Hello,
I am trying to mesure directly resistors 1 Mo and above (Fluke5450 calibrator, ESI standart resistors...) on a 3458A in 4 wires configuration with the well known Fluke cables 5440-7002/4 ; I am very surprised by the noise on the readings at full resolution (not coming from the 3458A : cal and certificate of conformity one week old), no neon bulb above the cables, no vibration and no HF noise coming from a SA, synthesyzer... Any idea ? Thank you.
For these high values of resistance, you should use 2W Ohm only.
4W gives no precision benefit @ R >= 1M, but increases measuring time and noise.
A PTFE cable assembly with guard / shield is recommended.
The 3458As accuracy specification is very poor in measuring 10M and above, but gives a rough idea about the value of the 5450A, for example.
See my article about the 5450A and measuring errors on high resistances.
OCOMP does only work up to 100k (on all instruments), so switch that feature off deliberately on the 3458A.
Frank