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[DPRG] Frequency-Voltage converter

Subject: [DPRG] Frequency-Voltage converter
From: Chuck McManis cmcmanis at mcmanis.com
Date: Fri Jan 30 17:00:02 CST 2004

Forgive for being dense, but what is it you are trying to do? Is it
         a) Measure capacitance ?
         b) Change a frequency to a voltage ?

There are F2V (and V2F aka VCO :-) chips, the F2V chips are sometimes 
confuse by harmonics and so its somewhat difficult to design accurately 
with them in a generalized sort of way. If you're building an FSK 
(frequency shift keying) modem they are great.

However, generally most of the things F2V chips used to do have been 
converted into a half dozen lines of DSP code and so most folks never use 
them any more. They used to be sold by Radio Shack ;-).

If you are measuring capacitance then you could do a lot worse than simply 
using an 8 pin ATMEL or PIC chip to measure the rise time of a fixed 
voltage. Both PICs and ATMEL chips have analog comparator versions that 
will switch at a specific voltage. The classic circuit is something like:

                  +---> Comparator Vref/2
          Rc      |
Vref---\/\/\/----+--->pic I/O
                  |
      Test Cap  =====
                  |
                Gnd

Basically you create a current source out of Vref/Rc (Rc is the calibration 
resistor). The I/O pin is used to ground the system (set output to 0) which 
fully discharges the capacitor, then you tri-state the I/O pin and start a 
timer, when the comparator triggers you have the delta-t to get to Vref/2 
and you know the current flow so you can compute C.

Alternatively you can generate a frequency sweep with the PIC feed into an 
R/C divider where you know R and C is unknown. At the frequency where the 
impedence of C == R the voltage of the output will be 1/2 the input. Now 
you know f and R so you can compute C.

--Chuck
At 01:03 AM 1/30/04 -0800, Sanjay Dastoor wrote:
>Hi all,
>
>I'm hooking up a circuit which requires me to measure capacitance - the 
>easiest method I've found is to use an astable 555 timer circuit which 
>outputs a square wave whose frequency is proportional to capacitance 
>(that's a mouthful to understand..sorry if it's dense).  Anyhoo, does 
>anyone have experience with chips that convert frequency to voltage?
>I've heard fairy tales of such chips, but can't seem to find any good info 
>on them.
>
>Sanjay
>
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