[volt-nuts] HP 3456A Input Impedance Check

[Cowham, Chuck]

Recently, I was curious about the HP 3456A input impedance.  The manual
states >1E10 ohms.  But, what is my 40+ year-old meter today?  Using a 10 V
source and a switchable 1x/10x scope probe I did a quick check.  When the
scope probe is switched to 10x a 9 meg series resistor is inserted between
the probe tip and the meter input.  Input impedance can be checked by
comparing 1x and 10x readings.

With autozero on, readings were 1x:  9.99995 V and 10x:  9.99990 to 9.99993.
Worst case, there was a 50 uV drop across the 9 meg resistor, which puts
input impedance at 1.8E12 ohms.  Quite amazing.

Then, wondering if the autozero was injecting charge into the input and
causing the readings to change with the probe at 10x, I checked again with
autozero off.  Now the readings for x1 and x10 are the same or at most one
digit (10 uV) different.  One digit difference equals 9E12 ohms.

I'm sure the dry winter air is helpful for this measurement.  I'll check
again in steamy August.

-- Chuck