Multiplying and Dividing voltages

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

Kevin Aylward snipped-for-privacy@anasoft.co.uk

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.

Here is a link to Analog Devices multiplier/divider page:

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

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

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