[volt-nuts] Fluke 845 Modification

Randy Evans randallgrayevans at yahoo.com
Sun Dec 29 21:51:50 EST 2013


The specs for the ICL7650on page 3 of the data sheet show a max bias current of 200 pa AT 85C (the ICL7650b is no longer available), higher than the LTC2054 at 150 pA at 150C.  The 845 will not be operating higher than 40C ambient, for a typical test lab environment, and the temp rise in the unit will be minimal since there will be less than 10 mW dissipation; e.g., 6V at less than 1mA current draw, excluding the recorder isolation amplifier (for now).

The typical value for the LTC2054  bias current is stated in the datasheet as 1 pA at 25C, and my experience with Linear Tech devices is that most units will be at or near the typical value. Assuming a 3 pA worst case value at 25 C and that the value doubles every 10 degree C, I would expect the highest bias current at 45C to be less than 12 pA, and most likely much lower. Selection for lowest bias current is certainly an option for my personal unit, if necessary (not likely but this design is not for a production unit).

I am interested in any feedback you might have on the bias current discussion of the two devices.  I agree a low bias current is highly desirable and I am trying to achieve that.  I am not wedded to the LTC2054 but it looks like a viable candidate and its very cheap so I could buy several and select if necessary.   


On Sunday, December 29, 2013 12:58 PM, Frank Stellmach <frank.stellmach at freenet.de> wrote:
The Fluke 845 is famous for its high resistance measurements, due to the 
differential principle.
But its leakage or bias current is not specified at all!
Anyhow, one has to pay a premium price for this ancient technology, 
perhaps having bad neon bulbs.

Well, all those new Super-Zero ChopAmps nowadays are specified for a 
typical Ibias @ RT only, like the LTC2054, or the MAX4238/4239. There's 
obviously no upper limit testing, and a specified 150pA max over 85°C 
temperature range is much too much!

Therefore I would opt for the ICL7650, guaranteed / tested 10pA max @ 
25°C, with slightly higher noise.
This OpAmp is still available from MAXIM, and from Intersil, again.

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