Doubt about PIC16F877 ADC reference voltage

[UroBoros]

PIC ADC ref Doubt!

hai
I have configured ADC in PIC16F877 for 10 bit mode
Ref given 5 volts. input signal applied 2.5 volts.Got ADC read as 512 .Ok

then I applied Ref as .8 volts and input supplied as .4volts
now the ADC shows 1200 instead of 512 which I (!!) expected.
Why ? With only Vref+ used cant i use any ref voltage from 0 to 5?
Please

Picstudent

 

[M!k]

Re: PIC ADC ref Doubt!

Take a look at the datasheet, electrical characteristics. There's a minimum voltage for VREF specified:

VREF = 2.0V ... VDD+0.3V Absolute minimum electrical spec. To ensure 10-bit accuracy.

Mik

 

[UroBoros]

Re: PIC ADC ref Doubt!

Hai
Thank you very much for pointing that .Got it.
Something is not clear for me .that is the range of Vref-!
To increase the resolution can I use Vref+ as 2 volts and Vref- as 1 volt?
or Vref- should always be - voltage? (less than 0)
thanks again
Picstudent

 

[C-Man]

Re: PIC ADC ref Doubt!

Hai
Thank you very much for pointing that .Got it.
Something is not clear for me .that is the range of Vref-!
To increase the resolution can I use Vref+ as 2 volts and Vref- as 1 volt?
or Vref- should always be - voltage? (less than 0)
thanks again
Picstudent

The difference between Vref+ and Vref- (when both are used) should be around 2.5 better 3 volts. Vref- can not be less than 0 volts, vref+ can not be more than VCC.

hope this helps

 

[UroBoros]

Re: PIC ADC ref Doubt!

Hai

That cleared the things up .Thank you.
So by no way in a PIC I cannot increas the resolution by trimming that span of input to just 1 volt or .5 volt?

thanks again
Picstudent

 

[C-Man]

Re: PIC ADC ref Doubt!

Hai

That cleared the things up .Thank you.
So by no way in a PIC I cannot increas the resolution by trimming that span of input to just 1 volt or .5 volt?

thanks again
Picstudent

I tried this many times but when you decrease span below around 2.5 volts you get a lot of problems with all PIC architectures. Have tried it with 12 16 and 18 PIC's.

Another big problem is that the sourcing impedance must be below 10 KOhms and in some PIC's below 2.5 KOhms to get the full 10 Bits resolution.

It does help if you place a 100nF capacitor directly at the pin of the PIC which is used as an ADC input.

best regards