How to make adcin stable?

[Charles Linquis]

You can connect Vref to Vdd, but it is easier just to set ADCON1 bits 5&4 to "0" which connects Vref to Vdd internally.

Also, READ YOUR DATASHEET. Here is a cut and paste directly from the A/D section of the 18F8722 datasheet:

The
maximum recommended impedance for analog
sources is 2.5 kΩ.

Other chips have similar recommendations.

Your 350K is WAAAAAY too high! If you really need a 350K pull-up, you will
have to buffer the analog signal with an op-amp to get accurate readings.


If you just want to sense the presence of an object, you would be better off using a digital input, and connecting the collector of your phototransistor to 5V through a 10K or 22K resistor.

[Ioannis]

I would suggest to use an opamp as a unity gain buffer befor feeding the ADC of the PIC.

As Charles stated 350K is too high for the ADC to work reliable.

Also to be sure that the PIC is working OK, use a trimmer in the place of the phototransistor and check the results. If they are stable and seem expected then you can be sure that PIC is OK. Of course it would be a very good idea to average the samples. Have a look at Darrels averaging routine at his site:

http://www.pbpgroup.com/modules/wfse...hp?articleid=7

Ioannis

[rogerroger]

Can anyone tell me why I am getting 16 bit results from this code? I expect at 5 volts in I should see 1023 but I see 65535.I am using a 16f616.



DEFINE ADC_BITS 10
DEFINE ADC_CLOCK 3
DEFINE ADC_SAMPLEUS 50
TRISA = 255 ' Set PORTA to all input
ADCON1 = 2 ' PORTA is analog
ADCIN 0, A0 ' PORTA.0 PIN 13

[Samoele]

change your ADCON1 =2 with

ADCON1=%10000010

[rogerroger]

Thank you for your reply. I made that change, I am now back to 8 bit results. I would like to have 10 bit results.

Thanks

[Samoele]

your variable A0 by ADCIN must be defined as word and no as byte

[rogerroger]

Thank you so much for your help!

This is all of the code,

A0 VAR WORD
DEFINE ADC_BITS 10
DEFINE ADC_CLOCK 3
DEFINE ADC_SAMPLEUS 50
TRISA = 255 ' Set PORTA to all input
'ADCON1 = 2 ' This gives me 16 bit results
ADCON1=%10000010 'This gives me 8 bit results
ADCIN 0, A0 ' PORTA.0 PIN 13
SEROUT PORTC.5,6,[" A0 = ",#A0,10]