[volt-nuts] Fluke 887 mod with LTC1050
randyevans2688 at gmail.com
Sat Apr 29 17:31:54 EDT 2017
The 8 pA bias current is well within the spec for the LTC1050 (+-10pA
typical, worst case +-50pA at 25C) but I was surprised about the lack of
variation. I also tried an "air bridge" wiring from the 887 filter to the
LTC1050 to avoid any leakage of the circuit board. It made no difference,
essentially eliminating board leakage. It could still be switch leakage.
Eventually I plan on just [putting a 1 megohm resistor from the LTC1050
positive input to ground and see if I still see the 8pA leakage. If I do,
then it's definitely the IC and not the switches.
On Sat, Apr 29, 2017 at 3:59 AM, Frank Stellmach <frank.stellmach at freenet.de
> 8pA is quite good a leakage / bias current for such a differential VM, and
> also for the LT1050.
> What makes me wonder, why three different OpAmps should all have exactly
> The variation from one OpAmp to the others should vary greatly, even if
> these are from the same batch.
> I don't remember your circuit, but maybe you have some sort of input
> protection on the non-inverting input, and that may cause this constant
> bias. Have you assembled the OpAmp completely on the PCB, or is the + input
> air-wired to the polarity switch S4E ? Ordinary PCB material may also cause
> leakage currents.
> I would also check the switches, if they are contaminated.
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