# MC56F8346 ADC showing different values when the same AC voltage given to 3 channels

Question asked by Praveen Palaparthi on Apr 5, 2017
Latest reply on Apr 12, 2017 by xiangjun.rong

Hi,

I am using MC56F8436 DSC in one of our application where I need to sense 3 phase AC voltage and current.

Nominal voltage:63Vrms on each phase

Nominal current:1A rms.

• I am using built in ADC 12 bit in simultaneous mode 8 channels are used

AN0=Phase-A voltage

AN1=Phase-B voltage

AN2=Phase-C voltage

AN3=Phase-D voltage same input as Phase-A.

AN4=Phase-A current

AN5=Phase-B current

AN6=Phase-C current

AN7=Temperature.

• In simultaneous mode AN0 and AN4 is converted first than followed by (AN1-AN5),(AN2-AN6),(AN3-AN7).

Issue is:

In my application for 2560 ADC value is equal to 63.5V rms.

When same the input is given to all the voltage channels and current channels,the ADC value seen in all three phases are different. Ideally speaking when same input is given to all the channels,all channels should show same ADC value.

Say AN0,AN1,AN2 and AN3 are shorted and applied AC voltage to these channels with offset 1.55V DC and 1.485Vrms.

note: 1.55Vdc and 1.485Vrms is equal to 63 Volts rms

I am taking 192 sample of sine wave with 50Hz freq which means I m triggering ADC at every 104 usec. Timer-C is used for this purpose.From these 192 samples I am actually calculating RMS voltage.

Final ADC value should read as 2540, but in one phase it is reading as ADC value 2543 in other phase 2545 and in third as 2547.

note: 2560 = 63.5Vrms, then 2540 is equal to 63Vrms.

I am not expecting exactly 2540 ADC value,understood that there will some variation with ideal value but at least  all the three phase should show same value say 2543 in all three phase.

Can you please help me why the ADC value seen is different in all the three phase when the same AC voltage is given.

With regards,

Praveen