Transcript Slide 1

Digitization
• When data acquisition hardware receives an
analog signal it converts it to a voltage. An A/D
(analog-to-digital) converter then digitizes the
signal and makes it ready for transfer to a
computer or to a display.
• Digitization of an analog signal requires two
separate operations.
– Define the number of points and the rate at which data
are acquired.
– Quantization--conversion of data into numerical form.
Signal Inputs into an A/D
converter
• Single-Ended
– All inputs are referenced to a common ground
• Adequate for high level signals
• Less expensive but problematic if grounding
problems exist. (Ground Loop Problems)
• Differential
– Differences between Hi input and Lo input are
measured directly without the influence of ground
loop interference.
• About 2 times the expense of single ended
inputs
• Needs 2 times as many wires
• Always use for thermocouples and low voltage
applications
Conversion Scheme
A/D (analog-to-digital)
converters
• Many specifications are quoted by hardware
manufacturers. Here, we’ll try to explain what
some of them mean in practice. For example:
–
–
–
–
Resolution
Linearity
Throughput
Gain
Resolution
• Resolution of an A/D converter is the number of
steps into which the input range is divided.
Resolution is usually expressed as bits (N) and
the number of steps is 2 to the power of N.
• Example: A converter with a 12-bit resolution
divides the range into 212 , or 4096 steps.
– A 0-10 Volt range will be resolved to 10V/4096 or
0.25 mV.
– A 0-100 mV range will be resolved to 0.0025 mV.
– A -10 to 10 V range is resolved by 20V/4096.
Throughput
• Throughput is the maximum rate at which
the A/D converter can output data values.
• A converter that takes 10 microseconds to
acquire and convert will generate about
100,000 samples per second.
Accuracy
• % Reading + Count
– Look at the scale being used. It will display the value,
as given by that scale.
– Multiply the readying by the % reading value.
– Add the “Counts” x the value of the least digit
presented.
The example in the manual:
V=134.2 mV
Accuracy = 134.2 x 0.008 + 2 x 0.1 = 1.3 mV
Gain
• On board amplifiers may permit you to
reduce the range and thus increase
resolution.
GPIB(General Purpose Interface
Bus)
• Also known as IEEE-488
• Started by HP (HPIB)
– 16 line parallel connection
• Advantages
– Fast data transfer rates
• Up to 1 MB/s
– Multiple devices (15) on each
GPIB
• Disadvantages
– Limited transmission lengths (2
m to 4 m) between devices
– Need GPIB adapter in PC