Measurement Computing DaqBoard 3000USB Series - Manual (Page 123)

Appendix B 938390 Signal Modes and System Noise B-3
Shielding
Using shielded TC wire with the shield connected to analog common will result in further noise reduction.
DaqBoard/3000USB Series boards have one analog common screw-terminal on TB7 and have several analog
common pins on the headers (see Chapter 2 pinouts). You can connect the shield of a shielded thermocouple to one
of the analog commons. When this connection is made the shield at the other end of the thermocouple is to be left
unconnected.
If a thermocouple shield is connected to the DaqBoard/3000USB Series board, leave the shield
unconnected at the other end of the thermocouple. Connecting the shield to common at both
ends will result in a ground loop.
TC Common Mode
The maximum common-mode voltage for a DaqBoard/3000USB
board is ±10 volts. Common-mode voltage is the DC or AC
voltage signal that is applied equally to both sides of a
differential input. Since thermocouples are measured using the
100 mV range, their maximum common mode voltage is ±6.0
volts.
If a thermocouple is connected directly to an engine component
in a motor vehicle, at a potential that is over the maximum
common-mode voltage, then very noisy or incorrect readings will
be seen. Thermocouple connections that are made directly to an
alternator or engine block may also result in high noise. Two
methods of reducing noise are:
(a) Run a ground line from the screw (or bolt), as
indicated in the first figure.
(b) Isolate the thermocouple leads with a set of
washers, one of which is electrically insulating
(such as mica), as indicated in the second figure.
Using a Washer Set and Heat Sink
to Isolate the Thermocouple
A thin layer of heat-sink
compound on the indicated
surfaces will improve
thermal conductivity.
The length of the insulating
shoulder washer’s hub must
not exceed the combined
thickness of the terminal ring
and mica washer.
*
The insulating shoulder washer
is non-metallic. It is made of
insulating material, such as
plastic
.
Running a Ground Wire to the
Battery’s Negative Terminal
*