Scope Probes Teardown and Diff Probe Investigation - Page 1
Forum section: electronics / general

author: kevyk
author_rank: Contributor
timestamp: July 03, 2011, 03:31:24 am
content: Since scope probes are so critical to oscilloscopes, and some probes often cost MORE than the damn scopes themselves (high freq differential probes for eg), it would be great to:1) Do a teardown of a few kind of probes like FET and differential2) Show why differential probes cost so much and what can be done to mitigate this (like design your own)

author: tekfan
author_rank: Frequent Contributor
timestamp: July 03, 2011, 01:37:04 pm
content: Here's a few ideas on probes and HF techniques from the late Jim Williams. Go trough the entire app note. You won't regret it.General discussion on different types of probes on page 69 and onward.A nice DIY fet probe is on pages 96-97.http://www.mit.edu/~6.331/an47fa.pdf

author: kevyk
author_rank: Contributor
timestamp: July 04, 2011, 01:32:48 pm
content: Excellent paper on all things scope probe-y. Thanks! Helps you understand the importance of how the scope and its probe(s) are actually PART of the circuit you are testing, not outside of it. Seems to ring a bit of the Heisenberg uncertainty principle.It would be great to do some Diff. probe tear downs still (up for it Dave?).

author: Unknown
author_rank: Unknown
timestamp: July 04, 2011, 09:23:20 pm
content: Probably the most detailed description is 'Oscilloscope Probe Circuit Concepts' by Joe Weber. It's availablehere. It's somewhat dated, but I'm not aware of a more up-to-date publication with the same amount of depth.

author: kevyk
author_rank: Contributor
timestamp: July 07, 2011, 12:27:19 am
content: Some really great info. Very outdated though - 1969 forhttp://www.slack.com/TE/TekConcepts/TekProbeCircuits.pdf,but interesting none-the-less.Thanks!

author: Unknown
author_rank: Unknown
timestamp: July 07, 2011, 04:27:28 pm
content: Definitely dated, not so sure about outdated. None of the theory has changed. Passive probes still work exactly the same. Even active probes still use many of the same principles, although nobody uses tubes or discrete transistors anymore. I would be very happy with a 2011 edition, but anything published since seems more like a sales brochure than a technical document. The service manuals for active probes back when they still published schematics (eg. Tek P6046) might be another place to look, but don't expect anything less than 25 years old.

author: kevyk
author_rank: Contributor
timestamp: July 08, 2011, 06:09:39 am
content: Right. Dated, not outdated...

author: kevyk
author_rank: Contributor
timestamp: July 11, 2011, 05:33:21 am
content: Dave, interested in a Differential Scope Probe teardown / explanation why they are so damn expensive... ?

author: EEVblog
author_rank: Unknown
timestamp: July 11, 2011, 05:47:42 am
content: My mate Doug Ford has already done this.See his excellent Silicon Chip article on scope probes, "THE SECRET WORLD OF PROBES"http://www.dfad.com.au/articles.htmlDave.

author: kevyk
author_rank: Contributor
timestamp: July 11, 2011, 06:14:13 am
content: Thanks you Dave! Wonderful article.

author: Unknown
author_rank: Unknown
timestamp: July 11, 2011, 10:17:35 pm
content: Quote from: kevyk on July 11, 2011, 05:33:21 amApart from the obvious issue of supply and demand, the complex part of a differential probe is getting a good common mode rejection rate, which is critical for diff probes. Good CMRR requires very accurate matching of the two channels over the frequency and amplitude range.A basic 10x probe has a tolerance in the order of 1%. A differential probe with a 60dB CMRR requires matching within 0.1%. This includes parasitics, which become more significant as the frequency increases. This makes attenuator design very tricky. The attenuator design is the major difference between the expensive brands like Tektronix and the cheap brands like Testec.

author: kevyk
author_rank: Contributor
timestamp: July 12, 2011, 05:03:51 am
content: The article athttp://www.dfad.com.au/articles.htmlis great but doesn't talk at all about differential probes. It amazes me that you can sped $2K - $5K on a high quality scope and $3K to $15K on a single differential scope probe...Ah... I know why... They use all solid gold components in probes and only rocks and horse glue in scopes

author: ejeffrey
author_rank: Super Contributor
timestamp: July 12, 2011, 07:17:45 am
content: At low frequency/DC it is pretty easy to get high CMRR. The larger the input impedance and the smaller the source impedance, the less carefully they have to be matched. This is how instrumentation amplifiers work -- they have an input impedance of megaohms to gigaohms -- essentially infinite compared to the ~250 ohm source impedance of the typical sensors you connect to them. Therefore it doesn't really matter if the impedances are balanced. The internal gain resistors still need to be matched, but in instrumentation IC amplifiers these can be laser trimmed to the required precision.At high frequency, the parasitic capacitance reduces the impedance dramatically. A measly 1 picofarad parasitic capacitance is 1600 ohm at 100 MHz. In this case, the source loading can usually not be neglected. For good CMMR, that 1 pF needs to equal between the two channels to very high accuracy.

author: Unknown
author_rank: Unknown
timestamp: July 12, 2011, 06:21:18 pm
content: High input impedance is fairly easy at low frequencies, but attenuation tends to be a problem at fairly low frequencies (assuming you need any attenuation). If the amplifier has an input capacitance of 9pF, you need 1pF in parallel to form a 10x compensated divider. Getting 1pF +/- 0.1% is not trivial.Resistors and capacitors also tends to have a voltage coefficient. The common mode voltage over them cause errors at higher voltages, even at low frequencies.

author: tnt
author_rank: Regular Contributor
timestamp: July 22, 2011, 08:28:24 pm
content: I recently modified a 1152A FET probe to work with a 3000-X series and what I can tell you is that opening the 'box' that connects to the scope reveals nothing. In this case, it wasn't even the compensation network, it was just power regulation and offset voltage generation. All the HF stuff happens in the probe head, sealed, using custom chips/dies. If you try to open the probe head, you essentially kill the probe ...

author: kevyk
author_rank: Contributor
timestamp: July 28, 2011, 03:05:12 am
content: I would think that a high quality properly front ended instrumentation op amp with a high CMRR over a wide bandwidth would be a way to create a nice differential probe and not cost more than the damn scope itself....What might make it easier is to use a powered FET right at the probe tip and feed its low impedance output to the instrumentation amp 2-3 feet up the cable. But then again, this might be a nightmare to design right.So back to the question about just using a properly front ended instrumentation op amp with a high CMRR.