Keithley 3761 User Manual

Also see for 3761: Firmware release notes

Contents

Keithley 3761 User Manual Manual pdf 16 page image

In this section:Cables ...................................................................................... 4-1External factors ........................................................................ 4-1High impedance measurements............................................... 4-2CablesShielded cables should be used with the 3761. The shield should be connected to circuit guard,analog LO. This helps prevent excessive radiation from the cables from interfering with anyequipment. The cable rating must not exceed the maximum allowable signal levels as defined in the3761 card specifications. For more information on cable requirements, go to the Keithley Instrumentswebsite (http://www.tek.com/keithley) and search for Switching Handbook or Low CurrentMeasurements Application Note.The test system response is affected by cable capacitance. This should be considered when thesources are connected to the 3761. Use a feedback ammeter for currents below 10 -5 A to increasemeasurement speed and decrease the effects of cable capacitance measurement.External factorsThe 3761 is specified for a resistive load. However, external circuit capacitances and inductances cancause excessive currents and voltages across the relay contacts. The currents and voltages must belimited to within the maximum allowable signal levels.• Capacitance: Charged capacitances in the circuit can cause excessive currents (surge currents)to flow through the relay contacts if the source is shorted out or if the output is switched tocapacitive loads. This excessive current can weld the relay contacts together. Therefore, useseries resistors, where needed, to limit the maximum current to within the contact ratings. Themaximum signal levels are stated in the specifications.• Inductance: The inductance of cables can produce high voltage and arcing across relaycontacts when switching. The peak transients must be limited to within the maximum signal levelsas defined in the 3761 specifications.When a channel is opened or closed, there is a charge transfer in the picocoulomb range. This is dueto the mechanical release or closure of the contacts, the contact-to-coil capacitance, and the straycapacitance between the signal and relay drive lines. The charge transfer causes a current pulse, andthe effect on the signal depends on the magnitude of the source being measured.Extreme environmental conditions can cause the offset current to exceed the 1 pA specification. If thecard has been exposed to high humidity or temperature (for example, during shipping), stabilize theboard within the specified environmental limits for 24 hours.Section 4Operation considerations