EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: superstock on July 19, 2018, 01:53:20 pm
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My Fluke 8800A doesn't return to zero DCV after measurement ranges 20V and under unless the probes are shorted together (floating). Everything else seems to work fine.
Is this a fault or were meters like this, I seem to recall my apprentice days shorting probes before measurements.
I have placed a 40M resistor across the terminals which sorts it out but should I need to do this.
Does anyone know if it's a fault or do they do this?
On the schematic the DC buffer circuitry all seems common so am I chasing a red herring?
Thanks peeps.
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This must be one of the most frequently asked questions on this forum. It is normal and is not a malfunction.
The reason for this symptom is that the meter has an extremely high impedance input for those lower voltage ranges. This has the benefit of much less loading of sensitive circuits to give a more accurate reading. The disadvantage is that stray electric fields can cause the reading to jump all over if nothing is connected. Even waving your hand near the input will likely cause a noticeable change.
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My Fluke 8800A doesn't return to zero DCV after measurement ranges 20V and under unless the probes are shorted together (floating). Everything else seems to work fine.
Is this a fault or were meters like this, I seem to recall my apprentice days shorting probes before measurements.
That's a GOOD thing.
What it means is the the impedance of the meter is so high that the electrons in the probe wires have nowhere to go after you disconnect them from the voltage.
ie. the meter is measuring (and displaying) the difference between the number of electrons in each probe wire. When you short the wires they can equalize and it will go back to zero.