[volt-nuts] HP-419 and Fluke 845 Modifications

acbern at gmx.de acbern at gmx.de
Wed Aug 6 13:24:23 EDT 2014


I have no personal experience with the 845, however am using the keithley 155 as nullmeter fo precision calibration. I had used the 182-M (a low noise version of the 182) and other digital gear before such as the 2182, and the 155 performs very well, is easy to use with not much noise and very stable. I have seen no reason to buy a 845 or modify the 155, it is very nice as it is.



> Gesendet: Mittwoch, 06. August 2014 um 04:30 Uhr
> Von: "R.Phillips" <phill.r1 at btinternet.com>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] HP-419 and Fluke 845 Modifications
>
> Hi Frank
> I have read your comments on the make-up and performance of the 845, so how 
> do you compare the Keithley 155 which I think was a later design ?
> Roy
> 
> 
> -----Original Message----- 
> From: Frank Stellmach
> Sent: Tuesday, August 05, 2014 10:33 PM
> To: volt-nuts at febo.com
> Subject: [volt-nuts] HP-419 and Fluke 845 Modifications
> 
> Hi Randy,
> to modify the 845 with a modern chopper OpAmp is a great idea, and will
> for sure improve its performance, rather than degrade it.
> 
> Fluke never specified the leakage currents of its chopAmps, neither in
> the 845, nor in the 332 / 335 or 334 MOSFET Chopamps..
> 
> The real replacement input resistance was never specified, only the
> isolation resistance, but not described, against which potential, or in
> which arrangement. Compare that to FET and Chopper OpAmp specifications!
> 
> And the optical chopper of the 845 goes into bipolar transistors, not
> even FETs were used!
> Therefore, the resulting input currents, caused by imperfections of the
> optical chopper, may also be in the high pAs, or nA, nobody ever has
> measured that, but will also give arithmetical 10E13 Ohm at voltage
> differences below µV level...
> 
> The great thing about the 845 is its 1kV isolation of input to monitor
> output, but this will not be changed, if you replace the bipolar /
> optical chopper amplifier stuff..
> 
> I think, nobody dared to modify the 845, or questioned it, because this
> instrument is a "myth", indicated by the price, you still encounter on
> ebay...
> 
> 
> For the Hp419, everything i described is  the same here, the difference
> of this circuitry is the lack of the isolation transformers only.
> 
> Therefore, go ahead, and maybe - please specify the bias currents before
> / after modification.
> 
> Frank
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