High impedance, low capacitance scope probe for open drain meausrements - Page 1
Forum section: electronics / testgear

author: euzer
author_rank: Regular Contributor
timestamp: April 03, 2019, 11:15:07 am
content: Any recommendation for a high impedance / low capacitance scope probe for testing 5V open-drain outputs with high pull-up resistance, e.g. 100k, such that its effect on the observed signal is minimal, as opposed to existing 10M/15pF probes supplied with scope?TIA.

author: Fungus
author_rank: Super Contributor
timestamp: April 03, 2019, 08:39:22 pm
content: For 5V you could build a little board with an op-amp (voltage follower) and look at the ouput.

author: nctnico
author_rank: Super Contributor
timestamp: April 03, 2019, 09:13:30 pm
content: Less than 15pf of input capacitance will be hard to beat. Perhaps look for a probe with a low capacitance at the tip. 10M Ohm isn't a problem. With an impedance at the load the error is in the 1% ball park which is better than your oscilloscope anyway.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 03, 2019, 09:49:11 pm
content: There are active probes which would satisfy your requirements. But they would be very pricey.For instance:https://www.keysight.com/en/pc-1659295/oscilloscope-single-ended-active-probes?cc=MX&lc=eng#https://www.rohde-schwarz.com/us/product/active_single-ended_probes-productstartpage_63493-73794.htmlhttps://www.tek.com/low-voltage-probe-single-endedhttps://teledynelecroy.com/probes/active-voltage-probes

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2019, 11:04:22 pm
content: Quote from: nctnico on April 03, 2019, 09:13:30 pmActually not at all hard if your scope has sufficient input sensitivity.Use a 100:1 probe and at least halve 10:1 probe tip capacitance and they cost just a few 10's of $ as opposed to an active probe $$$ !http://www.pintek.com.tw/files/pintek/CP3308R.pdf

author: David Hess
author_rank: Super Contributor
timestamp: April 04, 2019, 12:11:26 am
content: An active probe will be required to get a low enough capacitance. Below is an example from Bob Pease but you may require something faster.

author: rf-loop
author_rank: Super Contributor
timestamp: April 04, 2019, 04:56:06 am
content: I have usedthis.Not bad at all. But it is 1.5GHz RF probe, not DC coupled. With its price, $15 shipped not bad.It is just finished and perhaps somehow improved version ofElektor's article "Poor Mans 1 GHz" by David Jewsbury(pdf)It may also give some idea how to make quite cheap DC coupled low capacitance high impedance "poor mans" probe for predefined usage situation.So or so but active probe is one good solution.If cheap is not needed then there is available more in 1pF class:https://www.picotech.com/accessories/high-bandwidth-active-probes/tetris-1-ghz-active-probeOf course there is also cheap (active)differential probes, but these kind probes typically are not at all low capacitance. (example EEEVBlog HVP70 5.5pF)

author: Berni
author_rank: Super Contributor
timestamp: April 04, 2019, 05:26:43 am
content: Active probes have to be selected carefully because they often only have 1M or 100K DC resistance. The advantage of these probes comes above about 10MHz where the 20pF capacitance of a passive scope probe starts to load down the circuit significantly more than the actual probe resistance.But given that he is looking at 5V signals i think a DIY 100:1 scope probe will do just fine to get you to 100M input impedance. That is if your frequencies of interest are reasonably low in the few MHz range.If 100M still is not enough then the way to go is a good opamp in non inverting configuration. This gets you in to the GOhm range, with very careful opamp selections even in the TOhm range. Obviously the opamp will have limitations in terms of bandwidth, slewrate and voltage swing.Tho if you are interested in just static operation the tool for the job is a bench multimeter with a HighZ mode.

author: euzer
author_rank: Regular Contributor
timestamp: April 11, 2019, 10:02:42 am
content: Quote from: tautech on April 03, 2019, 11:04:22 pmThanks. It's showing asdiscontinuedhttps://www.rapidonline.com/pintek-cp-3308r-3000mhz-oscilloscope-probe-85-2235

author: rf-loop
author_rank: Super Contributor
timestamp: April 11, 2019, 03:04:00 pm
content: Quote from: euzer on April 11, 2019, 10:02:42 amThis "rapidonline" information is junk. It really is not 3000MHz probe.In this pdf (pintek) it ios right and it is of course 300MHz probe.But now.It is good to think...QuoteOk this Pintek have then 100Mega ohm DC resistance... so itvreally do not affect.But and then BIG but.Pintek 3308R input advertised "approximated" (perhaps they forget how to measure) capacitance is 5.5pF. For your circuit it is not at all low capacitance.Think this 100k pull up as source impedance and look what 5.5pF load do...in your hand is now "300MHz" probe what freq response (-3dB) in this case is 300kHzrisetime roughly 1.2µs!(there is many RC filter calculators with bodeplot etc:http://sim.okawa-denshi.jp/en/CRtool.php)