Differential probe construction

If you have one of the newer two channel scopes that feature is built in.

Bill K7NOM

Biggest problem is getting enough common mode rejection and sensitivity. A dual trace scope usually has a subtraction feature, but you have to keep the whole input inside the dynamic range of each channel. Then when you subtract them, you get the thing you really want to look at, but it's really small. And even excellent CMRR at the scale factor you need to use ain't enough by the time you subtract them.

If you try to crank up the gain, the individual channels saturate and make a mess of things.

Even if the scope has super CMRR, you still need perfect balance in the probing system. Making a homebrew differential probe will be an exercise in futility.

What you really need is a probe that's isolated so you can hook the "ground lead" of a single channel on something that's not really ground. But wait, such a thing exists. And I even have one for sale here:

There's a Tektronix A6902A for sale at:

mike

Hi Mike, Thanks for the info. It's still not what I'me looking for: Need to be used on SMPSU's and Power amps in bridged mode. The use of 2 channels is not such a bad idea, but what if you forget that you may not connect the ground? It happens faster than one should think (I know...) So, a probe that can easely connected to such a load and that do not cost to much. A schematic is preffered. Thanks, Marco

You have the same problem when you drive your car. If you forget to keep it on the road, you will have a crash. Remove the ground leads and you won't have anything to connect to ground.

You can also buy differential input plugins that use two probes. They're much better than two separate amplifiers, but still have resolution problems if the dc level is much greater than the amplitude of the signal.

mike