Measurement Tutorial 7
Keysight 34420A User’s Guide 287
Settling Time Effects
The meter has the ability to insert automatic measurement settling delays. These
delays are adequate for resistance measurements with less than 200 pF of
combined cable and device capacitance. This is particularly important if you are
measuring resistances above 100 kW. Settling due to RC time constant effects can
be quite long. Some precision resistors and multi-function calibrators use large
parallel capacitors (1000 pF to 0.1 mF) with high resistor values to filter out noise
currents injected by their internal circuitry. Non-ideal capacitances due to
dielectric absorption (soak) effects in cables and other devices may have much
longer settling times than expected just by RC time constants. Errors will be
measured when settling after the initial connection and after a range change. You
may need to increase the delay time before a measurement in these situations
(see page 100).
Errors in High Resistance Measurements
When you are measuring large resistances, significant errors can occur due to
insulation resistance and surface cleanliness. You should take the necessary
precautions to maintain a “clean” high-resistance system. Test leads and fixtures
are susceptible to leakage due to moisture absorption in insulating materials and
“dirty” surface films. Nylon and PVC are relatively poor insulators (10
9
ohms) when
compared to PTFE insulators (10
13
W). Leakage from nylon or PVC insulators can
easily contribute a 0.1% error when measuring a 1 MW resistance in humid
conditions.
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