ADLINK PCI-9524 - Manual (Page 71)

Operation Theory 55
ADLINK Technology Inc. PCI-9524
Copyright 2008 User’s Manual
ing pins. For channels that are not connected to a transducer,
feedback the excitation voltage to the corresponding remote-sens-
ing pins directly on the terminal board. Please refer to section 3.2
for details.
PCI-9524 applies simulated AC excitation to the load-cell trans-
ducers and must respond quickly toward the excitation voltage
change, otherwise amplitude attenuation will occur. In general,
calibrating your system and working at a fixed sampling rate not
exceeding 100-SPS is preferred for most high accuracy applica-
tions. Also, using a lower impedance transducer, shortening the
connecting cable, and increasing the wiring gauge will help to
improve the response time.
Load-cell transducers with inductive properties are not recom-
mended to be used with PCI-9524.
4.2.7 Thermal EMF, 1/f Noise and Auto-zero
Thermal electromotive force (Thermal EMF) is the most common
error in a low-level signal measurement system. A junction made
of dissimilar metals develops some voltage difference across it.
Working like a tiny thermal-couple, this phenomenon is also
known as the Seebeck effect. Common lead-tin solder junctions
can have 1 to 3µV/ºC temperature coefficients, and even tight con-
nected cooper-cooper junctions without oxidation will have as
much as 0.3µV/ºC temperature coefficients. For high-resolution
load-cell applications calling for a voltage resolution higher than
100 nano-Volts, obviously, a 1ºC fluctuation in temperature will
bury the signal of interest.