Discussion in 'General Electronics Chat' started by kppp, Mar 29, 2011.

1. ### kppp Thread Starter Member

Apr 23, 2008
14
0
hello all,
i am using PIC12F675 ic for ADC operation. Vref=5V resolution = 2^10 = 1023
when i give input from 0V to 5V it works fine with 1023 resolution.
i.e at the output 0V corresponds to 0V and 5V corresponds to 60V (on display)
but now i have input from 0V to 3V which i want to show as 0V to 60V on LCD.
i have calculation as: input Voltage ADC count * 60V / 1023 (it gives me 60V output at 5V input) ADC count at 5V = 3F0H
but when i want to show 60Voutput for 3V input, i have 2 ways to do:
1. reduce Vref to 3V which is not possible in hardware
2.reduce resolution to 614 (for Vref=3V) so formula becomes
input Voltage ADC count * 60V / 614
ADC count at 3V = 25BH
but is it a correct method?
pls suggest me the solution.
thank you all
regards

2. ### JDT Well-Known Member

Feb 12, 2009
658
85
What about an amplifier on the input with a gain of 5/3?

3. ### Markd77 Senior Member

Sep 7, 2009
2,803
594
A couple of other possibilities:
You can use a Vref on the GP1 pin - unless you are using all the pins already
You could power the PIC from a 3.3V regulator - unless it needs 5V for some other reason.
The op amp is an option but it needs to be rail to rail capable (unless you have a higher voltage available to power it) and it has to be accurate otherwise you lose the advantage.

4. ### uchugdx New Member

Mar 31, 2011
1
0
Hello Kppp,

You might want to use OPA835 from TI to drive the ADC in the microcontroller.

This part is currently sampling.

Here is the link for that

http://focus.ti.com/docs/prod/folders/print/opa835.html

Please note that TI has a very wide portfolio of precision as well as high speed amps that work very well with such microcontrollers

Also, we have e2e community on which you can post your questions and our system/apps engineer would address right away

http://e2e.ti.com/

Thank you

5. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,789
974
kppp,
What resolution do you need? That the real first question.