measure pF with Fluke87v? - Page 1
Forum section: electronics / testgear

author: daemonix
author_rank: Contributor
timestamp: May 24, 2014, 11:55:27 am
content: Hi,Im having problems measuring some caps 100pF and 22pF. The measurment jumps all over the place with 87v (random reading?). My ex330 seems to be doing a more or less correct job.Any ideas?thanks

author: electr_peter
author_rank: Unknown
timestamp: May 24, 2014, 12:18:21 pm
content: Hi,I think you have two options to measure small capacitances you mention:get a very short leads (see threadhttps://www.eevblog.com/forum/reviews/fluke-28ii-and-agilent-u1272a-can-do-pf-cap-measurements/- it suggests that short leads may help with small capacitaces)get a specialised LCR meter - regular DMMs are not suited for small capacitance measurements

author: daemonix
author_rank: Contributor
timestamp: May 24, 2014, 12:20:35 pm
content: So its normal for fluke not to measure while ex330 does right?

author: electr_peter
author_rank: Unknown
timestamp: May 24, 2014, 12:54:11 pm
content: I don't know if that is normal what you are seeing (I would think it is normal), it depends. From my experience I would not bet my design decisions on small capacitance measurements made by DMM because I find DMM not reliable enough for this task.Check the exact specifications of 87V and EX330 for capacitance measurements.What setup you are using? Try to use the same leads and everything to make identical setup.Also, check that in your vicinity there are no strong fields or any issues with electrical noise, grounding, etc. - your leads can pick up noise and convert it to false reading quite easily. I had issues when capacitance buttons did not work due to earth issues (house grounding was poor and huge stray voltages were floating around, messing small capacitance measurements).

author: daemonix
author_rank: Contributor
timestamp: May 24, 2014, 01:47:53 pm
content: thanks for your help

author: HighVoltage
author_rank: Super Contributor
timestamp: May 24, 2014, 03:46:25 pm
content: Even a good LCR meter with a 4 wire Klevin cable setup can have large variations, if you are not careful.Usually I lay the cables down, close to the DUT and then recalibrate the zero setting on the LCR meter.Then I move the lead clips to the DUT and hook them up.This helps a lot to get a stable reading.

author: TiN
author_rank: Super Contributor
timestamp: May 24, 2014, 05:53:49 pm
content: Proper probe setup and compensation is crucial for low capacitance measurements.That's why handhelds, including 87V are barely a tool for such applications, it's rather indicator tool, then measurement.I was bored enough to make a short experiment for you.Used tools:*Repaired HP 4263B LCR bridge meter, withmodified generic twizzer probe, after open and short self-calibration.* Fluke 87V* 2-lug banana jack with soldered 4 x 22pF NP0 0805 C0G capacitors, giving total 88pF spec*Repaired cameraMy 87V without anything connected to it was reading 0.26 nF, stable and steady. I nulled that by pressing REL Delta, as that's just parasitic capacitance of measuring path of terminals/meter itself.This is open baseline, reading 0.00 nF, stable. It's only 3.5 digits in this mode, lowest 10nF range, so it already gives idea that you are at very limit of an instrument itself.Now get 4263B powered on and ready.Here is probe tip for LCR bridge. It's 70 cm long, 4 coax probe with BNC ends for common terminal connection at LCR bridges.Here's display output. 10kHz test frequency at 1V level. This frequency gives most digits on HP/Agilent 4263B for capacitance measurement.You can see capacitors soldered to flat copper stubs between banana terminals. This is by no means best setup, but it should do the job.Best would be having full coax setup right to the capacitors.And result: 89.67pFNow plug that bodge probe thing into Fluke 87V:And result: stable 0.09 nF.Take my word, it's not bouncing or changing unless I touch terminals.So in two words: Handheld DMM ain't a right tool for pF and below range measurements.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 24, 2014, 05:58:52 pm
content: Good DMMs are fine with small caps. They usually struggle with big caps.Use short leads and zero the latent capacitance. To get a proper reading, keep the leads in the same initial position.My Gossen 26S (30,000 count) can read small ceramic caps within 2-3pF of their real value. Its published worst accuracy is 1%+6 pF.There is no fluctuation of the readings.The UNI-T 71D is also pretty good with small caps (ie 18pF), but fluctuates within 2-3pF of its median reading (Accuracy 1%+20pF).The 87V resolution is only 10pF, with an accuracy of 1%+2, for film capacitors. If your measurements fall within that range, you have nothing to worry about.

author: daemonix
author_rank: Contributor
timestamp: May 25, 2014, 09:31:21 pm
content: Quote from: TiN on May 24, 2014, 05:53:49 pmexcellent help!Thanks