Measurement Computing Personal Daq/3000 Series - Manual (Page 81)

Shielding
Using shielded TC wire with the shield connected to analog common will result in further noise reduction.
Personal Daq/3000 Series modules have one analog common screw-terminal connection for every 2 analog SE
channel connections. You can connect the shield of a shielded thermocouple to one of the analog common
terminals. 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 Personal Daq/3000 module, 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 Personal
Daq/3000 Series module 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 volts.
If a thermocouple is connected directly to a component in
the 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 the 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 following 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
Running a Ground Wire to the Battery’s
Negative Terminal
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 .
A thin layer of heat-sink
compound on the indicated
surfaces will improve
thermal conductivity.
*
Appendix A 937492 Signal Modes and System Noise A-3