Data
Acquisition Systems
A Selection Guide: Parameters
The Neff Selector
- a step-by-step tutorial designed to help you find the Neff product
that fits your data acquisition needs.
Parameters
When quantities of each transducer type are known
a system can be selected that supports the overall channel count,
the excitation requirements and the specific transducers that are
to be used. Issues include the use of constant voltage or constant
current excitation, transducer calibration, availability of remote
excitation sensing and the ability to monitor excitation levels.
Selection of other parameters such as analog bandwidth,
filter cutoff frequency and sampling rate are dependent upon expected
signal frequencies and signal waveshape. Are they basically sinusoids
or are they square waves or other complex shapes?
Measurement accuracy is dependent upon a number
of factors including gain and zero stability with time and temperature
changes, linearity, crosstalk between channels and noise. Discussions
of common-mode voltages sometimes cause eyes to glaze over but resultant
errors are real. The ability of a system to minimize the effects
of common-mode voltages is referred to as Common Mode Rejection
Ratio, expressed in dB. This ratio defines the effect a common-mode
signal at the input has on the system output. A single ended input
has no common-mode rejection; these voltages are simply added to
signal inputs so differential inputs are required for high performance
data acquisition. Some error sources are dependent upon amplifier
gain and filtering making a single, overall specification difficult
to assign. An error budget can be determined by combining errors
under specific conditions. Application
Note 505 offers a method of predicting uncertainty.
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