3-2 Volts and Ohms Measurements
Measurement overview
Volts measurements — Model 6514 can make volts measurements from 10µV to 210V
using three measurement ranges; 2V, 20V, and 200V.
Ohms measurements — Model 6514 makes ohms measurements by sourcing a test current
and measuring the voltage drop across the DUT. The resistance reading is then calculated
(R = V/1) and displayed. The electrometer can make ohms measurements from 10mΩ to 210GΩ
using nine measurement ranges; 2kΩ, 20kΩ, 200kΩ, 2MΩ, 20MΩ, 200MΩ, 2GΩ, 20GΩ,and
200GΩ.
NOTE Accuracy specifications for all measurement functions are provided in Appendix A.
Guarding
The purpose of guarding is to eliminate the effects of leakage resistance and capacitance, that
can exist between input HI and input LO. This leakage resistance and capacitance could
adversely affect high-impedance measurements.
Test circuit leakage
In a test circuit, leakage current can occur through the insulators of the terminals for the DUT
(device under test). In Figure 3-1, the test circuit consists of a current source in series with the
DUT. The objective is to make an accurate voltage measurement of the DUT.
In Figure 3-1A, a resistance leakage path through the insulators (RL1 and RL2) shunts cur-
rent around the DUT. If this leakage current is high in comparison to the DUT current, signifi-
cant measurement error will occur. To keep error <0.1%, the leakage resistance must be 1000
times greater than the resistance of the DUT. For example, if the nominal resistance of the DUT
is 100MΩ, leakage resistance must be >100GΩ.
Figure 3-1B shows how to use guarding to eliminate the effects of leakage resistance. With
GRD enabled, the driven guard, which is at the same potential as input HI, is connected to the
metal mounting plate (now known as the guard plate). With both ends of RL1 at the same poten-
tial, current will not flow through the insulator. With no current leakage path, all current flows
through the DUT allowing an accurate voltage measurement.
The above explanation also pertains to ohms measurements. The only difference is that the
test current is provided by Model 6514.
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