PIC16F Internal Opamp

[garg29]

Friends,
I am trying to use PIC16F1789 uC's internal Opamp as Differential amplifier. I want to measure 0 to 100 Ohms resistance. I have converted this resistance to 17.8mV to 33.9mV using simple wheatstone brdige. I have tried making differential amplifier using 1Mohm & 39Kohm resistors pair. Gain should be somewhat 25. FVR set to 4.096volt. but the result I am getting is 3.19 volt & 3.65 volts. which tells the gain is coming 107.6 & 179.2 . why the gain is changing & whether I can use the internal opamp as differential amplifier or not?

set OPA1CON = 0x80;


Any help would be helpful.

Hoping for a solution.

BEST REGARDS.

 

[milan.rajik]

Internal Op-Amp can only be used as Comparator.

Sorry, I was wrong. I checked the datasheet. OPA module is what you were referring. It can be used as an Amplifier. Post your circuit. I will test it.

 

[garg29]

one more thing I have initialized TRISA = 0b00110000;

 

[milan.rajik]

Pins RA4 and RA5 used for OPA ? Are they OPAxIN+ and OPAxIN- pins ? Have you disabled any other multiplexed function on these pin like ADC, Comparator, etc... ?

 

[garg29]

yes I have disabled ADC & all Comparators. is working but voltages are not proper

 

[milan.rajik]

Please post your circuit. What is FVRCON value ?

 

[garg29]

FVRCON = 0b10000011

 

[milan.rajik]

I think FVRCON should be 0b11001100.

 

[garg29]

FVRRDY is not settable & is always ‘1’ (Note 1)
CDAFVR<1:0> is for Comparator and DAC

 

[milan.rajik]

ADFVR<1:0> is for ADC I think.

FVRRDY is not settable & is always ‘1’ (Note 1)

Where is it mentioned Not Settable ?

 

[garg29]

some useful replies please......