Sigma-Delta Modulator on some breadboard

Thread Starter


Joined Jan 1, 2009
Happy New Year 2009 to everyone.

I've been fiddling with a first-order sigma-delta modulator (SDM) for a while now. I can't seem to make it work correctly. It sort of half-works, I'll try to explain what I mean.

Here's the setup I have on some breadboard. However, replace the feedback comparator (U4) with an analogue mux. So Q' goes to the mux selector which chooses either +2.5 V or -2.5 V. When simulating the circuit I get this output. Blue is input and red is output.

On breadboard the output in the second image is what I should get, but instead it works for the positive cycle of the sine wave, not the negative cycle. It's kind of hard to draw, so I'll describe as best as I can. In the second image, when the sine wave is positive, the output is mostly 1s. When the sine wave is negative the output is mostly 0s. But, when the sine wave is at ~0 V the output oscillates between 0 and 1. In my case on the breadbroad, I have mostly 1s on the positive cycle of the sine wave (good!), but for ~0 V and the negative cycle I have oscillation between 1 and 0. However, it should be oscillation followed by mostly 0s.

I would appreciate some thoughts on this as I have been stuck for a long time now.

Ron H

Joined Apr 14, 2005
How are you implementing the MUX? You should also post a schematic of the breadboard, not just the simulation.