MPC5744:ADC floating voltage reading

[ramachandran_di]

Dear Sir,

We are connecting 5 analog sensors,4 to ADC1 and one to ADC2 of microcontroller MPC5744.We provided a pull down of 20K in all lines (To avoid random voltages on floating ADC lines )to get the floating status as 0mV ideally. But we are seeing interesting observation as below. The analog sensor configured to ADC2 channel showing 100mv voltage continuous if the sensor disconnect abruptly and voltage will change to <10mV if we reset /refresh the controller. All other sensors configured to ADC1 channel was giving<10mv when disconnected abruptly.

And we tried below approach, all 5 sensors were reading <10mv when got disconnected.

From the behavior of the ADC values observed on ADC2 channel, We are assuming that the some voltages still there in the sample-hold capacitor in the internal circuitry of the micro controller and not able to discharge it completely. And one interesting point is that this is the only ADC 2 channel configured .So we thought of configuring few other channels currently configured for ADC3 to ADC2 since most of the ADC channels are multiplexed. And that approach worked. Now all 5 sensors are reading <10mv when got disconnected.

Could you please comments on these observations. Same HW circuit is used in all the 5 analog sensors.

Thanks

Divya

[PetrS · Accepted solution]

Hi,

this is already handling through Support Portal, so just pasting my feedback here...

It should be fine to know exact configuration you have. What channels/pins are measured.
You can try to use different channel on the ADC2 or use single channel measurement on different module, if you will see the same. My guess is this is caused by high source impedance on input, so if you measure only single channel on the module internal sampling capacitor does not discharge fast enough. Anyway you can try to enable presampling on ADC2, which should help I think. Or extend sampling time if it is possible for application. 

BR, Petr