Why measured input capacitance of Rigol DS1054Z depends on signal polarity? - Page 1
Forum section: electronics / beginners

author: RoGeorge
author_rank: Super Contributor
timestamp: October 14, 2023, 12:22:18 pm
content: I've connected a hand held (battery powered) capacimeterCM9601Ato measure the capacitance of a probe 1x that is connected to the oscilloscope.One way it measures 3.3pF. When the capacimeter terminals are swaped, plus with minus (connected reversed), the capacimeter measures almost 8pF instead of 3.3pF. Why so?To make things even more confusing, the DMM (mtx3283) gives different values (80pF normal polarity, 160pF for reversed polarity), very different from the 3.3pF normal or 8pF reversed measured with CM9601.Both instruments measure significantly bigger capacitance when the measuring voltage swing from 0V to a negative value, versus swinging between 0 and positive values.Is this expected for an oscilloscope, to present an input capacitance variable with voltage?

author: David Hess
author_rank: Super Contributor
timestamp: October 14, 2023, 10:57:18 pm
content: Quote from: RoGeorge on October 14, 2023, 12:22:18 pmA capacitance that low for a x1 probe is just wrong. It should be more like 120 picofarads.QuoteDo you mean that you swapped the leads going to the capacitance measurement? Or was the polarity of the voltage actually reversed? Was the oscilloscope on?I have made this measurement before, and measured the oscilloscope input directly, and swapping the leads did not matter. There is a change of capacitance over the input voltage range, but it should be insignificant because the raw input voltage range is only like 1 volt. At higher voltages, the input attenuator will isolate the capacitance change.Make sure that you are not overloading the input, and that the oscilloscope is turned on.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 14, 2023, 11:36:25 pm
content: The oscilloscope was on, and the waveform from the capacimeter was less than half screen in amplitude.The capacimeters (both) are using almost square wave to measure, but only one polarity, between 0 and +V. The oscilloscope was on DC coupling, so the waveform was in the upper half of the screen. When I swap the leads of the capacimeter, the waveform on the screen will appear in the lower half (from 0 to -V), while the capacimeter will display a much larger value.Indeed, the indicated capacitance was increasing a lot to higher channel sensitivity, when the preamplifier was on (a relay click is audible). Higher sensitivity was also saturating the input. With the probe switch at 10x, the capacitance was about the same, but I was interested in 1x, so I didn't try much with the probe on 10x.I'll measure again tomorrow, make a nice table with the results, and add here the waveforms seen on the oscilloscope.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: October 15, 2023, 04:09:44 am
content: Don't know much about your tool, but the capacitance of the tip of a o-scope probe will be small. The capacitance of the ground lead will include almost every wire in your house and even the Earth itself, as this lead is connected to the ground and neutral wires in your house as well as the planet you live on.

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2023, 09:36:54 am
content: Quote from: JustMeHere on October 15, 2023, 04:09:44 amThe capacitance of a 10x probe is small, on the order of the oscilloscope input, but the capacitance of a 1x probe is the capacitance of the entire probe cable and like 20 picofarads per foot.

author: iMo
author_rank: Super Contributor
timestamp: October 15, 2023, 11:05:59 am
content: Below the schematics of the inputs - there are diodes at the input it seems, thus the polarity could matter..

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2023, 04:04:23 pm
content: Quote from: iMo on October 15, 2023, 11:05:59 amDo not rely on Dave's schematic too much because it is incorrect in many places. For instance the top diode cannot be going to ground, the input series protection is completely missing, and the input network for the low frequency path has the wrong impedance.The shunt protection diodes do not have enough capacitance to cause what RoGeorge is measuring.

author: TimFox
author_rank: Super Contributor
timestamp: October 15, 2023, 04:17:58 pm
content: Note that protection diodes driven from a reasonably large amplitude square wave will probably conduct, which will give an incorrect measure of "capacitance" on a simple meter.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 16, 2023, 11:45:02 am
content: The oscilloscope+probe appears to be OK when a DC offset is applied.I've tried again today, this time with an AWG that generates a sinus with an adjustable DC offset. I've tried both, frequency sweep at various DC offsets, then constant frequency and measure Vpp on the screen at different DC offset in the signal. No differences observed, same results no matter the DC offset of the incoming signal.By the way, with Rigol DS1054Z oscilloscope and its stock probe connected to a 50\$\Omega\$ AWG output (AWG model Rigol DG4102), the Vpp becomes half at ~35MHz. For a series RC with R=50, corresponding Cprobe would need to be ~160pF in order to see half the generated Vpp (about 20MHz bandwith at -3dB).

author: radiolistener
author_rank: Super Contributor
timestamp: October 16, 2023, 05:03:56 pm
content: Oscilloscope input has semiconductor, but I think your issue is because your RLC meter measurement is wrong when you attach ground terminal of oscilloscope to a probe signal due to parasitic capacitance from osciloscope ground and mains...

