# Multiplying and Dividing Voltages

Discussion in 'Electronic Equipment' started by [email protected], Dec 15, 2004.

1. ### Guest

Hi mates.

Albert's here. I'm new here and i'm seeking your helps regarding the
following.

I'm working on a project which involves the arithmetic operations (+,
-, X, /) on voltages. Using ADC conversion is not applicable in my
case. The project is to capture 4 voltages from 4 testpoints from a
PCBA, ie A, B, C and D. After capturing the 4 voltages, we need to do
the following calculations:

Result = ((A+B)-(C+D))/(A+B+C+D)

For the addition and subtraction parts, i've got the idea of using
summing amplifier and inverting amplifier to perform the functions. If
you have other suggestion, kindly share with me. Fyi, the voltage

For the multiplying & division's parts, please give suggestion for the
circuit.
Thanks and hope to get your response soon.

Cheers,
Albert Leng

2. ### CJTGuest

Read the chapter on op amps.

3. ### Robert BaerGuest

Most especially concerning log and anti-log amps.
Hint: antilog( log(a) + log(b) ) --> a * b

4. ### Timo NeuvonenGuest

Most especially concerning log and anti-log amps.
In principle, yes. In practice, it may be difficult -or at least not
practical, depending on application.

Things to evaluate:
- in how many quadrants does the multiplier need to work (posite and
negative input voltages possible)
- how wide dynamic range is requided, 10dB (eg. 1V...3V) or maybe 80dB
(1mV...10V)
- which accuracy is required
- which temperature range is required (log amps tend to be error-prone with
varying temperature)
- required speed (~bandwith, which is somehow not-so-exact function with log
amps)

Possible workarounds to avoid log amps:
- if low accuracy and dynamics is enough, linearized model tuned to correct
operating point
- piecewise-linear approach
- digital approach (adc + digital math + dac)