Measurement Computing TempBook rev.4.0 - Manual (Page 26)

2-8 Installation, Configuration, and Calibration 04-25-01 TempBook User’s Manual
Differential Measurement Configurations
Floating Differential
Floating differential measurements are generally made when low-level signals must be measured in the
presence of relatively high levels of common mode noise. The most common example would be a non-
grounded thermocouple. When the signal source has no direct connection to the system analog common,
one must be provided. The easiest way to do this on the TempBook is to use the built-in biasing resistors
by closing the corresponding input channel DIP switches (S1 and S2) on the termination card. Discussion
of the card begins on
page 2-2 . As an alternative, a resistor can be connected between one of the two signal
lines (usually the lower in potential) and common. A resistor of 10 K Ω to 100 KΩ is satisfactory (less
noise with the lower values).
Floating Differential Configuration
Referenced Differential
Referenced differential measurement is used when measuring several voltages. However, the voltages
cannot share the same common node as a group of single-ended measurements might. An example is to
measure several currents via shunts that happen to share a common supply terminal along a bus. The
weakness in a single-ended hookup is that current flow through the common bus can introduce substantial
errors that can be indistinguishable from the real values measured. The need for the system common to be
connected to the common supply terminal is shown in the figure.
Referenced Differential Configuration