Hi,

I have encountered with some issues with ADC readings.

I’m using STM8S003F3 and reading 4 of 5 ADC channels (2, 3, 4, 6) and the readings are not as expected.

For testing the ADC readings, I use STM8S003-Discovery and with a voltage divider for all channels sourced by the 3.3VDC i.e.:

The voltage for channel 0 is 2.63VDC and I expect to read from ADC 815.

The voltage for channel 1 is 1.98VDC and I expect to read from ADC 613.

The voltage for channel 2 is 1.32VDC and I expect to read from ADC 410.

The voltage for channel 3 is 0.66VDC and I expect to read from ADC 206.

(The STM8S003-Discovery (STM8S003K3) has only 4 ADC channels.)

In the main loop, I use ADC reading as single channel single conversion mode, and read each time the next channel repeatedly from channel 0 to channel 3.

This is the initialization I have in my code:

               ADC_CR1_SPSEL = 0;                     

               ADC_CSR_CH = channel_0;         

               ADC_CR3_DBUF = 0;

               ADC_CR1_CONT = 0;

               ADC_CR2_ALIGN = ADC_ALIGNMENT;

               ADC_CSR_EOCIE = 0;

               ADC_CSR_AWDIE = 0;

Which:

               channel_0 is the first channel to be read.

               ADC_ALIGNMENT is set to be ADC_RIGHT_ALIGNMENT.

However, there are two phenomena:
1. There are a big difference between the actual reading and the desire (calculate) readings.

As you can see in the attached STMStudio screen shot, there peak of difference ADC reads about ~±20 bits(!).

Channel 0 –

Peak error – 22

Average error – 13

Channel 1 –

Peak error – 9

Average error – 2.5

Channel 2 –

Peak error – 22

Average error – 13

Channel 3 –

Peak error – 10

Average error – 3.5

2. Also, there is different in error rate between channels 0,2 and channels 1,3.

I have tried several ways to get better accuracy such as lower the ADC frequency reading by changing the prescaler for 3 (error rate is ~±12 bits) and read twice the same channel each time (error rate is ~±12 bits).

Is it common error rate, isn’t should be much lower error rate?

Is there other configuration I have to configure?

Please reply as soon as possible.

Thank you in advance.