Specifications

Preferred Measurement Connections

For any A/D Module to scan channels at high speeds, it must use a very short

sample period (< 10 µs for the VT1419A). 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 2-9 shows

recommended connections for powered transducers, thermocouples, and resistance

transducers. (See Appendix D for more information on Wiring Techniques).

HINTS 1. Try to install Analog SCPs relative to Digital I/O as shown in “Separating

Digital and Analog Signals” in Appendix D.

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 2-10 for schematic of Guard to

Ground circuitry on the Terminal Module).

4. The Terminal Module is shipped with the Ground-to-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:

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

2-9 A/C).

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 2-9 B/D).

5. 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.

6. Use good quality foil or braided shield signal cable.

7. Route signal leads as far as possible from the sources of greatest noise.

8. In general, don’t connect Hi or Lo to Guard or Ground at the VT1419A.

9. It is best if there is a dc path somewhere in the system from Hi or Lo to

Guard/Ground.

10. The impedance from Hi to Guard/Ground should be the same as from Lo to

Guard/Ground (balanced).

11. Since each system is different, don’t be afraid to experiment using the

suggestions presented here until an acceptable noise level is found.

Field Wiring

Preferred Measurement Connections

36 Chapter 2

IMPORTANT!

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