HP VXI 75000 C Series, User'S And Scpi Programming Manual

Page: 39 / 438

Field Wiring 39 Chapter 2
Preferred Measurement Connections
For any A/D Module to scan channels at high speeds, it must use a very short
sample period (<10
µ
second for the HP E1415). If significant normal mode
noise is presented to its inputs, that noise will be part of the measurement.
To make quiet, accurate measurements in electrically noisy environments,
use properly connected shielded wiring between the A/D and the device
under test. Figure 2-6 shows recommended connections for powered
transducers, thermocouples, and resistance transducers. (See Appendix E
page 361 for more information on Wiring Techniques).
Notes
1. Try to install Analog SCPs relative to Digital I/O as shown in
"Separating Digital and Analog Signals" in Appendix .
2. Use individually shielded, twisted-pair wiring for each channel.
3. Connect the shield of each wiring pair to the corresponding Guard
(G) terminal on the Terminal Module (see Figure 2-7 for schematic of
Guard to Ground circuitry on the Terminal Module).
4. The Terminal Module is shipped with the Ground-Guard
(GND-GRD) shorting jumper installed for each channel. These may
be left installed or removed (see Figure 2-8 to remove the jumper),
dependent on the following conditions:
a. Grounded Transducer with shield connected to ground at the
transducer: Low frequency ground loops (DC and/or 50/60Hz)
can result if the shield is also grounded at the Terminal Module
end. To prevent this, remove the GND-GRD jumper for that
channel (Figure 2-6 A/C).
b. Floating Transducer with shield connected to the transducer
at the source: In this case, the best performance will most likely
be achieved by leaving the GND-GRD jumper in place (Figure 2-6
B/D).
3. In general, the GND-GRD jumper can be left in place unless it is
necessary to remove to break low frequency (below 1 kHz) ground
loops.
4. Use good quality foil or braided shield signal cable.
5. Route signal leads as far as possible from the sources of greatest
noise.
6. In general, don’t connect Hi or Lo to Guard or Ground at the
HP E1415.
7. It is best if there is a D.C. path somewhere in the system from Hi or
Lo to Guard/Ground.
8. The impedance from Hi to Guard/Ground should be the same as from
Lo to Guard/Ground (balanced).
9. Since each system is different, don’t be afraid to experiment using the
suggestions presented here until you find an acceptable noise level.
IMPORTANT!