Differential probe construction

[VMI]

Hi,
I looked around but nowhere I whas able to find a schematic of a
differantial probe.
It's main use is to measure with a scope on smpsu's and the like.
I know we can remove the ground lead from the scope, but where then is the
protection left in the case of...
Anny ideas or info?
Thanks,
Marco

 

[Bill Janssen]

VMI wrote:

Hi,
I looked around but nowhere I whas able to find a schematic of a
differantial probe.
It's main use is to measure with a scope on smpsu's and the like.
I know we can remove the ground lead from the scope, but where then is the
protection left in the case of...
Anny ideas or info?
Thanks,
Marco




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

Bill K7NOM

 

[mike]

VMI wrote:

Hi,
I looked around but nowhere I whas able to find a schematic of a
differantial probe.
It's main use is to measure with a scope on smpsu's and the like.
I know we can remove the ground lead from the scope, but where then is the
protection left in the case of...
Anny ideas or info?
Thanks,
Marco


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:
http://nm7u.tripod.com/homepage/te.html

mike


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Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[VMI]

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

 

[mike]

VMI wrote:

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?

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

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

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/