Voltage dip at ADC pin of mcu

[praveenrawat]

Hello Sir ,

I am facing a issue with my design 

problem statement:-I am converting a voltage from 12V to 4.7V with the help of zener diode

combination ,and the voltage i am reading directly to S32KMCU 

But once i am conncting the trace to the ADC pin for measurement of voltage 4.7V i am not getting the voltage excetly 4.7V i am getting around 1.8V

and Once i disconnect the trcae to the MCU i am getting expected voltage 4.7V

can you please suggest possible solution

[PavelL]

Hello,

Can you share the exact MCU type and the pin, where is 4.7V connected?

If the supply voltage of the connected pin is lower than 4.7V, then the voltage will be limited to the supply voltage. Or the pin is not in highZ state, it might be in a logic low level. You can check that as well.

Best regards,

Pavel

[praveenrawat]

MCU is S32K341,PIN is PTA15/ADC1

the ADC VREF  supply voltage is 5V.

But I have not powerd the MCU .I have just connected the trace to ADC pin for measurement .

[PavelL]

Hi,

in that case, the MCU is powered from the pin (via diode between pin and VDDx) and the voltage drop is a logical consequence. If I were you I would try to avoid this behavior.

Best regards,

Pavel

[praveenrawat]

Okey noted

So how will you connect the 4.7V to MCU ADC pin in order to not get dip in voltage 

1:- using buffer circuit 

2:-using a voltage dividers 

Any suggestions from your side

Regards 

Praveen 

[PavelL]

Hi,

please look into attached AN4731 - it's been created for older MCUs, but the information are still valid. If i were you I would use a resistor to limit the current and/or a switch controlled by MCU. 

Best regards,

Pavel

[praveenrawat]

Noted

Thanks for your support