Thank you Charles,
The Dec thread looks interesting, maybe a good solution. But was not
looking forward in replacing the amp circuit. I think I'll try a optical
isolated version of 335D circuit. This MOSFet is easy to apply to the
null meter circuit without a major overhaul. If successful I'll share my
simple mod. If the performance fails, I'll try the Dec 2013 method.
Dallas
On 5/6/2014 2:06 PM, Charles Steinmetz wrote:
Dallas wrote:
Did Fluke ever make the 845ab Null Meter with the optically-isolated
Bilateral Analog FET chopper?
I have never seen an 845 with FET chopper, despite the engineering
change specification most 845 owners have seen. I've heard of one or
two 845s that may have been repaired (by Fluke) with a FET chopper
assembly (perhaps using 335D parts ??), but I'm not sure any 845s left
the factory that way.
These days, there are integrated chopper-stabilized ("zero offset") op
amps that can do better than chopper amplifiers (e.g., LTC1050,
LTC1052, LTC2054, LTC2057, LTC7652, ICL7650, MAX4238/4239). Randy
Evans started two threads here back in December 2013, one on modifying
the Fluke 887 and one on modifying the 845. He made available his
notes, which were quite thorough. Check the archives starting on Dec.
22, 2013.
Best regards,
Charles
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