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Al l - D i g ital Measurements
Planning the Measurement
Digital measurements are performed the same as for analog measurements except that there is no
anti-alias filtering and the physical connections are through digital signal analyzer pods instead of
coaxial wire and BNC connectors. Some features must be interpreted differently when the channel
is digital, like view input, input range, and overflow. Also, digital measurements require more
consideration of sample frequencies and data clocks.
The six following steps outline the planning process for a typical digital measurement on a
microprocessor-based system:
1.
Use a signal flow diagram to select measurement points.
connected to a shared data bus. Thus, the measurement points are determined by the selection
of the clocks and qualifiers which identify the data; not necessarily by the physical connection
point. Measurement points are not limited to existing I/O ports. Additional measurement
points can be created by modifying the microprocessor software to write data to (or read data
from) non-existent ports. This technique allows multiple test points to be placed inside the
software algorithm.
Figure
14-2
shows an example of a signal flow diagram. In this example there are three test
points. Only two of the test points,
all three test points are on same microprocessor data bus. Because the test points share a
common bus, only one data pod needs to be connected to capture one or
of
data. The data is identified using the data clock and qualifiers. Using qualifiers, test points
can be selected without changing the physical connections.
ADC
TPl
and
TP3,
1--------------------------------------------------1
microprocessor system
i
i
i
TP1
TP2
Figure 14-2. Signal Flow Diagram
Digital
and
All-Digital Measurements
The input data pod(s) are usually
correspond to actual I/O devices. Physically,
two
I
I
DAC
TP3
Mixed Measurements
channels-worth
14-3
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