Fluke 8808A User Manual page 98

8808A
Users Manual
Making Low-Level Current Measurements
There are many applications where obtaining the utmost accuracy in low-level current
measurements is critical. For example, determining the leakage current of a battery
operated device in its standby mode is critical in determining the time before battery re-
charge is needed. Traditional multimeters make these measurements using a shunt
method shown in Figure A-3. The shunt resistor converts the current to be measured to a
voltage, which is called the burden voltage. Since the internal impedance of the current
source is in parallel with the shunt resistor, the current flowing in the shunt resistor is less
than the actual value, thus causing an error.
Another method of low-current measurement is the feedback resistor method shown in
Figure A-4. The feedback resistor converts the current to be measured into a voltage. The
high gain operational amplifier forces the burden voltage to approximately zero and thus
reduces the error associated with the simple shunt measurement approach. The zero
burden voltage measurement method used in the Meter gives a more accurate
measurement of low-level (leakage) currents.
A-8
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IN
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Figure A-3. Shunt Method of Low-Level Current Measurement
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IN
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Figure A-4. Zero Burden Voltage Low-Level Current Measurement
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Feedback
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