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Proteus simulation was ok, but my PIC is broken!

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M

Mago-Anthony

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877 VREF QUESTION.

HI,

I HAVE DONE AN ADC CONNECTION OF THE +VREF PIN TO +12V TO EXTEND THE ADC RANGE (SCHEMATIC SIMILAR TO SAMPLE OF PROTEUS). THE SIMULATION IN PROTEUS WAS OK, BUT IN THE CIRCUIT MY PIC IS BROKEN! :( WHY IN PROTEUS HAS DONE THIS EXAMPLE IF WITH OVER OF 5V TO +VREF PIN PIC BROKE?
THANKS TO ALL
 

Re: 877 VREF QUESTION.

Proteus simulation is only a taste of how may your design run in the real world. but somethings are different in practice. for example there is no bouncing in buttons in proteus.

proteus simulated your design theorotically and it ran ok but in the real world you can't use vref voltages above the VDD level.
 

Re: 877 VREF QUESTION.

All PIC inputs don't support voltage above VDD and below GND !!!

It's the same thing with all ICs. Don't never apply voltage outside the supply range on any input.


CD:)
 

877 VREF QUESTION.

What you should do instead is to connect two precision resistors as a voltage divider at your ADC input. This is the method used in all digital voltmeters. You can use this method for voltages much bigger than 12 V if you like, just select the right combination of resistors. Also make sure to use a proper impedance, voltmeters usally have 10 Mohm impedance, this should work in most cases.
If you need high precision without using high precision resistors, you can connect a precision voltage reference to the input of your devices and then make a calibration routine in your pic software.
 

Re: 877 VREF QUESTION.

When trying the solution ME has suggested you must use an opamp as a buffer because the ADC inputs of a PIC have a very low impedance (see datasheet only a few 10 KOhms).

This is absolutely necessary unless you use a very low impedant voltage divider .

best regards
 

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