Preferred Measurement Connections - HP E1313A Manual

Preferred Measurement Connections

IMPORTANT!
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Chapter 2
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For any Scanning A/D Converter module to scan channels at high speeds, it
must use a very short sample period (<10 µsecond for the HP E1413). 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 2-8 shows recommended
connections for powered transducers, thermocouples, and resistance
transducers. (See Appendix F for more information on wiring techniques).
1. Use individually shielded, twisted-pair wiring for each channel.
2. Connect the shield of each wiring pair to the corresponding Guard
(G) terminal on the terminal module (see Figure 2-9 for schematic of
Guard to Ground circuitry on the terminal module).
3. 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 2-10 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
50/60 Hz) 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 2-8 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 2-8 B/D).
4. 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.
5. Use good quality foil or braided shield signal cable.
6. Route signal leads as far as possible from the sources of greatest
noise.
7. In general, do not connect Hi or Lo to Guard or Ground at the
HP E1413.
8. It is best if there is a DC path somewhere in the system from Hi or Lo
to Guard/Ground.
9. The impedance from Hi to Guard/Ground should be the same as from
Lo to Guard/Ground (balanced).
10. Since each system is different, do not be afraid to experiment using
the suggestions presented here until you find an acceptable noise
level.
Field Wiring 43