# Multiplying and Dividing voltages

Discussion in 'Electronic Basics' 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.

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

Cheers,
Albert Leng

2. ### Kevin AylwardGuest

Ah... There is an interesting circuit that will directly compute:

differential_I = Ik.(I1 - I2)/(I1 + I2)

So, let I1=A+B and I2=C+D

http://www.anasoft.co.uk/EE/circuits/calculate.gif

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

3. ### BanGuest

Albert,
As you wrote, do first the sums with 2 opamps and then you can use a
multiplier/divider to do the division. A very good unit is the AD538 and
also AD539 or MPY100, but these are expensive. For a more economical unit
have a look at the AD633, MLT04 and MPY634. I found the SSM2164 useful and
use it in many of my designs, even if it requires some tricks to do a
temperature and DC-stable division.

4. ### John PopelishGuest

Here is a link to Analog Devices multiplier/divider page:
http://www.analog.com/en/subCat/0,2879,773%5F862%5F0%5F%5F0%5F,00.html

5. ### Bob EldredGuest

What is the accuracy required and what is the bandwidth of the signals? I
know you have said that you don't want to go the ADC route, however than may
give you the best performance for the money. I would explore using a
microprocessor with built in ADC's such as the PIC 16F818 or something
similar and couple the eight bit output to an inexpensive eight bit DAC to
regenerate an analog result. That processor can run at 20MHz and can accept
up to eight analog inputs, you need four. Once converted, the resulting
digital signals would be added and subtracted in software, a trivial
process. An 8 or 10 bit division algorithm in software would be used to
develop the quotient and the result outputed as an 8 bit signal to a digital
port on the processor. That would feed a DAC chip if required. Without
actually writing the code, I'm sure it could be done in a couple of hundred
steps and could easily run it a PIC at a 25KHz rate or so. A processor can
also do other magic tricks like signal averaging or filtering, add or
subract offsets and other things like auto zeroing to improve signal
quality.
Bob

6. ### SlowOnTheUptakeGuest

Couldn't you use op amps for log and anti-log curcuits to accomplish
the MULT and DIV?

7. ### Bob EldredGuest

Yes you could. These circuits are tricky and need to be temperature
compensated, but would work. Another method is pulse height pulse width.
Bob