In this section:
Cables ...................................................................................... 4-1
External factors ........................................................................ 4-1
High impedance measurements ............................................... 4-2
Cables
Shielded 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 any
equipment. The cable rating must not exceed the maximum allowable signal levels as defined in the
3761 card specifications. For more information on cable requirements, go to the
Keithley Instruments
website
(
http://www.tek.com/keithley
) and search for
Switching Handbook
or
Low Current
Measurements Application Note.
The test system response is affected by cable capacitance. This should be considered when the
sources are connected to the 3761. Use a feedback ammeter for currents below 10
-5
A to increase
measurement speed and decrease the effects of cable capacitance measurement.
External factors
The 3761 is specified for a resistive load. However, external circuit capacitances and inductances can
cause excessive currents and voltages across the relay contacts. The currents and voltages must be
limited 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 to
capacitive loads. This excessive current can weld the relay contacts together. Therefore, use
series resistors, where needed, to limit the maximum current to within the contact ratings. The
maximum signal levels are stated in the specifications.
•
Inductance:
The inductance of cables can produce high voltage and arcing across relay
contacts when switching. The peak transients must be limited to within the maximum signal levels
as defined in the 3761 specifications.
When a channel is opened or closed, there is a charge transfer in the picocoulomb range. This is due
to the mechanical release or closure of the contacts, the contact-to-coil capacitance, and the stray
capacitance between the signal and relay drive lines. The charge transfer causes a current pulse, and
the 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 the
card has been exposed to high humidity or temperature (for example, during shipping), stabilize the
board within the specified environmental limits for 24 hours.
Section 4
Operation considerations
