DE-5000 strange result with reference capacitance - Page 1
Forum section: electronics / testgear

author: sbixby
author_rank: Newbie
timestamp: June 28, 2018, 11:37:24 pm
content: I am generally a novice, so I am not sure if these results are meaningful or not.I have a new DE-5000, purchased mostly for learning & fun with random loose components, less for diagnostics and repair. At my job (as a software engineer, not EE), we have a set of IET Labs capacitance reference modules, in 50, 100, 200 and 500pf. I know little about them, my EE coworker uses them for calibration of equipment installations.Today we thought we'd compare the DE-5000 to his Fluke LCR (model unknown at the moment). For 50, 100 and 200pf, the readings were pretty comparable between the two units.For 500, though, it got very weird. It kept showing ~2.5pf when we hooked it up, with the TL-21 probe set and guard, or with the coax set he uses with his Fluke. I ran the calibration with the TL-21 to check and still got the 2.5pf reading. Then it gets weirder -- sometimes when I hook up without the guard line, it does read -- ~525pf - if I then connect the guard line, it goes back to 2.5. Other times, all combinations of hookups still show 2.5pf.I tried auto mode, Cs mode, Cp mode, all of the frequencies, re calibration, etc -- but still the same results.Can anyone shed some info on what might be happening?

author: bugi
author_rank: Regular Contributor
timestamp: June 29, 2018, 05:50:37 am
content: The thing is, I have not found anywhere a description how that "guard" line is supposed to be used (though I can make an educated guess).Internally, it is connected to the meter's "ground" (i.e. meter's own 0V plane, wherever that ends up being connected). Check the continuities with the measurement connections to the guard connection in your setup, there should not be such connection.

author: nanofrog
author_rank: Super Contributor
timestamp: June 29, 2018, 09:29:37 am
content: Quote from: sbixby on June 28, 2018, 11:37:24 pmThis does sound strange.Could you post a photo of the Fluke coax test jig you used?Not sure how this was connected to the DE-5000.Did you open the TL-21 and DE-5000 and check for loose solder joints or intermittent shorts?Are the PCB's dirty?Did you make sure the ref capacitors were discharged prior to testing (the DE-5000 does NOT have a discharge circuit, so they must be done manually). *Testing non-discharged capacitors can kill it, as there's no protection between the inputs and the IC (such a circuit would reduce accuracy).*In the meantime, you could retest using +/- 4mm banana connectors to connect to the ref capacitors and see if the results are constant as you retest (eliminate any test fixture).Do note the TL-21 used as-is (no guard wire), is a 4 wire measurement. Which means it applies a current and measures the resulting open-circuit voltage. But once you insert the guard wire, it becomes a 3 wire measurement, which works by applying a voltage and measures the short-circuit current.Quote from: bugi on June 29, 2018, 05:50:37 amHere's a couple of resources you might want to take a look at:Minimize Measurement Errors in Component Test FixturesIET LCR Measurement Primer(.pdf)

author: Zucca
author_rank: Unknown
timestamp: June 29, 2018, 09:56:20 am
content: Manual range selection?

author: evava
author_rank: Regular Contributor
timestamp: June 29, 2018, 11:15:38 am
content: Quote from: nanofrog on June 29, 2018, 09:29:37 amCan you please elaborate on it ?Schematic and short description/explanation.Never heard of it...P.S. I only found thishttps://books.google.cz/books?id=ek7miVCuGXwC&pg=PA610&lpg=PA610&dq=guard+wire+three+wire+measurement&source=bl&ots=flHabzMamC&sig=362m_A5OZXKCnDuUarqBmhOoGDk&hl=cs&sa=X&ved=0ahUKEwil_aKY3PjbAhUFyKQKHXUhBb8Q6AEIRzAH#v=onepage&q=guard%20wire%20three%20wire%20measurement&f=false, but I am not sure if it applies to your case..

author: nanofrog
author_rank: Super Contributor
timestamp: June 29, 2018, 01:02:30 pm
content: Download theIET LCR Measurement Primerlinked above, and start reading from pg. 21 to 25 (covers 2 wire, 4 wire, and 3 wire measurement techniques).

