a greater measure of confidence
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Using Electrometers & Picoammeters for
Low-Level Current Measurements
Measuring DC Amps Where the DMM Can’t
Using E lEctrom EtErs a nd Picoam mEtErs for low-lEv El cUrrEnt mEa sUrEmEnts measuring DC amps Where the Dmm Can’t
A greAter meAsure of confidence
Low-Level Current Measurement Applications
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High-resistance Measurements of Materials
– Measuring high-resistance of materials by sourcing high voltage and measuring low
current with a Model 6517B Electrometer and a Model 8009 Surface Resistivity Box
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Leakage Measurement of Capacitors
– Measuring capacitor leakage by sourcing high voltage and measuring leakage
current with a Model 6517B Electrometer
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Simplified Insulation Resistance Measurements
– Measuring insulation resistance by sourcing high voltage and measuring low current
with a Model 6517B Electrometer or a Model 6487 Picoammeter
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Testing Breakdown Voltages and Resistances
– Powering devices and measuring breakdown voltages and resistances by sourcing
voltage and measuring low currents with a Model 6487 Picoammeter
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Simplified Two-Channel, Powered Measurements
– Aligning and monitoring ion beams and optics, testing multiple devices, and testing
multi-pin devices with a Model 6482 Dual-channel Picoammeter
What is an Electrometer?
Like digital multimeters, (DMM,) Keithley electrometers are
instruments that measure charge, currents, voltages, and
resistances. However, electrometers measure beyond the
capabilities of standard DMMs by measuring charges with 10fC
resolution, currents with 100aA resolution, and resistances up to 200TΩ.
Electrometers are used where there is a need for extreme sensitivity or where
there is a need for multiple types of sensitive electronic measurements.
What is a Picoammeter?
Measuring low DC currents often demands a lot more than a DMM
can deliver. Generally, DMMs lack the sensitivity required to measure
currents less than 100nA. Even at higher currents, a DMM’s input
voltage drop (voltage burden) of hundreds of millivolts can make
accurate current measurements impossible. The low voltage burden
of a picoammeter makes it function much more like an ideal ammeter
than a DMM, so it can make current measurements with high
accuracy, even in circuits with very low source voltages. Keithley
picoammeters combine the economy and ease of use of a DMM with
low current sensitivity near that of an electrometer.
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Using E lEctrom EtErs a nd Picoam mEtErs for low-lEv El cUrrEnt mEa sUrEmEnts measuring DC amps Where the Dmm Can’t
A greAter meAsure of confidence
Simplified High Resistance Measurements of Materials Using a Keithley Electrometer
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High Resistance Measurements
Resistance is most often measured with a DMM, which can make measurements up to about
200MΩ. However, in some cases, resistances in the gigohm and higher ranges must be
measured accurately. These cases include such applications as characterizing high megohm
and gigohm resistors, determining the resistivity of insulators, and measuring the insulation
resistance of printed circuit boards. These measurements are made by using an electrometer,
which can measure both very low current and high impedance voltage.
Common High Resistance Measurements
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Insulation Resistance – Insulation resistance (IR) is the ratio of a DC applied voltage between two electrodes and
the total current between them. Examples of insulation resistance measurements include measuring the leakage
between traces on a printed circuit board or the resistance between conductors in a multi-conductor cable.
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Volume Resistivity Measurements – Volume resistivity is the electrical resistance through a one centimeter cube
of insulating material and is expressed in ohm-centimeters.
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Surface Resistivity Measurements – Surace resistivity is the electrical resistance between two electrodes on
the surface of an insulating material and is expressed in ohms (usually stated as ohms per square for clarity).
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Both volume and surface resistivity measurements are obtained by taking wresistance measurements and then converting them to
resistivity values by taking geometric considerations into account. Resistivity measurement setups are shown below in Figure 1. Both
volume and surface resistivity measurements can be improved by using test fixtures like the Keithley Model 8009 (shown in Figure 2)
and the Model 6517B Electrometer (shown in Figure 3).
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Figure 1. Resistivity measurements
Figure 2. Model 8009 Resistivity Test Fixture
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Figure 3. Model 6517B Electrometer/High Resistance Meter
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Using E lEctrom EtErs a nd Picoam mEtErs for low-lEv El cUrrEnt mEa sUrEmEnts measuring DC amps Where the Dmm Can’t
A greAter meAsure of confidence
Simplified Capacitor Leakage Measurements Using a Keithley Electrometer
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Capacitor Leakage Measurements
Capacitors are very important in all areas of electronics. From timing circuits to sample and hold applications, we depend on capacitors to
act in a nearly ideal fashion. In many cases, however, complex electrochemical interactions cause capacitors to fall short of perfect. One of
the less ideal properties that a capacitor has is leakage, or insulation resistance (IR).
Capacitor leakage can either be expressed as insulation resistance, expressed in megohm-microfarads (computed by dividing the resistance
value by the capacitance) or by leakage current at a specific voltage. The Model 6517B Electrometer is particularly useful for this application
because it can display either resistance or leakage current and will source up to 1000V DC.
Capacitor leakage is measured by applying a fixed voltage to the capacitor under test and measuring the resulting current. The leakage
current will decay exponentially with time, so it’s usually necessary to apply the voltage for a known period (the “soak” time) before
measuring the current. Improved performance will result if a forward-biased diode (D) is included in the circuit, as shown in Figure 4.
The diode acts like a variable resistance, low when the charging current to the capacitor is high then increasing in value as the current
decreases with time. The series resistor can be much smaller since it is only needed to prevent overload of the voltage source and damage
to the diode if the capacitor becomes short-circuited.
For statistical purposes, a quantity of capacitors is
often tested to produce useful data. Obviously, it is
impractical to perform these tests manually, so some
sort of automated test system is required. Figure 5
illustrates such a system, which employs a Model 6517B
Electrometer/High Resistance Meter and switching cards
that are installed in a switching mainframe. The Model
6517B is particularly useful for this application because it
can display either resistance or leakage current and will
source up to 1000V DC.
Figure 5. Capacitor leakage test systemFigure 4. Capacitor leakage test circuit with diode
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