Consistency Check with HP3458AHighVoltage kindly lent me his 752A to check it and then to calibrate the HV ranges of my 5442A.
The 752A is from about 1993, 29 years old.
![](https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/?action=dlattach;attach=1499713;image)
That vintage bears the risk that the resistance of the switch contacts increased considerably, busting its specification of 0.2ppm uncertainty @ 10:1 ratio.
The intrinsic problem with such Primary Ratio Standards is that they can not be calibrated by “something better” instrument. It’s written in the manuals, that they are working correctly “by design”, elaborate error analysis, and by auto-adjustment.
Therefore, I tried to check the 10:1 ratio by use of the 3458A. Its transfer uncertainty on its 10V DC range is specified to 0.55ppm, but its 10:1 transfer capability by linearity (INL + DNL) might be even accurate to about 0.3ppm of output, following hpj 4/1989, p.23.
The 752A was auto-adjusted following the setup from the manual addendum and using an 845AR w/o problems.
I tested the 10:1 transfer by measuring the 10V input directly at its INPUT jacks, as well as the 1V output, both with reversed polarity, which is critical concerning thermoelectric voltages. I repeated this procedure several times, but always found errors far beyond 0.2 or 0.3ppm ratio uncertainty.
![](https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/?action=dlattach;attach=1499719;image)
So my conclusion was that the switches should have excessively higher resistance, after all these years.
HighVoltage allowed me to open the instrument with greatest care and white gloves, of course, to measure the resistances of the switch contacts. To my big surprise the switches were all ok, as described in the next chapter.
So I investigated further and finally discovered that the bias current in the 10V range of my 3458A caused these errors. It’s specified to have max. 20pA.
The bias current was determined between -10 .. +10V of input voltage, by measuring the voltage drop over an external 1MΩ resistor.
![](https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/?action=dlattach;attach=1499725;image)
These bias current measurements changed over several sessions, probably over temperature, time, humidity and so forth, up to 20pA max. bias current.
![](https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/?action=dlattach;attach=1499731;image)
My 845AR has an input impedance of 1MΩ. By closing and opening the input short, the difference in µV readings gave about 100.. 200fA of bias current. Reading carefully the manual, chapter 2-31 “Increasing Input Resistance”, I deduced that the bias current w/o that 1MΩ resistor would typically be 3µV / 1000MΩ ~ 3fA.
The 752A has quite a high impedance of 400 kΩ, so I calculated the ratio error of the divider loaded by a bias current, see xls file below.
The error formula is: ε = -R
1* I
B / U
in ![](https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/?action=dlattach;attach=1501096;image)
A 20pA bias current at 360kΩ and 10V input creates -0.72ppm of ratio error, so this obviously is the root cause.
The 3458A can be used, anyhow, for measuring 100V and 1kV at 10V output, as the ratio error is 10 times smaller in this case. Used as a Nullmeter, it still has e.g. +8pA bias, creating errors when using the 752A in its classical bridge configuration for calibrating 1kV, 100V, 1V and 100mV versus an external 10V standard.
![](https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/?action=dlattach;attach=1499740;image)
Therefore, any Nullmeter to be used must have guaranteed bias currents well below 1pA.
Here’s another table for my Keithley 182A nV DVM. This shows, that even this class of nV-meters might not be the right choice.
![](https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/?action=dlattach;attach=1499746;image)