Guarding - Keysight B2980B Series User Manual

How to Perform Measurement
Voltage Measurement
The filter function is used to average measurement result data, calculate moving
average, and so on. For the precise and reliable measurement, it is recommended
to use the filter function, see
information.

Guarding

Guarding is effective when a voltage source under test has the high output
resistance.
Figure 3-10
of triaxial cable has Common potential and the same voltage as the measured
voltage is applied between center and inner conductors. The insulation resistance
of cable is a finite value, so the voltage divided by the ratio of the output
resistance of voltage source under test to the insulation resistance of cable is
measured. If the output resistance is too large, an error that cannot be ignored
may occur in the measured data.
Also, there is an electrostatic capacitance between the inner shield and the center
conductor of triaxial cable, so the measured voltage is settled according to the
time constant determined by the electrostatic capacitance and the output
resistance. If the output resistance is too large, the settling may require the longer
time.
If the guard is used, the buffer amplifier keeps the potential of the inner shield at
the same potential as the center conductor of triaxial cable. So, the voltage is not
applied between both end of insulation resistance and capacitance of cable, and
these affect can be ignored. Therefore, you can make the precise and fast
measurement even for the voltage source under test that has the large output
resistance.
100
"Measurement Filter" on page 213
shows the theory of guarding. If the guard is not used, the inner shield
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