Fluke 8808A User Manual - Page 98

Making Low-Level Current Measurements, A-3. Shunt Method of Low-Level Current Measurement

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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 recharge 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. IIN A RInternal RSHUNT VOUT Figure A-3. Shunt Method of Low-Level Current Measurement eue28.eps 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 be rather low and thus induces an error. The zero burden voltage measurement method used in the Meter gives a more accurate measurement of low-level (leakage) currents. IIN RInternal RFeedback A VOUT Figure A-4. Zero Burden Voltage Low-Level Current Measurement eue29.eps A-8

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8808A
Users Manual
A-8
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.
A
V
OUT
I
I
N
R
SHU
N
T
R
Internal
eue28.eps
Figure A-3. Shunt Method of Low-Level Current Measurement
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 be rather low and thus
induces an error. The zero burden voltage measurement method used in the Meter gives a
more accurate measurement of low-level (leakage) currents.
V
OUT
I
I
N
R
Feed
b
ack
R
Internal
A
eue29.eps
Figure A-4. Zero Burden Voltage Low-Level Current Measurement