Fluke 9100 - Page 235

Section 4: Using the Model 9100: Capacitance Function 4.13-7
Final Width = 215mm
4.13.5.2 Hardware Configurations
When increasing or decreasing output Capacitance, using any method: if the new
Capacitance is too large or small for the present hardware configuration, then if OUTPUT
is OFF there will be no noticeable effect as the hardware reconfigures.
If OUTPUT is ON, it will be temporarily turned OFF so that the hardware can reconfigure,
then ON again at the new capacitance. No warning will be given. This interruption should
cause little disturbance to the reading on any UUT.
Note that because the current passes through external test leads connecting the capacitor
to the terminals, the voltage sensed across the terminals will include the lead volts drops,
so the result here will also include a small quadrature element due to the lead resistance.
4.13.5.3 Configuration for Capacitance Measurement in UUTs
Instruments which measure Capacitance generally use a method which drives a current
(Ic) as a stimulus through the test circuit (usually a capacitor) and measures the voltage
(Vc) developed across it. The stimulus can be a constant, ramping or AC current. Internal
circuits then calculate the Capacitance digitally, dependent on the type of stimulus.
Whatever the form of the stimulus current, the resulting voltage Vc will satisfy the basic
capacitive equation: C x dv/dt = i (instantaneous values), and the voltage sensing device
is designed to measure the form that the result will take. Digital calculations will derive
the value of the test capacitance, knowing the stimulus.
The 9100 assumes that a method such as that of Fig. 4.13.1, is employed by UUTs.
Fig. 4.13.1 UUT Configuration for Capacitance Measurement
Test
Capacitance
Ic
Voltage
Measurement
Capacitance
Calculation
and Display
Stimulus-
Current
Source
Vc
Hi
Com
UUT