Section 5: Typical applications
Model 3761 10-Channel Low Current Multiplexer Card User's Manual
5-2
3761-900-01 Rev. B
/
January 2018
Application considerations
Voltage applications include those where the 3761 can be used to switch a single pole of low voltage
sources. When measuring low currents, consider the following sources of noise:
•
Triboelectric currents
are caused by friction between a conductor and insulator when a coax
cable flexes due to vibration or temperature fluctuation. To minimize noise, use cables such as
the Keithley Instruments Model 3761-BNC-SMA. This low-noise cable is ideal for low-level
measurements of voltage and current.
•
Piezoelectric currents
are caused by mechanical stress to the insulating materials of
connectors. Minimize stress on the insulators and use a connecting cable such as the
3761-BNC-SMA. This cable features BNC and SMA connectors with low piezoelectric effects.
•
Electrochemical effects
generate current between PCB conductors due to contamination of the
card surface. To minimize card contamination, handle the card by the edges only.
To prevent overheating or damage to the relay contacts, do not exceed the signal level
specifications of the 3761 card. Maximum switched and carry current and voltage levels are
100 mA and 30 V in voltage mode. The card can switch low power AC (typical bandwidth up to
1 MHz). Maximum switched and carry current and voltage levels are 100 mA and 30 V peak
using resistive loads.
Direct measurement of capacitor leakage or insulation resistance
If you want to measure the leakage current or insulation resistance of a capacitor, the direct method
of measurement lets you apply almost any test voltage to the capacitor under test. The direct method
uses the 3761 in current mode, operating the card at zero volts even though the capacitor test voltage
is large.
The leakage current measurement depends on the capacitor dielectric and the applied voltage. A
direct measurement of the insulation resistance is shown in the next figure for three channels of the
3761. In this test configuration, a picoammeter (IM) is connected to one of the two output connectors
of the 3761 card, a shielded test fixture is used to contain the capacitors under test, and a 3762 is
used to route the test and soak voltages for the capacitors under test.
To avoid inaccurate capacitor leakage measurements, do not enable the 3761 channels until
you verify that the capacitors under test are fully charged.
Make sure that each capacitor under test has a series resistance and an individual discharge resistor.
The discharge resistors are used to drain the electric charge on the capacitors when the test
completes.
If you are making measurements with capacitor values greater than 1 µF, use the
indirect
method
(on page 5-4) with a voltmeter measuring the voltage drop across a resistor
connected in series with the capacitor under test.