author: bugi
author_rank: Regular Contributor
timestamp: June 29, 2018, 02:51:09 pm
content: Quote from: nanofrog on June 29, 2018, 09:29:37 amI could be missing some point, especially since at least the first article is still quite vague, but with that start...Alas, those fail to apply with the DE-5000.The first one seems to describe guarding the measurement wires (for which the DE-5000's guard connection would be very clumsy way, since there are already coax cables and their shields connected to the device 0V plane, which is also the guard's connection in DE-5000, so not much extra benefit for having the socket?) Also, being connected to simply ground, it does not work as "guarding" in the sense the first link describes (or I think it tries to describe; I was already familiar with this concept, and could immediately recognize DE-5000 is not using the guard that way.)The IET's description claims "The guard terminal is electrically different from the instrument ground terminal, which is connected to chassis ground".. (Weirdly enough, the figure 4 specifically draws ground and case connection) In DE-5000 the guard is directly connected to the 0V plane, and through that to e.g. DC input negative terminal. Not a real "ground" though, but still, could cause troubles. Also, DE-5000's own manual states "GUARD provides a shield to reduce noise for device under test (DUT), test leads, and other equipment. " That is, noise reduction, not the stray effect removal as explained in the primer, or reducing leakages etc. as in the first article.The noise reduction is what I was expecting, but that is not explained better. (The cables are already shielded, so only thing left to shield could be... case of the DUT, but it needs to be open for measurement, so... EDIT: unless it is a special unit like the one shown by sbixby below.)Thanks for the two links anyway, have to read with a proper thought later.

author: sbixby
author_rank: Newbie
timestamp: June 29, 2018, 04:05:22 pm
content: A follow-up:This coax cable fixture used by my company in the field works pretty well with the Fluke LCR meters we have. However, when I try to CAL with the DE-5000, I'm not able to get past the OPEN calibration, it goes to FAIL. So, my results might be understandably skewed. The coax/BNC fixture:Using this fixture and the ref cap - but no guard:I thought I took a picture with the guard attached -- it gets a result of ~2.4pf, but otherwise similar to the photo.Then, using TL21, with guard connected:Guard disconnected:Other questions asked: "Manual range..." I couldn't find any range override on the DE-5000, unlike my DMMs, so not sure on that.Solder shorts/dirt -- I don't know yet, I will pull it apart sometime soon and have a good look.If there are actual circuit problems on the DE-5000, I don't (yet) have the skills to diagnose.Thanks for the follow-ups. If it looks like the unit has problems, I'll try to get a replacement. It was ordered from Amazon, so that shouldn't be too difficult.

author: bugi
author_rank: Regular Contributor
timestamp: June 29, 2018, 05:15:04 pm
content: Considering the connection you show in the photos, I think the coax is nearly useless; the adapters map one terminal to coax center, another to coax shield, so you are not getting the real shielding against external effects that way, and since you're using the banana sockets with (assuming here) ordinary banana pair to coax adapters, you are not having 4-wire connection, either (and the standard box doesn't support 4-wire either). So you could just as well just stick two banana cables, from meter to the single-banana-to-coax adapters.Note that DE-5000's manual does not mention support for the 3-wire measurement like explained earlier. But that would still not explain why you're getting so bad result.I don't have anything similar to test my unit with, so I made a simple mock up, 680pF (measures as ~633pF) ceramic, and two 2.2pF ceramics to simulated case parasitics (in the way described in that SCA-500's datasheet). Attaching guard wire to the "ground" point reduces measured value by about 2pF. However, my measurement connection for that was the almost full 4-wire with direct TL-21. I wonder if I have banana-to-hooks...With banana-to-hook connections (from DE-5000 straight to DUT), measurements naturally vary a lot depending on how the wires go, my hands etc., but 666.3pF seems to come often. Connecting guard wire drops it again by just ~3pF. In both measurements, it was direct on, default settings, etc.Maybe test your DE-5000 the same way I did, if you happen to have a simple test board and few suitable capacitors.

author: nanofrog
author_rank: Super Contributor
timestamp: June 29, 2018, 05:31:08 pm
content: I'm not seeing any photos.Once visible to me, I'll take a look later and see if I can help sort anything out.