Specifications

VT1419A Guard
Connections
The VT1419A guard connection provides a 10 kW current limiting resistor between
the guard terminals (G) and VT1419A chassis ground for each 8 channel SCP bank.
This is a safety device for the case where the Device Under Test (DUT) isn’t
actually floating, the shield is connected to the DUT and also connected to the
VT1419A guard terminal (G). The 10 kW resistor limits the ground loop current,
which has been known to burn out shields. This also provides 20 kW isolation
between shields between SCP banks which helps isolate the noise source.
Common Mode
Voltage Limits
Be very careful not to exceed the maximum common mode voltage referenced to
the card chassis ground of ±16 volts (±60 volts with the VT1513A Attenuator
SCP). There is an exception to this when high frequency (1 kHz - 20 kHz) common
mode noise is present (see “VT1419A Noise Rejection” below). Also, if the DUT is
not grounded, then the shield should be connected to the VT1419A chassis ground.
When to Make
Shield Connections
It is not always possible to state positively the best shield connection for all cases.
Shield performance depends on the noise coupling mechanism which is very
difficult to determine. The above recommendations are usually the best wiring
method, but if feasible, experiment with shield connections to determine which
provides the best performance for a given installation and environment.
NOTE For a thorough, rigorous discussion of measurement noise, shielding and
filtering, see “Noise Reduction Techniques in Electronic Systems” by
Henry W. Ott of Bell Laboratories, published by Wiley & Sons, ISBN
0-471-85068-3.
Noise Due to Inadequate Card Grounding
If either or both of the VT1419A and Agilent/HP E1482 (MXI Extender Modules)
are not securely screwed into the VXIbus Mainframe, noise can be generated. Make
sure that both screws (top and bottom) are screwed in tight. If not, it is possible that
CVT data could be more noisy than FIFO data because the CVT is located in A24
space, the FIFO in A16 space; more lines moving could cause noisier readings.
Wiring and Noise Reduction Methods
Appendix D 373
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