G-2
Applications Guide
Model 6487 Reference Manual
Measurement considerations
Some considerations for making accurate amps measurements are summarized as follows.
Additional measurement considerations are covered in Appendix C of the Model 6487
User’s Manual. For comprehensive information on precision measurements, refer to the
Low Level Measurements handbook, which is available from Keithley Instruments.
It is critical that the picoammeter perform the measurement without interfering with the
flow of current in the circuit, possibly affecting operation or inducing additional errors.
Voltage burden is the terminal voltage of a picoammeter and ideally this voltage should be
zero (no resistive or offset effect). Some meters, such as DMMs, utilize a shunt resistor to
measure the voltage drop across a known resistance and a typical burden of 200mV is not
unusual.
The Keithley Model 6487 uses an alternative approach referred to as a feedback picoam-
meter, where the voltage burden is simply the input voltage of an operational amplifier.
Furthermore, since the input voltage of the operational amplifier is the output voltage
divided by the gain (typically 500,000) the voltage burden is in the microvolt range.
Leakage currents and guarding
Leakage currents are generated by high resistance paths between the measurements circuit
and nearby sources. These currents can considerably degrade the accuracy of low current
measurements. Some ways to reduce leakage currents are to use good quality insulators,
maintain cleanliness on the surface of insulators, reduce humidity, and use guarding.
Guarding also reduces the effect of shunt capacitance in the measurement circuit.
One way to reduce leakage currents is to use good quality insulators when building the test
circuit. The best insulator is air. If possible, use air as the insulator for the sensitive node
(the part of the circuit between the source of current and the ammeter HI input). Because
this connection must eventually be mechanically supported, good insulators should still be
used whenever it is necessary to make physical contact to the sensitive node. Some good
quality insulators are Teflon, polyethylene, and sapphire. Avoid materials such as pheno-
lics and nylon. Refer to Keithley’s Low Level Handbook for additional information on
choosing the best insulator.
Humidity may also degrade low current measurements. The amount of water an insulator
absorbs will vary depending upon the insulator. It is best to choose an insulator on which
water vapor does not readily form a continuous film. Sometimes this is unavoidable if the
material being measured absorbs water easily, so it is best to make the measurements in an
environmentally controlled room. In some cases, an insulator may have ionic contami-
nants and, especially in high humidity, a spurious current may be generated.
