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MPC5744:ADC floating voltage reading

Question asked by Divya R on Mar 6, 2020
Latest reply on Mar 9, 2020 by Petr Stancik

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.