Why does a meter show a measurement when there is "nothing" connected to it - Page 1
Forum section: electronics / beginners

author: accesscontrolforum
author_rank: Contributor
timestamp: December 14, 2014, 12:55:24 am
content: In regards to my item I have listed:https://www.eevblog.com/forum/buysellwanted/hp-agilent-3478a-true-rms-multimeter-5-12-digit-dmm-hp-ib-interface-w-new-leads/I had someone ask:In the third picture in the e-Bay listing, the meter is reading 1.87696 VDC, but nothing is connected. Can you explain why? I thought perhaps the signal was being fed in from the rear terminals, but the "front/rear" switch looks like it is set to "front".Thanks,*******my response was:This is not uncommon. It does not denote a malfunction. I could be wrong but I would image it is the capacitance in the air?I know that it is not a malfunction. But why does it do this?

author: dannyf
author_rank: Super Contributor
timestamp: December 14, 2014, 12:58:13 am
content: Interference on a high impedance input.

author: c4757p
author_rank: Super Contributor
timestamp: December 14, 2014, 01:00:53 am
content: Yup. My 3468A drifts around more than any other meter I've seen when nothing's connected to it - the inputs have very high impedance, so the input capacitance happily stores any charge that comes its way, with nowhere for it to drain off.Actually a bit of a problem with the model - it can start to cycle: charge builds up, voltage climbs - autorange switches up to the next range, which has a lower impedance and discharges the built-up charge - autorange switches back down. Repeat ad infinitum, putting wear on the relays. I try to manually up-range it if I'm going to leave it on for a while.

author: dannyf
author_rank: Super Contributor
timestamp: December 14, 2014, 01:02:25 am
content: QuoteI don't believe such a thing exists,

author: accesscontrolforum
author_rank: Contributor
timestamp: December 14, 2014, 01:08:12 am
content: I was thinking like a lighting bolt. The earth is negative and the ionosphere? is positive and it holds a charge/discharges through the air?So the air between these two points has a potential difference?What do I know I just a a taco for the first time?

author: c4757p
author_rank: Super Contributor
timestamp: December 14, 2014, 01:13:54 am
content: Quote from: accesscontrolforum on December 14, 2014, 01:08:12 amwat

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 14, 2014, 11:55:05 am
content: Hi,charge and capacitance out of thin air?Nonsense!Simply read the specifications and study the schematics!The DCV specification tells you, that in 30mV, 300mV and 3V DC modes, the input impedance is > 10^10 Ohm.Vice versa, this means, that the input currents are:< 30mV/10^10Ohm => <0.3pA or < 3pA or < 30pA, respectively.For instruments like 34401A or 3458A,this input current is specified directly (<20pA), instead of that calculational input resistance.This input / leakage current charges a capacitance INSIDE the instrument.That may be same stray capacitance like on the PCB, or simply C101 = 470pF, as you can find in the schematics. You just have to look..The leakage current itself also originates internally. It's simply the the sum of leakage currents of both protection diodes to + and - 3.5V, and the bias current of the buffer OpAmp, all inside the input multiplexer / buffer ASIC U102, see input path in the appended picture.You may also measure this input current by setting the instrument to fixed 300mV range, Autozero Off, shorting the input. Then remove the short and start your stop watch. If the display reads a certain delta U, stop the time delta t and calculate the input current by:I = 470pF * dU / dt.That's all basic electronics engineering, no Harry Potter magic, I think.PS: You shalt never leave such high impedance inputs open, especially in autorange mode.Frank

author: photon
author_rank: Frequent Contributor
timestamp: December 15, 2014, 05:39:46 am
content: Quote from: Dr. Frank on December 14, 2014, 11:55:05 amThanks for the explanation which is simple and new. The OP's question seems to have been raised in this and other blogs before with the explanation given, as far as I know of, that it is caused by charge trapped by the high input impedance. Your explanation is new in that, if I understand you correctly, it is caused by leakage current. The high impedance plays the role of lowering the amount of current required to cause the meter to measure a non-zero voltage.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 15, 2014, 06:00:07 am
content: Evaluating the reading of a DMM (or any active, sensitive circuit) with open input is simply not a valid test of anything.You are literally measuring "undefined".A valid test of zero or drift would be to SHORT the input.

author: photon
author_rank: Frequent Contributor
timestamp: December 15, 2014, 06:59:45 am
content: Quote from: Richard Crowley on December 15, 2014, 06:00:07 amThe OP's question was to explain why his 3478A was reading 1.87696 V. He knew the reading was not valid but did not know whether there was a problem with his meter or not. This explanation tells why it was not his meter. Next time the OP will know to short the inputs of his meter before taking a picture to sell it. Also it explains why, at higher voltage settings (lower input impedance), the reading is 0.