author: RoGeorge
author_rank: Super Contributor
timestamp: October 16, 2023, 06:25:40 pm
content: Could be, but seems unlikely.Both the capacitance meters I've tried were floating, powered from battery. The DMM was sitting on the bench (made of pressed fiber wood, not metal and not grounded), while the CM9601 was sitting in its cardboard box (usually it is unused, so I only opened its box and measure, didn't bother to pull it out of its cardboard box). Only the oscilloscope is powered by mains.Meanwhile I've thought maybe the capacimeter was trying to push constant current through the oscilloscope input, and that might reveal any asymmetry, while the test with the AWG was very low impedance and of controlled voltage, but I don't have the time to investigate any further. The question was more out of curiosity.Can anybody who happen to have a battery powered capacimeter and a DS1054 oscilloscope repeat the measurement, please?With probe at 1x, does the capacimeter show the same value when the tip and the shield of the probe are swaped while measuring? Mine shows 2-3 times more capacitance when the minus of the capacimeter is connected to the probe tip and the plus of the capacimeter is connected to the probe shield.

author: StillTrying
author_rank: Super Contributor
timestamp: October 18, 2023, 02:12:56 pm
content: Could the CM9601 be getting confused with much of the capacitance on X1 being >1m away and through the probe's ~250R center core resistance, and then the scopes 1M in parallel.You could do a CM9601 test with ~1m of coax, swapping the inner and outer polarities to see if you get the same capacitance, and what effects stay capacitance or having the outer earthed is having.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 18, 2023, 06:38:10 pm
content: - the oscilloscope itself shows ~22pF either way, measured at the BNC connector, without probe. Something happens when measuring through the probe, particularly when the probe is connected to the oscilloscope.- with a 1 meter 50 ohm coax plugged into the powered oscilloscope oscilloscope (1V/div), it shows 11pF normal and 16pF reversed.- when the same coax is unplugged from the oscilloscope, it measures 100pF normal and 105pF reversed.It seems like whenever the hot wire from the capacimeter is connected to the outside shield, a 1 meter shielded cable (probe or coax) measures with about 5pF more.Maybe it is the ground influence of the surrounding after all.

author: tszaboo
author_rank: Super Contributor
timestamp: October 18, 2023, 09:42:18 pm
content: The oscilloscope probe has a lossy coax cable. It's still "50 Ohm", but the DC resistance is like 300 Ohm on the center pin, while the shielding its almost 0.

author: David Hess
author_rank: Super Contributor
timestamp: October 19, 2023, 01:12:26 am
content: Quote from: RoGeorge on October 18, 2023, 06:38:10 pmSomehow it is measuring the common mode capacitance instead of the differential capacitance. For capacitance meters where this matters, the terminals are usually marked.I do not understand though why the measurement of just the oscilloscope is the same either way.I have at least three different instruments which can measure capacitance at this level, but only older Tektronix oscilloscopes. I have made this measurement before and never noticed that polarity matters.

author: StillTrying
author_rank: Super Contributor
timestamp: October 19, 2023, 02:50:24 am
content: Quote from: RoGeorge on October 18, 2023, 06:38:10 pmI'd expect that.And if the ~1m of coax was resting on a metal desk I'd expect another ~4pF when reverse measured, but no difference when not reversed.I can't really explain it of course, but I think of it as thehot endnow having to partially drive the metal desk as well as the outer.QuoteOdd for the cap. to be so low when connected to the scope.The next experiment to try would be a 1M on the far end of the coax, it wouldn't surprise me if theright wayandreverse wayresults are similar to the strange incorrect X1 scope readings.The CM9601 measuring very low capacitance by simply putting a voltage across the cap and then timing the discharge through a ~20M would mostly explain the strange results you're getting with the scope probe, the scope's 1M would make the cap. appear to be ~20 times smaller.I doubt it's doing that, can't find any PDFs on it."Is this expected for an oscilloscope, to present an input capacitance variable with voltage?"The scope's input impedance would have to be stable with voltage to show accurate wave shapes on X1 when there was some impedance in the source, and completely useless when the probe is on X10.Slightly related to scope input impedance changes, my CML+ has bad impedance changes between AC/CD coupling 30-170Hz.https://www.eevblog.com/forum/chat/scope-acdc-coupling-at-60hz/msg2530800/#msg2530800

author: RoGeorge
author_rank: Super Contributor
timestamp: October 19, 2023, 10:41:26 am
content: Quote from: David Hess on October 19, 2023, 01:12:26 amThe capacimeter's terminals are marked, indeed, I mean red for plus when measuring polarized capacitors. I do not have the proper schematic, but I've found on a forum a schematic from another hand held capacimenter, MY6013A. One user there was saying that's the same schematic as in the CM9601A (but I didn't open the CM9601A to double check that info).

author: David Hess
author_rank: Super Contributor
timestamp: October 19, 2023, 02:12:22 pm
content: Quote from: RoGeorge on October 19, 2023, 10:41:26 amThe important part on the schematic is where it shows that the negative terminal connects to chassis ground. Chassis ground couples to earth ground, so if the shield of the probe or oscilloscope is connected to positive, then the capacitance between the chassis or shield to earth ground to the chassis ground of the capacitance meter is measured.I think the DE-5000 LCR meter for instance has a separate chassis ground (guard) from the two measurement inputs. My ESI impedance bridge and my multimeter have chassis ground connected to one terminal like is shown in your schematic.But that still does not explain why the probe showed a large difference in capacitance and the oscilloscope input did not, so something else is going on. My guess is that the resistance of the probe center conductor is confusing the capacitance meter.

author: mawyatt
author_rank: Super Contributor
timestamp: October 19, 2023, 03:20:38 pm
content: Quote from: David Hess on October 19, 2023, 02:12:22 pmYes the DE-5000 has a separate ground from the negative sense terminal and produces a bipolar test signal.Best,