Issue with ADC leakage

[MFaro]

I using stm32F4 discovery board. I am having issues with the ADC. The voltage signal changes after I give that signal as Input to ADC. This may be because if leakage current of Analog Pin.

Can you guys suggest the way-out from this.

Regards

Musab

[Danish1]

With the information you've supplied, the only suggestion we can give you is "buffer your analog signal with e.g. an op-amp".

But maybe, if you give us more information, we could help you better.

1) Are you considering DC levels or AC + DC? And if AC, what is the highest frequency present in your signal?

2) What impedance is your analogue voltage signal?

3) What voltage level do you see if it is not connected at all to the discovery board?

If it's outside the range 0 to Vdd, then you need to bring it into that range.

4) When precisely does the voltage change: When you wire the connection to the discovery board, when you select Analog mode on the pin, when you tell the ADC to use that pin as its source, when you start an ADC conversion.

5) What does the voltage change to? (If, when you connect to a pin, the pin is configured as digital output, one would expect the digital output to pull the level).

Help us to help you,

Danish

[Ozone]

> This may be because if leakage current of Analog Pin.

This "leakage" is most often caused by insufficient understanding of the working principles of a multiplexed SAR ADC.

If you configure the sampling time too short (and/or have a too high input resistance), remaining charge of the S&H capacitor will "leak" to the next channel.