10-07-2012, 12:33 PM
PRINCIPLES OF DATA ACQUISITION AND CONVERSION
PRINCIPLES OF DATA ACQUISITION AND CONVERSION.pdf (Size: 50.67 KB / Downloads: 17)
Data acquisition and conversion systems are used to acquire
analog signals from one or more sources and convert these
signals into digital form for analysis or transmission by end
devices such as digital computers, recorders, or communications
networks. The analog signal inputs to data acquisition
systems are most often generated from sensors and transducers
which convert real-world parameters such as pressure,
temperature, stress or strain, flow, etc., into equivalent
electrical signals. The electrically equivalent signals are then
converted by the data acquisition system and are then utilized
by the end devices in digital form. The ability of the
electronic system to preserve signal accuracy and integrity is
the main measure of the quality of the system.
How Many Samples per Cycle?
The answer to this question depends on the allowable average
error tolerance, the method of reconstruction (if any),
and the end use of the data. Regardless of the end use, the
actual error of the discrete data samples will be equal to the
throughput error of the data acquisition and conversion
system plus any digital errors contributed by a digital computer
or other digital end device.
A FEW A/D CONVERTER POINTS
A brief discussion of A/D converter terminology will help
the reader understand system resolution and accuracy a little
better.
Accuracy
All analog values are presumed to exist at the input to the
A/D converter. The A/D converter quantizes or encodes
specific values of the analog input into equivalent digital
codes as an output. These digital codes have an inherent
uncertainty or quantization error of ±1/2LSB. That is, the
quantized digital code represents an analog voltage that can
be anywhere within ±1/2LSB from the mid-point between
adjacent digital codes. An A/D converter can never be more
accurate than the inherant ±1/2LSB quantizing error. Analog
errors such as gain, offset, and linearity errors also
affect A/D converter accuracy.
Selecting the Resolution
The number of bits in the A/D converter determines the
resolution of the system. System resolution is determined by
the channel(s) having the widest dynamic range and/or the
channel(s) that require measurement of the smallest data
increment. For example, assume a channel that measures
pressure has a dynamic range of 4000psi that must be
measured to the nearest pound. This will require an A/D
converter with a minimum resolution of 4000 digital codes.
A 12-bit A/D converter will provide a resolution of 212 or
4096 codes—adequate for this requirement. The actual resolution
of this channel will be 4000/4096 or 0.976 psi.