Download Problem: Error in Low Voltage, Low Current Measurements

A
G R E AT E R
M E A S U R E
O F
C O N F I D E N C E
Problem: Error in Low
Voltage, Low Current
Measurements
Potential Cause: Voltage Burden
One of the biggest sources of error in
measuring low currents from low voltage
sources is voltage burden. Unlike an ideal
ammeter, a shunt style ammeter (for example, in a DMM) has some resistance, and
current flowing through it will create a voltage drop. This voltage burden, which can
reach several hundred millivolts, subtracts
directly from the source voltage and thus
reduces the measured current. If the source
voltage is already low, then this causes substantial errors.
For example, in a semiconductor circuit
the source voltage may be a single junction
voltage drop of 0.7V. Assuming that the
source resistance is 10kΩ and the voltage
burden is 200mV, then the current through
the meter will be:
0.7V
– 0.2V
__________
10,000
= 50µA
Compare this to the ideal case where
there is no voltage burden:
0.7V
– 0.0002V
_____________
= 69.98µA
10,000
This is an error of 29%.
Remedy
Electrometers and picoammeters, because of their feedback design, have very
low voltage burdens—typically in the tens
to hundreds of microvolts. If a picoammeter
with a voltage burden of 200 microvolts is
used in our example, then
This is an error of 0.03% with the electrometer/picoammeter’s feedback technique
instead of the 29% with DMM’s shunt style
current measurement. The low voltage burden of the electrometer and picoammeters
allows a much more precise current measurement.
Specifications are subject to change without notice.
All Keithley trademarks and trade names are the property of Keithley Instruments, Inc.
All other trademarks and trade names are the property of their respective companies.
Keithley Instruments, Inc.
28775 Aurora Road • Cleveland, Ohio 44139 • 440-248-0400 • Fax: 440-248-6168
1-888-KEITHLEY (534-8453) • www.keithley.com
© Copyright 2004 Keithley Instruments, Inc.
Printed in the U.S.A.
Problem: Error in Low Voltage, Low Current Measurements
No. 2559
1004
October 2004
1