VXi VT1422A 사용자 설명서

The VT1422A guard connection provides a 10 k

Ω current limiting resistor 

between the guard terminals (G) and VT1422A 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 VT1422A guard terminal (G). The 10 k

Ω resistor 

limits the ground loop current, which has been known to burn out shields. 
This also provides 20 k

Ω isolation between shields between SCP banks 

which helps isolate the noise source.

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 “VT1422A 
Noise Rejection” below). Also, if the DUT is not grounded, then the shield 
should be connected to the VT1422A chassis ground.

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 the 
particular installation and environment.

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.

If either or both of the VT1422A 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.