# Difference circuit not acting as expected

1. ## Difference circuit not acting as expected

Hello,

I am trying to create a simple difference circuit using an op-amp (X-Y), but the output is not what I expect. The figure above is a simplified block diagram of the circuit. I am feeding two signals into a stand alone phase comparator, and I need to subtract the two output. In this case, R1 and Rf are equal at 1K for a gain of 1.
When I test the difference circuit alone, at DC it works as expected using variable power supplies. However, when I hook up the difference circuit into the circuit above, i get a strange result.

Ignore the yellow trace. The blue trace is the probing the D output directly out of the phase comparator. The Purple trace is probing the U output directly out of the comparator. The red trace is the Oscilloscope's own math function performing D-U. The green trace is the output from the difference circuit following the phase comparator. The math function (when not zero) calculates a value ~.75V. However, the output from the difference circuit is so small, and has such a large DC offset (>3V), that just to see any difference computation happening i had to AC couple the signal and move to 50mV / division. Why do you think my difference circuit is behaving this way?
Just a note on the phase comparator. The part number: MCK12140DG. Perhaps I should be buffering the inputs?

Sami

•

2. ## Re: Difference circuit not acting as expected

You don’t really give us a lot of information. Like WHAT your opamp is—Don’t you think that’s an important bit of information? And Power supplies? And you know that ECL needs to be terminated, right? I don’t see that.

•

3. ## Re: Difference circuit not acting as expected

Hi,

It seems you work with digital signals. If so, then use ICs designed for digital operation.
(An OPAMP is designed fir analog signals and thus not the best choice for digital signals. Expect distorted rise fall signals, ringing, delays caused by saturated input and/or output state)

Use digital logic ICs and/or a comparator.

Klaus

•

4. ## Re: Difference circuit not acting as expected

Looks like single supply OP in saturation.

--[[ ]]--