LMC7660 + 16f877 Ref Problem

[Chris]

Ok i have a lmc7660 hooked up supplying -4.8 volts to a pair of 1na116pa
instrumentation amps, i'm also bleeding off that line to the -ref on the
pic, problem is if i measure the voltage on the output of the lmc7660 i
get .8 volts if i disconnect only the pic end i get -4.8 volts, whats
up? is the pig draining too much? how can i get around this???

Thanks

 

[dont know]

Ok i have a lmc7660 hooked up supplying -4.8 volts to a pair of
1na116pa instrumentation amps, i'm also bleeding off that line to the
-ref on the pic, problem is if i measure the voltage on the output of
the lmc7660 i get .8 volts if i disconnect only the pic end i get
-4.8 volts, whats up? is the pig draining too much? how can i get
around this???

Thanks

you answered your own question. Why are you connecting a negative
voltage to a PIC?

 

[Chris]

Connecting the neg voltage from the lmc7660 to vref- on the pic isn't
this were i should be grabbing the neg reference???

 

[dont know]

Connecting the neg voltage from the lmc7660 to vref- on the pic isn't
this were i should be grabbing the neg reference???

I will let any "pic" expert answer that

 

[John Popelish]

Connecting the neg voltage from the lmc7660 to vref- on the pic isn't
this were i should be grabbing the neg reference???

The PIC reference inputs are constrained to be at or between the
supply rails. If you apply a voltage outside the rails, protective
diodes between the input and the supply rails become forward biased.
This limitation also applies to all the analog inputs and logic inputs
to the PIC. You can have an analog section that deals with voltages
outside this range, but you will have to convert any signals that are
to go to the PIC so that they remain between its rails.