Fluke 9100 - Page 237

Section 4: Using the Model 9100: Capacitance Function 4.13-9
Final Width = 215mm
Capacitance Function Action
Consider the switches at position 'A'. The UUT drives its stimulus current I
C
into C
IN
via
the Hi terminal, and draws I
C
via the Lo terminal. The value of C
IN
can be one of five
possible values, selected automatically to accommodate the range of stimulus currents.
The system gain is set by the 'Total Capacitance Demand' set value (C
T
), transferred by
DAC to control the gain of amplifier 'G'. The final amplifier is switched in decade values.
The overall result is an output voltage (V
C
) satisfying dv
c
/dt = i
c
/C
T
, placed across the Hi
and Lo terminals, while sourcing the same instantaneous values of i
c
, drawn by the UUT
from the Lo terminal.
The value of the virtual capacitance with the switches in position 'A' is given by:
C
T
= (C
IN
/G) x (R
SET
/R
STD)
With the switches in position 'B', for a particular range of I
C
values, the value of the virtual
capacitance is given by:
C
T
= (C
STD
/G) x (R
SET
/R
IN)
4.13.5.5 4-Wire Connections for Capacitance Function
In order to protect the 9100 output, fuses and low-value protection resistors are placed
internally in series with each of the current-carrying wires. Also, as noted earlier, the
external current-carrying wires have their own resistance. The Lead-Impedance
Compensation Bridge is used to compensate for these impedances between the voltage
V
C
and the UUT input.
In Fig. 4.13.2, the 9100 and UUT are shown in 4-wire connection. One pair of wires
passes
I
C
(Hi and Lo at the 9100), while the second pair (Hi Sense and Lo Sense) senses
the voltage at the UUT input terminals.
The bridge receives the sensed voltage from the UUT terminals. If this is not exactly the
same as V
C
, the bridge compensates for any volts drops in the protection circuitry and
interconnecting leads, maintaining V
C
at the UUT input.
It is most important that a four-wire connection be used for Capacitance function. The
Model 9105 leadset, supplied with the 9100, provides a four-wire connection. It is
recommended that the leadset be fitted, using its four short banana leads, whenever the
9100 is being used in Capacitance function.
When a four-wire connection is being used, the '
4 WIRE' screen key beneath the LCD
display should be selected (light characters on dark background). Four-wire operation is
already selected in the default condition of the Capacitance function.
Refer to pages 4.13.2/3.
Caution:
The greatest error exists when 2-wire connections are being used, with 4-wire selected.