Differential probes

[akis]

I was (again) in need of some and a look on ebay they are more expensive than my digital scope (I got a "cheap" OWON)!

Has anyone got experience with building their own board?

[mmagin]

What sort of things do you want to measure?  Some things to consider:

• Do you need isolation for safety?
• How much bandwidth?
• How small of a signal do you expect to deal with?
• Desired input impedance?

[DanielS]

How easy or hard, cheap or expensive that is going to be depends on your requirements.

I needed a simple 100+kHz non-isolated differential probe to do resistive current measurements and I ended up using AD629 chips with a monolithic 24V-to-2x15V DC-DC converter chip - much simpler than cobbling up my own using discrete opamps. (I did try using opamps first but had horrible offset drifts and gave up after wasting a few hours on that idea with the part I had on-hands.)

[PTR_1275]

I was looking at differential probes for the safety / isolation factor. I pretty much decided on the Pintek brand. I ended up placing a relatively lowball bid on a fluke scope meter and won it. Cheaper than a single differential probe, more portable and 100MHz.

It is a bit slow and clunky to use, and I found out when reading the manual that the 2 channels have a common common terminal, so it can't be used to measure 2 completely separate channels, but good enough for me right now. I will probably still get a diff probe in the future.

It is a fluke 105B series 2, only thing missing was the battery  (new one should be here today or tomorrow) and the rubber holster.

[akis]

What sort of things do you want to measure?  Some things to consider:

• Do you need isolation for safety?
• How much bandwidth?
• How small of a signal do you expect to deal with?
• Desired input impedance?

Up to 10MHz would be more than enough.
Smallest signal would be say 10mV because my scope already has a lot of noise of its own
Input impedance say 10M

[mmagin]

Up to 10MHz would be more than enough.
Smallest signal would be say 10mV because my scope already has a lot of noise of its own
Input impedance say 10M

I got lazy and bought a differential amplifier plugin (and it only goes to 1 MHz) for my ancient scope.  So, this is mostly speculation on my part and knowing the things I would have to investigate if I were doing this:

You certainly CAN have some amplification in your differential probe to bring up a signal above the noise inside your scope.
Provided you are only dealing with small signals and isolation is not necessary, the "easy" thing would be to just use a instrumentation amplifier IC for a few dollars, though you only get up to a few MHz at best that way.  For example AD8421 seems to be one of the faster instrumentation amps.

I think if you want to go beyond that, you're into some more sophisticated design if you want decent CMRR and wide bandwidth (down to DC). 

[sarepairman2]

you can't go wrong with the old tektronix probes.

[Someone]

You certainly CAN have some amplification in your differential probe to bring up a signal above the noise inside your scope.
Provided you are only dealing with small signals and isolation is not necessary, the "easy" thing would be to just use a instrumentation amplifier IC for a few dollars, though you only get up to a few MHz at best that way.  For example AD8421 seems to be one of the faster instrumentation amps.

I think if you want to go beyond that, you're into some more sophisticated design if you want decent CMRR and wide bandwidth (down to DC).

It doesn't seem to need much sophistication, just some well chosen parts. The tradeoff of input impedance and noise, bandwidth, and stability are interesting.

Has anyone got experience with building their own board?

I've got some boards on they way by slow post (dirtyPCBs) and will share them with everyone once its been tested.

[akis]

Just to clear things out. if I understand it correctly, "differential" probes means that they are truly floating, and that they share nothing in common with the other pair, or with the ground or with anything else on your bench. Which means you can pretty much plug them anywhere without shorting anything out. Is that correct?

[tautech]

Just to clear things out. if I understand it correctly, "differential" probes means that they are truly floating, and that they share nothing in common with the other pair, or with the ground or with anything else on your bench. Which means you can pretty much plug them anywhere without shorting anything out. Is that correct?

Correct, within their specifications of course. 

Buying a scope is always the start of further spending on probes of various sorts, just in order to use it to its full potential.
Easy to spend far more on probes than a scope itself. 

[tggzzz]

Just to clear things out. if I understand it correctly, "differential" probes means that they are truly floating, and that they share nothing in common with the other pair, or with the ground or with anything else on your bench. Which means you can pretty much plug them anywhere without shorting anything out. Is that correct?

Not necessarily. Some probes are designed for differential signals and need high speed rather than high voltage or isolation.

BTW, in addition to imput resistance, you should also specify input capacitance.

[Mechatrommer]

Just to clear things out. if I understand it correctly, "differential" probes means that they are truly floating, and that they share nothing in common with the other pair, or with the ground or with anything else on your bench.

no, they must share a "common voltage", physically or mathematically and hence "differential voltage" between them will be defined . that will determine the state of the common mode and differential voltage limit of the diff probe. if the common mode voltage rating is exceeded? the probe input fet or bjt will be screwed. if two points have very different common modes (or gnd references, ie as you have put it, "truly floating"), they will manifest in differential voltages, and if that is exceeded? the probe is screwed. even if you try to probe 2 different grounds, for eg earth's ground and mars' ground, the probe will be screwed, because the difference is way off the chart. so technically for differential probing, no 2 points are truly floated, they must be brought to one common term, usually the probe's ground, resistively or capacitively coupled to the inputs.

Which means you can pretty much plug them anywhere without shorting anything out. Is that correct?

correct, albeit the 2 signals are brought common to the probe's gnd, nonetheless at some safe impedance/coupling level, safe if the common mode and differential votage specs are not violated.