Problem with a voltmeter: the output is not stable

[lats]

Voltmeter

hi
m designing a voltmeter in which i m using pic16f877 (Ref: ). the adc in it is a 10 bit adc.m using it to show the volatge level from 0-1023 volts and 50 hz ac frequency. I have fed and ac input signal and a voltage divider network along with a peak detector having opamp LF353. the problem is that the output is not stable.there is fluctuation in the volatge on the lcd screen whereas there is no fluctuation in power line.m attaching the diagram.

Regards

 

[manenef]

Re: Voltmeter

Just check on the sensitivity of your ADC, otherwise in some ADCs just a small noise signal will cause it to register some values!

 

[lats]

Voltmeter

thanks but if adc is sensitivite how to make it correct?m asking that ,i know there is some sensitivity problem but wht and how can it be corrected

 

[manenef]

Re: Voltmeter

Some ADs have a pin whereby you can connect a a nulling restistor to reduce sensitivity. Please do chech on the manual of your specific ADC to get infomation on how to reduce sensitivity.

 

[lats]

Voltmeter

As i have mentioned in diagram i'm using PIC1f877's internal ADC, and i can't find such a pin in that, if u have some other solution then tell me.

 

[IanP]

Re: Voltmeter

If you can, connect 5V from voltage regulator (through 1-10kΩ resistor) directly to the ADC pin and check if the display is stable ..

Next, you can do two things around the peak detector:
- increase the value of the cap from 0.47µF to, say, 2.2-4.7µF (tantalum),
- add R-C filter between the output of the peak detector and ADC (try ≈1kΩ and 22µF tantalum) ..

Regards,
IanP

 

[lats]

Voltmeter

Thanks Ianp for replying. Actually if i take the calculated values of voltage divider resistors, the display shows almost 5-10% less value than actual voltage, if i change the resistors values then it shows correct value but the circuit losses linearity. why is it happening so?