ADLINK PCI-9524 User Manual
Page 72
56
Operation
Theory
ADLINK Technology Inc.
Copyright 2008
Table 4-2: Temperature Coefficient of different metal junctions
Junction Type
Temperature Coefficient ( µV/ºC )
Copper–Copper <
0.3
Copper–Gold 0.5
Copper–Silver 0.5
Copper–Lead-Tin Solder
1 to 3
Copper–Brass 3
Copper–Aluminum 5
Copper–Nickel 10
Copper–Copper Oxide
> 500
Wiring made to connect the load-cell and PCI-9524, inevitably cre-
ates multiple metal junctions. When there are temperature differ-
ences between these junctions, the thermal EMF will not be able
to cancel out each other, and generates an offset error that fluctu-
ates with ambient temperature change. The worst problem of ther-
mal EMF is that it creeps slowly in a very low frequency range,
typically below 1Hz, rendering any post digital filtering impractical
due to the extremely long settling time therefore required.
Besides thermal EMF, they are other noise sources that reside in
semiconductor devices, exhibiting 1/f noise properties; i.e. noise
density increases as frequency of interest decreases.
The auto-zeroing technique used on PCI-9524 helps to remove
systematic offset errors in the signal chain, including thermal EMF
drift and 1/f noise from the transducers, cabling, wiring, signal con-
ditioning and amplifiers.
For noise rejection response when auto-zero is enabled, please
refer to Figure 4-4. The solid line denotes SINC responses of ADC
signal gain running at 60 SPS; the dotted line denotes the simulta-
neous noise attenuation at both near-DC (0Hz) and near ADC's