanks for reply. Please refer below image for opamp signal conditioning.

Impedance value is not specified in opamp datasheet but generally it should be 50 to 200 ohm.

Please note that same circuit works fine with other adc channels, which is strange.

Initially i also thought its sample time issue and tried to increase it but no success. but when i increased over all conversion time (>7us) it started to give correct count.

I suspect issue is with adc clock frequency & specific adc channel port structure.

Regards,

Tejas

Re: MK20FX512VLQ12 ADC1_SE11 problem

xiangjun_rong — Fri, 21 Apr 2017 05:55:17 GMT

Hi, Tejas,

From the schematics of ADC part, I think it is okay from analog conditional part, the OP AMP output impedance is low, it is okay.

I think the ADC clock has impact on the ADC accuracy, Maybe the ADC clock is not proper, pls refer to the 6.6.1.2 16-bit ADC electrical characteristics in data sheet for the ADC clock frequency.

Hope it can help you

xiangjun Rong