Dr. Frank:
I have to disagree with you, null meters was/are still are an important instrument, I still use them and I know others who also use them.
Mr. Pettis, I did not make any statement about the importance of null meters.
I simply referred to the fragmentary specification, which lacks the bias current of this instrument.
This instrument is mystified in the metrology community, also by its 2nd hand price, like having 'infinite resistance', and that only THIS instrument can be used in the commonly known bridge arrangement, like with standard cells, or precision dividers as 752A, 720A, and also for the calibration of the 5450A.
There is a procedure in the Fluke manual, paragraph 4-22 which tells how to measure the leakage resistance and at null, a properly operating null meter does achieve an input resistance of near infinity at balance, no DVM or auto-zero Op Amp is going to get anywhere near that plus there is more bias currents from the DVM and auto-zero op amps than a null meter at balance.
The null meter is the superior instrument for use with bridges, I use them in my lab all the time, all of my bridges use null meters, whether on board or external.
Sorry, Mr. Pettis, that is exactly this false mystification, overrating, or simply a misconception about the 845A, that I want to deflate.
I refer to the original manual of the 845AR from about 1975, as the online versions are incomplete.
The specification on page 1-1 only mentions :
"INPUT ISOLATION
Better than 10^12 ohms at less than 50% r.h. and 25°C regardless of line, chassis, or recorder grounding... with driven guard, isolation improves .. to 10^13 ohms."
This only parameter about high ohmic behavior, is further described on page 4-5, in paragraph 4-19 'Leakage Resistance Test', with Fig. 4-10.
It is evident, that this parameter and associated test describe the isolation resistance between the chassis ground, recorder ground and power ground, which are all shorted together, versus the Guard and Null- Input / Common only.
It makes absolutely no statement about the bias current of the Null-amplifier-Input, nor its input resistance.
The null-amplifier also does not achieve 'near- infinity resistance at balance'.. that statement I've read very often, and it's wrong, or simply not appropriate.
Instead, the 845A input resistance is simply a fixed 10MOhm in the low voltage ranges, from 1mV down to 1µV.. and that's exactly so specified in the manual.
What you obviously mean instead, that in the case of perfect balance, there will not flow any current across the bridge-legs, as zero Volt divided by 10MOhm gives zero current..
Speaking about 'Infinite Resistance' is simply not the correct parameter to describe 'zero loading' of the bridge.
Instead the flowing cross-current, created by the possible bias current of the null amplifier, has to be considered.
Well, and this important parameter of all things is NOT specified.
Any other FET chopper amplifier, like in DVMs, or also in the 7650/7652A OpAmps create bias currents on the order of several pA, up to 20..50pA.
This can be measured quite easily, also on the 845AR/AB.
Maybe, that this photo chopper principle is really superior, but this is to be proven, not to be assumed.
A 2nd remark :
The 845A draws current for the case of non-perfect balance, which is a used case in bridges, when you want to monitor differences and their drifts, like a standard cell versus a fixed 1.018V output.
If you measure 1mV difference, that would at least give 100pA, plus the possible bias current of the chopper.
In comparison, a 3458A draws 20pA at most, for all voltages between 0V and 10V.
For this used case, of differential mode, modern DVM are superior over the 845A.
... They can achieve a voltage measurement of as little as 10nV resolution, a 3458A cannot do any better than 50nV resolution and the null meter is continuous unlike the DVMs. I've tried using DVMs in place of null meters, not near as good, I've used auto-zero op amps for null detectors, good but still lacking a bit. I even use a vacuum tube null detector for special purposes which actually outperforms the solid state units.
The 845A can't resolve 10nV, but the 3458A resolves very well 10nV, in its 1V and 100mV range.
I think, you mix up here a different parameter, and that's noise.
Low noise nV - meter, do not have necessarily very low input bias currents, no, in contrary!
There exists analogue NV meter, but also nV DVMs, like the HP34420A, and the Keithley 182A/2182A.
These are superior over the 3458A in aspect of noise, but they both have higher input currents.
I have several 845 units and two 419As, while the 419 is a bit easier to work on inside, I still prefer the 845, for a few years, ESI put the 419As in their 242 bridges (mine has one, it hasn't worked in some time), ESI dropped it (unknown reason), I'm using a Keithley 155 as an out board detector right now. If you're wondering, I simply didn't have time to fix the 419s and 845s are all busy.
A null meter can make a world of difference when attempting to make such sensitive measurements, forget the DVMs and put the null meter back where the procedures call for them.
It's fine, that you own these old instruments, I also would like to have one, as these have some advantages over many DVMs, like high isolation, guarding, high common mode operation, and battery mode.
But anyhow, your appreciation for them is not appropriate.
Otherwise, these instruments would still be in production by FLUKE (and the other manufacturers), as FLUKE still sells bridge type instruments, like the 720A, 752A, and so on.
Fluke themselves instead promote their 8508A, and prove, that its performance as a null-meter in such bridge application can very well replace these old analogue instruments.
The 3458A also makes a very good null-meter.
It has a guard also, and the Isolation Resistance to case ground is also 10^12 Ohm, like the 845A.
It is specified having < 20pA bias current, which the 845A is lacking.
I determined this bias several times in my bridge applications really being less than 10pA, depending on the common mode voltage used.
It also has very low noise down to 0.01ppm @ 10V range, that is on the order of < 100nV rms. The 0.1V and 1V ranges deliver 20nV noise, only.
Please visit our DMM noise comparison test here in the forum.
These parameters are fully sufficient for the different bridges that I had to calibrate.
Maybe an even lower bias current, and the battery mode like the 845A would give an improvement.
So I would like to ask you, please really
measure the bias current of your 845A, and present your results here.
Frank
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