Multiplying using opamps?!?!?!?!

Im doing a project where I need to convert between various types of units, namely litres, gallons (US) and pounds of fuel. I have a PIC chip working it all for me very happily, but however, I have to look at different options. I was wondering if anyone could help me by informing me about if and how opamps can be used in this way, or indeed if there are any other ways of doing it.
If you have any nice circuit diagrams could you email them to me??

(danddan1101@hotmail.com)

Ta.

You're probably going to regret asking this question :lol:

To multiply with an opamp is simply the gain of the opamp. So to multiply by two, give the opamp a gain of two! - as easy as that!. You just need to set your gains to the correct values.

Opamps were designed for use in analogue computers, a google search will probably find you plenty of details.

that is good for a fixes multiplcation factor

ie x*G and G is fixed

but for x*y as imputs to an OPAMP, there is two ways of doing it

1) convert both input analogue voltages to the log of (via a log amp) then add them together
since x*y = log(x) + log(y).

Then inverse log the result to get the answer.

However, this relies on three log amps with exactly the same characteristics (ie three transistors in the feedback path of three opams with the same characteristic) - not likely


2) Via PWM. Use one signal (say x) to define the the peak voltage. Use y to define the mark-space duty.

Then integrate the output and you have the answer of x*y

both results are very crude. Multiplying two singals together is not what Analogue is good at unfortunetly

Another possible method that's half analog, half digital is a multiplying DAC. These parts let you multiply an analog voltage by a digital value. Its basicly just a programmable attenuator.