[PIC] Differential adc in pic16f1786

[Dinuwilson]

i need to read -2v to +2v voltage range. i am using pic16f1786 12 bit adc in differential mode.

is it possible to do with this in this micro controller

i am using 2 v +vref from FVR buffer in the micro controller

i am confused about -vref. what it mean? should i need to use this for my application?

for testing i am configured +vref from fvr 2v reference and -vref as ground. when i check the value from the adc, if 1 put 0 mv from my differential input, adc output is 25 count when i increase +1mv,+2mv,+3v.... count increase gradually up to 4096. so i can trace up to 2v

same configuration when i put -1mv my adc count is 23
-2mv '' 20
- 3mv '' 17

-11mv '' 1

when i put -12mv ,-indication of the adc ADRESH register get activated , i am using 2's compliment method

so up to -11mv negative indication of the ADRESH is not activated (showing as in +ve side)



please help me to solve this problem. i tried a lot , but i can't find out anything....

 

[jayanth.devarayanadurga]

You can't give -Ve voltage to ADC pin. ADC will get destroyed. Vref- is for lower reference voltage and Vref+ is for upper reference voltage. If Vref - is 2V and Vref+ is 4V then ADC range is from 2V to 4V so, 2V adc gives 0 reading and for 4V it gives 1023 if 10 bit adc. Also between Vref - and Vref+ there should be a min difference of 2.5V. Shift your voltage so that -2V becomes 0V and 2V becomes 4V and try.

 

[Dinuwilson]

thank's for our replay....+vref and -vref i understand.. is any chance to read the -values in pic16f1786? in datasheet +4096 to -4096 they are saying,that's why i am confusing

 

[jayanth.devarayanadurga]

ADNREF: ADC Negative Voltage Reference Configuration bit
1 = VREF- is connected to external VREF- pin(1)
0 = VREF- is connected to VSS

ADPREF<1:0>: ADC Positive Voltage Reference Configuration bits
11 = VREF+ is connected internally to FVR Buffer 1
10 = Reserved
01 = VREF+ is connected to VREF+ pin
00 = VREF+ is connected to VDD


Negative reference voltage means not actually negative but if your Vref+ is 4V Vref - is 2V, then 2V is less positive than 4V. It is lower reference.

 

[Dinuwilson]

ok..i got it sir,,, thank's. if i need to read negative voltage levels what i need to do?

 

[jayanth.devarayanadurga]

You have to shift the voltage using a Clamper. Shift the level 2.5V up so that -2V becomes 0.5V and 2V becomes 4.5V. It should not exceed 5V. If it exceeds then you have to scale the voltage. Are you measuring AC or -ve DC?


If AC is given then you can use this circuit.


Set Vref+ to 4.2V and use this

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Code C - [expand]
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if((val >= 0) && (val <= 1950))val = (val * (-0.001026));
else if((val >= 1950) && (val <= 3900))val = (val * 0.001026);

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[Dinuwilson]

ok......i wiill try it out...sir

 

[jayanth.devarayanadurga]

See attached files. Change freq of AC to 50 Hz in Proteus. You have to set it in VSINE.

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Code C - [expand]
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//10 bit adc, Vref+ = 4.2V
 
adcval = ((ADC_Read(0) / 243.0) - 2.0);
 
//12 bit adc, Vref+ = 4.2V
 
adcval = ((ADC_Read(0) / 975.0) - 2.0);

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