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2.2.6
Sync Out
2.3
Measurement
Resolution
and Accuracy
2.3.1
Timed Mode
© Copyright 2024
United Electronic Industries, Inc.
•
Dividers for Digital Outputs: Dividers can be applied to signals that
are routed to the digital outputs. Each digital output can separately be
configured with its own divider value. Available divisors are 2, 4, 8, and
16. The output signals can also be inverted. The output pulse will always
have a 50% duty cycle.
The VR-608 has an internal Sync bus (Sync[3:0]) that allows signals to be sent
to the sync lines on the backplane. With the exception of the alarm signals, all of
the signals that can be sent to the digital outputs can also be sent to the sync out
lines. This means that sync lines can be routed to other sync lines. The sync bus
signals can also be routed to the digital outputs.
A multiplexer is used to route all sync out signals. The multiplexer is also used
when signals need to be routed from a channel to a digital output outside of the
channel's block, e.g., when a Z-pulse signal from channel 1 is routed to DOut 2.
Note that the sync signals have priority over the multiplexer lines. This means
that in a configuration where both a sync out signal and a digital output signal
need to be routed through the multiplexer, only the sync out pulse will occur.
Signals from the same channel can be routed to a sync out line and a digital
output as long as the digital output is in the channel's block.
The DNx-VR-608 accuracy is largely based on the chassis master clock. The
66 MHz master clock offers the following accuracy:
•
10 ppm initial accuracy
•
15 ppm temperature dependent drift over the -40 to +85 °C operating
range
•
5 ppm drift over the first year, less per year thereafter
For the calculations here, we will assume a flat 30 ppm accuracy of the 66 MHz
clock, or a period of 15.15 nanoseconds. Unlike most analog measurements,
the count mode of the DNx-VR-608, along with the input frequency, have a great
impact on overall measurement accuracy and resolution.
Timed Mode counts the number of teeth detected during a specified time interval
and returns the velocity in teeth per second or RPM. In this mode, the accuracy
is determined by the input frequency and the duration of the sampling. The
rough equation to determine resolution is simply:
Resolution
For example, if the input frequency is 10 kHz, sampled for 1 second, the
resolution is one part in 10,000 or 0.01%. If the input frequency is 1 kHz and the
sample duration is 0.1 seconds, the resolution is one part in 100, or 1%. To
compensate for crystal error you need only add a flat 0.003% (for the 30 ppm
master clock error).
Accuracy
=
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input frequency
February 2024
DNx-VR-608 Variable Reluctance Interface
100
=
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input frequency
100
sample duration
Chapter 2
Functional Description
sample duration
+
0.003%
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