Model 6517B Electrometer Reference Manual Section 4: Basic measurements
6517B-901-01 Rev. C / August 2015 4-31
Current measurement considerations
Ohms measurements are performed by forcing voltage and measuring current (FVMI). Thus, accurate
measurements require accurate current measurements. Capacitive inputs increase preamplifier
noise, resulting in increased noise across the voltage source terminals (refer to Other measurement
considerations (on page 4-35) for additional measurement considerations).
Charge measurements (Q)
The Model 6517B is equipped with four coulombs ranges to resolve charges as low as 10 fC (10
-14
C)
and measure as high as 2.1 µC. In the coulombs function, an accurately known capacitor is placed in
the feedback loop of the amplifier so that the voltage developed is proportional to the integral of the
input current in accordance with the formula:
The voltage is scaled and displayed as charge.
Basic measurement procedure
After measuring high voltage in the volts function, it may take a number of minutes for input current
to drop to within specified limits. Input current can be verified by placing the protection cap on the
INPUT triaxial connector and then connecting a jumper between COMMON and chassis ground.
With the instrument on the 20 pA range and zero check disabled, allow the reading to settle until the
input bias current is within specifications.
Auto discharge
The Model 6517B has an auto discharge feature for the coulombs function. When enabled, auto
discharge resets the charge reading to zero when the charge reading reaches the specified level.
After the integrator resets, the charge measurement process simply restarts at zero.
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