Agilent 3458A DMM reading for "air voltage" - Page 1
Forum section: electronics / general

author: dl1640
author_rank: Regular Contributor
timestamp: November 24, 2017, 05:58:18 am
content: Hello,I'm just curios why my 3458A reads about -7V when nothing is connected on the terminals.Everytime after it warmed up it reads about -7V "air voltage".How is your 3458A doing?

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 24, 2017, 06:14:05 am
content: That doesn't say much. It is an open high impedance input with some input current and of course some impedance. That leads to a voltage.On the two 3458As here the voltage is in the range of a few hundred mV with open inputs.On my 34401A (in high impedance mode) the voltage rises over 12V, the 34401A switches than to the 100V range which shorts this voltages and everthing starts over again.

author: dl1640
author_rank: Regular Contributor
timestamp: November 24, 2017, 06:21:09 am
content: danke

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 24, 2017, 08:42:46 am
content: Quote from: dl1640 on November 24, 2017, 05:58:18 amEssentially, in the high impedance mode it is the voltage where the input current is near zero. The input current varies with the input voltage mostly due to the protection diodes leakage, and if at some voltage the sum of all leakages is close to zero, that is where the meter readings are homing with an open input.CheersAlex

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 24, 2017, 08:56:33 am
content: As an example I attached the input current over voltage from one of my 34401As.

author: dl1640
author_rank: Regular Contributor
timestamp: November 24, 2017, 09:33:25 am
content: Quote from: e61_phil on November 24, 2017, 06:14:05 amI just checked the spec, the 3458A has >10G Ohm input impedance below 100V range.If I switch to 100V or 1kV range the open circuit reading will normally near zero.I have another 5.5 digit DMM and it has >1G Ohm input impedance for 10V and below, and 10M Ohm for 100V range and above. And it is reading something below 100V range and almost read a zero for 100V and above.It seems it is really impedance related.For 3458A mine is reading a -7V while your two are reading less than 1V, mmm...it looks each DMM has very different behavier under such condition.

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 24, 2017, 10:27:15 am
content: Quote from: dl1640 on November 24, 2017, 09:33:25 amI checked it again. The few 100mV were in my mind, only.One is reading ~+52mV, the other one reads ~-660mV.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 24, 2017, 10:44:16 am
content: My 3458A usually reads around 100 mV when nothing is connected but I have also seen 4V to 5V on the display.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 24, 2017, 03:17:44 pm
content: Would there be any dielectric absorption associated with the input? IOW, if you put a shorting strap on it for half an hour or so, then remove it, is the voltage the same?

author: Henrik_V
author_rank: Regular Contributor
timestamp: November 24, 2017, 04:49:37 pm
content: My 3457 go up close to 4V...It's always a good idea to check for leaking input protection diodes ... a little ZAPP ... and a diode might start leaking.One easy quick test (I think I already mentioned it here) is a battery (1,5/3.x/9V what fit the range) in series with a 10Meg (100Meg) resistor... (and a switch to short-cut the resistor , and for the premium version a DPDT switch to swap polarity)Should result (coarsly) the input impedance at both (!) polarities. If a diode is leaking to much it can be identified.Still have a Keithley 196 with some leaking diodes (FETs) waiting for repair... (OK, a dumpster diving device, also needed a power supply fixing first) . In spec on a 5700 but failed the test above.

author: Cerebus
author_rank: Super Contributor
timestamp: November 24, 2017, 11:43:30 pm
content: Quote from: Conrad Hoffman on November 24, 2017, 03:17:44 pmI very much doubt it. There are a couple of class 1 82pF ceramic caps on the input and that's it. Class 1 caps are usually pretty good on dielectric absorption, there are some brands that are significantly worse than others but IIRC the ones on the 3458 are AVX which normally perform well in that respect.

author: Vtile
author_rank: Super Contributor
timestamp: November 26, 2017, 02:48:36 pm
content: Quote from: dl1640 on November 24, 2017, 09:33:25 amThis is also seen in my modest 100 megaohm FETVOM, but mainly with leads attached, but then it picks-up electrostatic fields of random type. With that they are easily visible while with digital meter it only gets in phase "should I refresh my screen".Also my modest Keithley 197A in 200mV and 2V ranges (>1GOhm) do the same, the voltage run up by time until the circuit is closed.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 26, 2017, 02:59:16 pm
content: Quote from: Cerebus on November 24, 2017, 11:43:30 pmI would not expect much dielectric absorption from the class I caps. The more likely sources are cables and the FR4 board.However also the input bias current is not expected to be so super stable - that pA range current can easily depend on temperature, humidity and maybe the moon.

author: dl1640
author_rank: Regular Contributor
timestamp: November 27, 2017, 02:56:48 am
content: Quote from: Conrad Hoffman on November 24, 2017, 03:17:44 pmHi Conrad,The reading still climb to about -7V after removing the shorting strap.