Resistance Measurement - Fluke 8062A Instruction Manual

8062A
Instruction Manual
A
Common
3-8.

Resistance Measurement

Resistance measurements are made using a ratio technique as shown in
Figure 3-6. When the resistance function is selected, a series circuit is
formed by the ohms source, a reference resistor from the voltage divider
(selected by the range switches), and the external unknown resistor. The ratio
of the two resistors is equal to the ratio of the voltage drop across each of
them. Since the voltage drop across the reference resistor and the value of the
reference resistor are known, the value of the second resistor can be
determined. Input protection during resistance measurements consists of a
thermistor and a double-transistor clamp.
The operation of the a/d converter during a resistance measurement is
basically as described earlier in this chapter, with a few exceptions. During
the integrate period the voltage drop across the unknown resistor charges the
INTEG capacitor. During the read period, the voltage across the known
resistor (stored on the flying capacitor) discharges the INTEG capacitor. The
length of the read period is a direct indication of the value of the unknown
resistor.
3-10
Current
Shunt
DC
AC
Figure 3-5. Current Measurement
True RMS
AC
Converter
HI
Inputs
to A/D
Converter
LO
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