First, what Ed said. :)
Second, you're looking for leakage on the board, but you won't find it there, so don't tear things apart in a hunt for what is a ghost. The leakage is coming from the protective clamping diodes and/or the gate of the input FET. For the latter, one can expect a gate leakage current that is very, very approximately, roughly 6-7 orders of magnitude below IDSS. So, a 10mA IDSS (a reasonable guess for the input FETs) gets you leakage of the order that you are inferring.
Third, what Ed said.
--Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
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On 4/19/2022 21:56, romeo987 wrote:
I have a pair of DM501As that I use as my every day go-to DMMs on my bench. They work well. I have noticed, though, that with an open input on the 200mV range, one reads about +11mV, and the other about +3mV. Goes to zero when the input is shorted. On the 11mV unit, this corresponds to about 1.1nA on board leakage into the A/D input node and the associated 10Mohm input resistance. When I isolate the A/D input and ground it through 10Meg, the reading is around 0.01mV. So it is not the A/D chip itself.
The schematic shows a chain of switches in the S1 assembly, between the input attenuator assembly and the A/D. By watching the display as I switch ranges and short the input, it appears that the leakage injection is into that chain of switches. I isolated the relatively long pc track from the last switch (S1-N) to the A/D (R1613) and it looks like the injection is somewhere in the S1 assembly itself. I sprayed PC cleaner in, around and through the switches liberally, but after drying it - same result. But mere dirty switches aren't enough - there has to be a source of +V somewhere close by. To isolate possible switch leakage from other parts of the circuit, I have grounded, in turn, all of the feeds from the ohms, AC and temperature circuits at the respective switch terminals, with no difference to the display. I can't readily see any other sources.
And now I am out of ideas.
So I am appealing to the wealth and breadth of knowledge in this forum for any suggestions. What else can I do to identify the leakage source?
Is switch removal REALLY as simple as the manual suggests "...by carefully unsoldering the connections and pulling....". Dozens of pins have to be completely cleanly unsoldered for this to work... I really do not want to go there just yet.
Yes, I realise that this parameter is not specified in the manual, and this effect does not detract from the usability of the instrument. It's because I can :-)!
Roman
