Campbell Scientific AM25T 25-Channel Solid State Multiplexer - Manual (Page 23)

Appendix A. Single-Ended
Measurements
Single-ended measurements with the AM25T are not recommended. This is not due to any
problem with the AM25T. Single-ended measurements have some problems that
differential measurements avoid. The problems with single-ended measurements are:
• Reduced noise rejection.
• Ground reference and Ground Loops.
A differential voltage measurement measures the voltage difference between two inputs.
Current is not allowed to flow through either input. A single-ended voltage measurement
measures the voltage of an input with respect to datalogger ground. Current can flow to
ground if there is a voltage potential to drive it. When current is flowing, the voltage will
change along the current path. For example, if a thermocouple’s measurement junction is
in electrical contact with a point on a car that is at +1 millivolt with respect to datalogger
ground, that voltage will be added to the thermocouple’s output, about a 2.5 degree error
on a copper-constantan thermocouple. Likewise, current flowing through the datalogger’s
ground can cause the ground reference to vary slightly along the terminal strip.
Despite the shortcomings, single-ended voltage measurements are possible with the
AM25T.
A.1 Directions for Single-Ended Measurements
If single-ended measurements are required, take the following precautions.
1. Use only shielded wire (this advice is just as pertinent for differential
measurements). Tie the sensor shields to the datalogger earth ground
through the multiplexer’s ground terminals.
2. Electrically insulate the measurement junctions of thermocouples. This
must be done to avoid errors caused by different ground potentials.
3. With a CR10(X), connect the AM25T ground to analog ground (AG)
instead of ground (G). Make sure any other power returns are connected
to ground (G).
When making single-ended measurements with a 21X,
power the AM25T, and any other peripherals, with a
separate power supply. Connect a single wire between a
21X ground and the negative terminal of the external
battery. CAUTION
A-1