Keithley 3761 User Manual

Also see for 3761: Firmware release notes

Contents

Keithley 3761 User Manual Manual pdf 19 page image

Section 5: Typical applications Model 3761 10-Channel Low Current Multiplexer Card User's Manual5-2 3761-900-01 Rev. B / January 2018Application considerationsVoltage applications include those where the 3761 can be used to switch a single pole of low voltagesources. When measuring low currents, consider the following sources of noise:• Triboelectric currents are caused by friction between a conductor and insulator when a coaxcable flexes due to vibration or temperature fluctuation. To minimize noise, use cables such asthe Keithley Instruments Model 3761-BNC-SMA. This low-noise cable is ideal for low-levelmeasurements of voltage and current.• Piezoelectric currents are caused by mechanical stress to the insulating materials ofconnectors. Minimize stress on the insulators and use a connecting cable such as the3761-BNC-SMA. This cable features BNC and SMA connectors with low piezoelectric effects.• Electrochemical effects generate current between PCB conductors due to contamination of thecard 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 levelspecifications of the 3761 card. Maximum switched and carry current and voltage levels are100 mA and 30 V in voltage mode. The card can switch low power AC (typical bandwidth up to1 MHz). Maximum switched and carry current and voltage levels are 100 mA and 30 V peakusing resistive loads.Direct measurement of capacitor leakage or insulation resistanceIf you want to measure the leakage current or insulation resistance of a capacitor, the direct methodof measurement lets you apply almost any test voltage to the capacitor under test. The direct methoduses the 3761 in current mode, operating the card at zero volts even though the capacitor test voltageis large.The leakage current measurement depends on the capacitor dielectric and the applied voltage. Adirect measurement of the insulation resistance is shown in the next figure for three channels of the3761. In this test configuration, a picoammeter (IM) is connected to one of the two output connectorsof the 3761 card, a shielded test fixture is used to contain the capacitors under test, and a 3762 isused to route the test and soak voltages for the capacitors under test.To avoid inaccurate capacitor leakage measurements, do not enable the 3761 channels untilyou 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 testcompletes.If you are making measurements with capacitor values greater than 1 μF, use the indirectmethod (on page 5-4) with a voltmeter measuring the voltage drop across a resistorconnected in series with the capacitor under test.