ADC in a micro - actual use of -Vref

[gophert]

Thanks Ron for the details



Maybe around 20 years ago I've seen one literally melting on a protoboard. Brave, it worked until the last moment.

You can also use an op amp configured as a differential amplifier. A 2.9V reference.

As shown the op amp will output 0 to 4.0v with your input range to 3.7V max. Then set internal voltage ref to 4.096. You get almost the whole range of ADC at 1mV/bit on 12 bit ADC.

Be sure to use an op amp with output to ground.

 

[large_ghostman]

You can also use an op amp configured as a differential amplifier. A 2.9V reference.

As shown the op amp will output 0 to 4.0v with your input range to 3.7V max. Then set internal voltage ref to 4.096. You get almost the whole range of ADC at 1mV/bit on 12 bit ADC.

Be sure to use an op amp with output to ground.

View attachment 97131

Sorry I am a bit lost here, his chip is 10 bit ADC and the ref is VDD or external, as far as I know it cant be set to a value unless using VDD or external ref. OR I am missing something??

 

[granddad]

ATF .. If you need 1mV resolution my vote is for Mike+ post #6 would be the way to go given the PIC18F 10 bit ADC and no internal +ref. see no reason why you could not use one TL431 for both +Vref and -Vref .

 

[large_ghostman]

ATF .. If you need 1mV resolution my vote is for Mike+ post #6 would be the way to go given the PIC18F 10 bit ADC and no internal +ref. see no reason why you could not use one TL431 for both +Vref and -Vref .

There is an internal +REF but its tied to VDD, its important to point that out as it needs to be set. If he is going to use one external REF then why not 2? Even with a opamp many packages have 2-4 opamps in a single package

 

[be80be]

Wrong pin i removed post

 

[atferrari]

To all that replied, thanks.

I will try two stable references, most probably two TL430 or 431 and will apply oversampling, considering that the ADC in the 18F4431 it is just 10-bits resolution.