Campbell Scientific SDM-SW8A 8-Channel Switch Closure Input Module - Manual (Page 18)

SDM-SW8A Switch Closure Input Module
width becomes a limitation at 70 Hz (0.35/0.005sec), or about 126 MPH. The
SW8A's maximum input frequency and 5 ms minimum pulse width
specifications are not a limitation given the 014A's maximum calibrated speed
of 100 MPH.
8.3 DC Voltage Pulse
Voltage pulse transitions from below 0.9 V to above 4.0V, not exceeding
±20V, with a minimum pulse width of 5ms, are counted accurately at any
frequency less than or equal to 100 Hz.
8.4 Duty Cycle
Duty cycle is not an exact measurement due to the SW8A input filtering and 2
ms sampling frequency. Signal magnitude also affects duty cycle
measurements. Optimum duty cycle measurements result if an integral number
of cycles are measured per datalogger scan interval, and the scan interval is at
least 1 second.
Input Filtering and Sample Frequency Error - In a "worst case" analysis,
input filtering will distort the time that the signal is high by ±2.5 ms. If the
input filtering is at worst case, the 2 ms sampling frequency can create an error
of ±2 duty cycle samples on any measurable cycle.
Equations given below estimate the "worst case" duty cycle measurement error
for a 50% duty cycle and the minimum/maximum measurable duty cycle for a
given frequency. The error limits are calculated, assuming a sample interval of
a single cycle. The error may be significantly reduced by allowing the SW8A
to measure duty cycle over several cycles. As shown by the equations, the
error decreases with decreasing frequency.
50% Duty Cycle - The "worst case" duty cycle measurement error for a 50%
duty cycle at a given input frequency is
±ERROR = Hz * 0.4
where ERROR = Actual Duty Cycle ±Measured Duty Cycle.
For example, a 50% duty cycle at 10 Hz could be measured as 46% to 54% in
the worst case.
Minimum/Maximum - The measurable minimum/maximum duty cycle is
defined by the 5 ms pulse width specification of the SW8A. For example, at a
10 Hz frequency, the minimum and maximum duty cycle that can be measured
is 5% and 95%, respectively (0.005/0.100 * 100). The "worst case" duty cycle
measurement error for the minimum/maximum measurable duty cycle is
±ERROR = Hz * 0.3
For example, the minimum measurable duty cycle for a 1 Hz signal is 0.5%.
The duty cycle measurement could range from 0.2% to 0.8%.
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