ADC: first injected channel creates voltage peak

[wolfgang23]

Posted on July 22, 2014 at 10:53

Hello!

I have a problem with the ADC on my STMF4Discovery Board.

I am reading two analog voltages with ADC1 in triggered injection mode.

The voltage on the pin with injection rank 1 always has a peak at the injection event. Even when i change the ranks of my two channels, the one with rank 1 always has the peak.

Here are some Scopes:

CH1 (Yellow) is ADC_CHANNEL_8

CH2 (Cyan) is ADC_CHANNEL 15

Here ADC_CHANNEL_15 has rank 1:

Here ADC_CHANNEL_8 has rank 1:

Do you have any idea how i could fix this?

Thanks in advance!

#adc #noise #injected #discovery

[wolfgang23]

Posted on July 23, 2014 at 10:07

Also, the first ADC channel always has more noise on it.

[ivani]

Posted on July 23, 2014 at 13:58

What you see is the charging process of the sampling capacitor of the ADC. As the two channels has almost the same voltage the capacitor is almost charged before the second conversion.

The noise measured in the first channel is most likely due to insufficient sampling time. Try to increase it in the corresponding SMPR register of the ADC.