Fluke 8080 "flukes" - Page 1
Forum section: electronics / projects

author: aluck
author_rank: Regular Contributor
timestamp: May 25, 2013, 04:42:16 am
content: We've got a nice new calibrated Fluke 8080 in our lab. It's all nice and dandy until we started to measure voltage. Here is it:- power on - show no voltage (some uV-mV)- ground probe on device ground - no voltage- power probe on some +500 mV point - shows about 500 mV- power probe OFF board - STILL 500 mV(!!!), wait for 15 secs - STILL 500 mV (!!!!!!)- power probe onto some other point that is being measured by ADC, and is about 0 - ADC goes up to 500 mV (!!!!!) like Fluke is draining back some voltageIs it a normal behavior for a lab meter? Never used one before.

author: SeanB
author_rank: Super Contributor
timestamp: May 25, 2013, 09:16:20 am
content: Connect a 10M resistor across the input and try again. You must have low humidity there and combining this with a meter with 10Gohm input impedance and around 100pf of capacitance for input and leads it will do that.

author: Unknown
author_rank: Unknown
timestamp: May 25, 2013, 10:43:26 am
content: I'm not familiar with the Fluke 8080, do you mean 8808A?As SeanB already stated, bench meters will usually have an input impedance of 100 pF // > 10 Gohm (note the greater than). Note that (long) leads will add to this parasitic capacitance. If you charge this cap, than it will take at least a second for it to discharge (RC = 1s for 100 pF // 10 Gohm). Why is this not a problem for normal measurements? Source resistance. A battery might have a source resistance of less than an Ohm. The input of a thermocouple amplifier might have a source resistance of 1 Mohm. Discharging the 100 pF cap through 1 Mohm would only take 0.1 ms. The resistance of dry air is almost infinite at these voltages, so if the leakage through the meter is low enough, then the 'cap' can easily maintain its voltage. Inputs of meters can often even get charged by stray fields or bias currents. Apparently leakage and bias currents are so well balanced that the net result is zero discharge. Does the same happen after shorting the leads or connecting them to some other voltage, like 1.5V?Lowering the input resistance (putting a 10 Mohm resistance in parallel with the input) is one way to fix it. Upping the range to one that has a normal 10 Mohm input impedance is another. Usually meters will not maintain a stable value, but just drift all over with their leads disconnected. Since the majority of circuits have a reasonably low source resistance, this is not generally a problem. And the rest would be loaded down by the 10 Mohm input impedance of other meters anyhow.That ADC input has to be very high impedance, or does it measure the 500 mV only for a few samples? If the meter is actually able to source a significant amount of current, then something would be very odd.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 25, 2013, 12:01:03 pm
content: heyas being said - high input impedance resulting with some false readout , my 3478A does the same . when set to auto ranging and no input , it shoes 0.5V ! , I think one of the members here said the meter is actually piking up some EMF and slowly charge up , my 34401 though doesn't act the sameanyhow , for >100mV simply put it on the 3~1V range , for any lower , you are gonna need that 10G resistance ( your DUT will have some Mohms of impedance hence no worries )Cheers !