Section 13: Model 3750 multifunction control card
Series 3700A Switch and Control Cards Reference Manual
13-12
3700AS-909-01 Rev. B / December 2011
Output loading precautions
In addition to the maximum specified loads for the output voltage and current, note several other
precautions:
•
Excessive amounts of capacitance present on the voltage output nodes can affect their normal
behavior. For example, exceeding the specified output capacitance for the voltage output can
cause a stability problem and result in a noisy output. The Model 3750 voltage output stage is
compensated for a significant amount of capacitance that far exceeds the normal expected
amount due to cables, circuits, and loads.
•
Another consideration for the voltage output is load current. Inevitable resistance in the series
path of the voltage output experiences a voltage drop when current flows. At levels of 10 mA or
more, this can be a significant portion of the accuracy specification.
For example, only 0.1 Ohms of stray resistance causes 1 mV of error at 10 mA of load current.
To avoid this additional error, keep circuit loading to a minimum and use short heavy connections
where possible. For voltage drops that are constant, programming additional voltage can help
compensate.
•
For current outputs, excessive amounts of series resistance can also cause a problem if the
output voltage rises to near the compliance level. At voltages above the specified compliance
level, the output may experience higher offsets and ultimately result in a clamped value.
•
Because both outputs are electrically isolated from earth potential, they can float or be driven to
some arbitrary reference point. Do not exceed the maximum ratings stated for the card on any of
the channels under all operating conditions.
Hardware configuration
To configure digital I/O pull-up resistors and VEXT sources, you must remove and partially
disassemble the Model 3750 as shown.
CAUTION
Be sure to use proper anti-static procedures while handling the Model 3750. Take care not to stress
or flex the printed circuit board and do not touch other circuitry.
1. Remove the top shield cover:
•
Unscrew the number 4-40 screw (1) as shown in the "Shield removal" figure below.
•
Slide the top cover in a direction away from the D-sub connectors, disengaging the cover from the
printed circuit board.
•
Lift the top shield cover off of the printed circuit board.
2. Set jumpers per options listed below.
3. Replace the top shield cover.
•
Slide the top cover in a direction toward the D-sub connectors, engaging the cover onto the printed
circuit board, and securing with the number 4-40 screw (1).
4. The card can now be returned to service.