Making a differential scope probe, how tough?

Why not battery power, fully floating, and a fiberoptic link to the receiver box on the scope? Hmmm, maybe I should manufacture those!

John

Hi!

NO! [Scratching the old memory cells] "Medical electronics" developing an input stage for ECG recorder. The signal is in range of 0.5-5 mV on a human body noisy with about 1V line frequency voltage. You need about 8 hands to trim all the trimmers each time you blink your eye. So the "expensive?" solution was to go into microelectonic solution of first amplifier stage with the input and feedback resistors monolitic with the division ratios certified by Vishay to x/ppm. Only then the rest of the circuit became trivial. So if you can get hold of such a stage it will save you a lot of frustration. But it is your "craziness", so good luck!

"We always learn!" - a Hassidic proverb.

Stanislaw Slack user from Ulladulla.

Your probably capable of just such a thing, which would be pretty cool. Of course it would cost a bundle.

One question, does having an ill defined relationship to ground make the input behavior hard to tune?

What proportion of the typical gadget's cost is composed of regulatory compliance? Ie., UL, FCC, EMC, CE, RoHS, IEC, etc. certification costs, and/or the costs in time or consultant fees to engineer compliance?

That's the killer.

If you were happy with about 100KHz, Analog Devices and Burr Brown (TI) do some nice isolation amps like this:

Dave :)

Not really. 850 nm VCSEL lasers and silicon photodiodes are fairly cheap.

The circuit would still have a local ground/Farady cage, probably a metal box or equivalent, tucked inside a plastic case.

I wouldn't do any of that, and none are needed for the US market anyhow.

John

Sometimes 100kHz is pretty useful. I've still rarely been upset by only having a 100kHz current probe at my disposal.

It isn't sounding very encouraging about the 25MHz diff voltage probe though.

I wasn't talking about the parts cost. I suppose if you are correct about not needing to address compliance issues, then it might not be so bad.

It is a very cool idea though. Now tell me, would it make more sense to digitize each input separately, then subtract digitally, or actually implement an analog differential amp, then digitize a single ended result?

What rate of oversampling would you use? I'd like to see at least 10x if not 20x. At these speeds, it wouldn't be easy to get more than 8-10 bits. Hmm, maybe that means the analog diff front end is still needed.

Heck, even I can do an optical data link. Done one before with free space transmission off a running engine piston!

But discrete analog and high-end stuff isn't my thing.

Interesting. None are needed for the "instrument" market, or any market, such as consumer products? Are these certs all just voluntary?

Wait a minute, I thought at least FCC part 15 or something was required for anything with a clock running at more than some dismal speed?

Thanks for comments.

Are you wanting high voltage differential probe or logic level diff probe...2 very different animals. Assuming the former, it's not too difficult to do something like Tek 5200. You attenuate both inputs with matched resistors to get the voltages down to reasonable values, connect to an instrumentation amplifier and a line driver. Tek 5200 uses readily available Intersil ICs, if I recall correctly. Paul Mathews

If you're interested in measuring the source to gate voltage, you may be in luck.

If the source is a relatively low-impedance point, you can just use a single-ended amplifier, that eliminates HALF of the amplifiers and balance adjustments.

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Another way is to just buy an old Tek 465B with battery pack, hang it from the ceiling with fishing-line, and just use it with a regular scope probe, hooking the ground lead to the source. Should be less than 10pf to ground.

Regards,

George

If it's fully floating, there's no "differential" input; there's the input, and there's the return, the "ground" clip, the circuit common of the probe. And I wouldn't digitize... probably go FM, which is how the Tek scopes do it, I think.

A differential input needs *three* leads, but a scope probe only needs two.

Last time I looked, test equipment is exempt, and you'd have to get caught anyhow.

John

Just the capacitance of the enclosure to the universe will be a lot more than that. And the ground side will squirt massive ammounts of hum and noise into the circuit under test.

John

Hello Stanislaw,

ECG inputs are usually fully isolated. Have to be.

Hello John,

You could also use a couple of tiny iso transformers. One wideband for the signal, the other to drive the DC restore.

oops, you're right, a one-meter sphere is around 60pf, the Earth is around 700uF, so a Tek scope is going to be 20-40pF I'd guess.

So jsut add another kludge-- sense the source voltage, buffer it, and feed it to a screen cage around the scope. That will take care of the capacitance and the noise. Watch the fingers though.

A common-mode choke/balun can help a lot, or wrap the probe lead around a ferrite toroid a few times. That at least improves the hf cmrr a bit.

John

John Larkin wrote in news: snipped-for-privacy@4ax.com:

If you just want a floating scope input,get a TEK A6902. Or order the service manual for the P52XX and build one yourself the way you want.

Nothing to do with measuring the signal and CMRR. Safety = 50 uA ( that's my keyboard spelling micro) at line frequency can (does NOT have to ) throw the heart into pachanga dance (medically called fibrillation).

This part of medical electronics many times IS the major time consuming part of designing an intensive care room equipment. There are NO excuses.

Have fun

Stanislaw Slack user from Ulladulla.

^^^^^^ This rhymes with "sludge". It's KLOOGE! >:-[

The times I've needed differential probing, I just used the 454, with two of the same model of probes, and put the vertical on A-B. :-)

Cheers! Rich

Nope:

That works for *really* slow signals where CMRR isn't an issue.