Section 4: Using the Model 9100: Resistance Function 4.7-7
Final Width = 215mm
4.7.5.2 Hardware Configurations
When increasing or decreasing output resistance, using any method: if the new resistance
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, the new hardware will be reconfigured as quickly as possible to
minimize the disturbance to autoranging UUTs.
When moving from one hardware configuration to another, the 9100 will automatically
select the 'best' current setting for the new output value, based on the previous current
setting.
4.7.5.3 Configuration for Resistance Measurement in UUTs
Instruments which measure resistance generally use a method which drives a 'pseudo-
constant' current (Ir) through the test circuit (usually a resistor) and measures the voltage
(Vr) developed across it. Internal circuits then calculate the resistance digitally, using a
form of Ohm's Law:
R = V/I
The 9100 assumes that this form of measurement is employed by the UUT. A simplified
illustration is shown in Fig. 4.7.1:
Test
Resistance
Ir
Voltage
Measurement
Resistance
Calculation
and Display
Pseudo
Constant
Current
Source
Vr
Ω
Com
UUT
Fig. 4.7.1 UUT Configuration for Resistance Measurement
Note that because Ir passes through external test leads connecting the resistor to the
terminals, the voltage sensed across the terminals will include the lead volts drops, so the
result here will also include the resistance of the external leads.
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