294
Chapter 7 Tutorial
Measurement Fundamentals
Sources of Error in Resistance Measurements
External Voltages
Any voltages present in the system cabling or
connections will affect a resistance measurement. The effects of some of
these voltages can be overcome by using offset compensation (
as
described on the previous page
).
Settling Time Effects
The internal DMM 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 k
. Settling due to RC time constant effects can be
quite long. Some precision resistors and multifunction calibrators use
large parallel capacitances (1000 pF to 0.1
F) with high resistance
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, after a range change, or when using
offset compensation. You may need to increase the channel delay time
before a measurement in these situations (
for more information on
channel delay, see page 105
).
High-Resistance Measurement Errors
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
ohms). Leakage from nylon or PVC insulators can easily
contribute a 0.1% error when measuring a 1 M
resistance in humid
conditions. The table below shows several common insulating materials
and their typical resistances.
Insulating Material
Resistance Range
Moisture Absorbing
P
TFE
Nylon
PVC
Polystyrene
Ceramic
Glass Epoxy (FR-4, G-10)
Phenolic, Paper
1 T
to 1 P
1 G
to 10 T
10 G
to 10 T
100 G
to 1 P
1 G
to 1 P
1 G
to 10 T
10 M
to 10 G
N
Y
Y
N
N
Y
Y
34970A Refresh UG.book Page 294 Wednesday, February 17, 2010 12:34 PM
Summary of Contents for 34970A
Page 16: ...16 34970A Refresh UG book Page 16 Wednesday February 17 2010 12 34 PM ...
Page 20: ...34970A Refresh UG book Page 20 Wednesday February 17 2010 12 34 PM ...
Page 21: ...1 1 Quick Start 34970A Refresh UG book Page 21 Wednesday February 17 2010 12 34 PM ...
Page 39: ...2 2 Front Panel Overview 34970A Refresh UG book Page 39 Wednesday February 17 2010 12 34 PM ...
Page 59: ...3 3 System Overview 34970A Refresh UG book Page 59 Wednesday February 17 2010 12 34 PM ...
Page 87: ...4 4 Features and Functions 34970A Refresh UG book Page 87 Wednesday February 17 2010 12 34 PM ...
Page 217: ...5 5 Error Messages 34970A Refresh UG book Page 217 Wednesday February 17 2010 12 34 PM ...
Page 241: ...6 6 Application Programs 34970A Refresh UG book Page 241 Wednesday February 17 2010 12 34 PM ...
Page 254: ...254 34970A Refresh UG book Page 254 Wednesday February 17 2010 12 34 PM ...
Page 255: ...7 7 Tutorial 34970A Refresh UG book Page 255 Wednesday February 17 2010 12 34 PM ...
Page 324: ...324 34970A Refresh UG book Page 324 Wednesday February 17 2010 12 34 PM ...