Having accuracy problem with PIC16F877A ADC

Hi,

I am having accuracy problem with PIC16F877A ADC. I am using 2 ADC channels for analog signal input and switching one after reading value on other channel continuously in ADC interrupt. I have set Vref- on 1Volt and Vref+ on 3 references are fixed and showing no variations. after scaling the readings i am putting result on seven segment displays and pwm channels for each analog input. both channels giving variation in reading of around 2~3BIT LSB at fixed input signal. I am using dc signals for input.

Thanks

A reading that varies by 2 or 3 represents just 6mV at the Vref voltages given. Getting DC voltages that are ripple free is very difficult especially in a digital environment. I suspect it is simply noise. Try putting 100nF ceramic capacitors on the two input pins.

Another possible source of error is not allowing sufficient acquisition time.

Mike.

Pommie said:

A reading that varies by 2 or 3 represents just 6mV at the Vref voltages given. Getting DC voltages that are ripple free is very difficult especially in a digital environment. I suspect it is simply noise. Try putting 100nF ceramic capacitors on the two input pins.

Another possible source of error is not allowing sufficient acquisition time.

Click to expand...

I am using 20MHz clock,TAD = 64Tosc, giving 300us delay before channel switching and giving 300us after channel selecting which I think is enough time for acquisition. I have tried 100nF caps as well

How fast do you need to sample... If its only 4 / 5 times a second then you can oversample.... read and sum the ADC 24 times.. then dividing by 6, will give a reasonable 12 bit precision.

10 times a second is desirable. you mean take average of 24 samples stored in array?

What is your source impedance?, switching channels can be slow, as the sample and hold has to charge/discharge, which is dependent on the source impedance. If it's switched too fast, then the higher one reads low, and the lower one reads higher.

I am using LM324 opamp buffer for feeding the adc channels

How are you feeding your PIC with the 1 V and 3 V?

I have found that the load on Vref+ is quite bad during sampling. A capacitor of 10 µF rather than 10 nF improved stability.