negative V into A/D on 16F877?

[JimS]

16f877a adc negative vref

Hi,
I'm trying to read in a pH sensor that outputs anywhere from +1.6v to -1.6v by using a PIC16F877's A/D converter. Unfortunately, I can't figure out how to deal with the negative voltage range. Can I just apply a -1.6v to RA2/AN2/VREF- and have the ADC give me my 10bits scaled from -1.6v to +1.6v?

thank you

 

[sc/mp]

16f877 quale vref max

No you can't

You must shift voltage before apply it to the pic.
Use an AOP to sum the PH voltage with a constant one.
You can also use two resistors so that the middle point
is always > 0V

 

[Sobakava]

It may burn adc of pic for higher voltages.

 

[RegUser_2]

Isn't the PIC ADC a capacitive type?

If so it'll accept negative inputs to the ADC and you will have to tell (program) the ADC that you want bipolar input.

 

[sc/mp]

Here are max ratings voltage for 16f876 :

 

[ebenni]

here is another method,

 

[KamW]

Why not use any single supply, 5V opamp configuration, with 2.5V ref. so -1.6 -> 0.9, 1.6 -> 4.1

regards
KamW

 

[bunalmis]

PIC ADC input voltage must be in 0 to Vcc range.
You can use voltage adder using single opamp, If you want to measure negative voltages.

If your input signal levels -1.6 to +1.6v you can add 1.6v
now your range 0..3.2v and you can apply the pic.