Campbell Scientific CR6 CR6 Measurement and Control System - Page 345

Pay Attention to Specifications, 1.3.7.2 Input Filters and Signal Attenuation

Page 345 highlights

Section 8. Operation Table 80. Switch Closures and Open Collectors Switch Closure on U or C Terminal Open Collector on U or C Terminal Figu 8.1.3.7.1 Pay Attention to Specifications • Pay attention to specifications. Take time to understand the signal to be measured and compatible input terminals and CRBasic instructions. 8.1.3.7.2 Input Filters and Signal Attenuation U or C terminals are equipped with pulse-input filters to reduce electronic noise that can cause false counts. The higher the time constant (τ) of the filter, the tighter the filter. The table Time Constants lists τ values. Input filters attenuate the amplitude (voltage) of the signal. The amount of attenuation is a function of the frequency passing through the filter. Higherfrequency signals are attenuated more. If a signal is attenuated enough, it may not pass the state transition thresholds required by the detection device as listed in table Pulse-Input Terminals and Measurements ). To avoid over attenuation, sensor-output voltage must be increased at higher frequencies. For example, table Low-Level Ac Amplitude and Maximum Measured Frequency (p. 346) shows increasing voltage is required for low-level ac inputs to overcome filter attenuation on U terminals configured for low-level ac: 8.5 ms time constant filter (19 Hz 3 dB frequency) for low-amplitude signals; 1 ms time constant (159 Hz 3 dB frequency) for larger (> 0.7 V) amplitude signals. For U or C terminals, an RC input filter with an approximate [TBD µs time constant precedes the inverting CMOS input buffer. The resulting amplitude reduction is illustrated in figure Amplitude Reduction of Pulse-Count Waveform (p. 346). For a 0 to 5 Vdc square wave input to a pulse terminal, the maximum frequency that can be counted in high-frequency mode is approximately 250 kHz. Table 81. Time Constants (τ) Measurement U terminal low-level ac mode U terminal high-frequency mode U terminal switch-closure mode C terminal high-frequency mode C terminal switch-closure mode τ See footnote of the table Filter Attenuation of Frequency Signals (p. 346) 1.2 3300 0.025 0.025 345

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Section 8.
Operation
Table 80.
Switch Closures and Open Collectors
Switch Closure on
U
or
C
Terminal
Open Collector on
U
or
C
Terminal
Figu
8.1.3.7.1 Pay Attention to Specifications
Pay attention to specifications.
Take time to understand the signal to be
measured and compatible input terminals and CRBasic instructions.
8.1.3.7.2 Input Filters and Signal Attenuation
U
or
C
terminals are equipped with pulse-input filters to reduce electronic noise
that can cause false counts.
The
higher the time constant (τ) of the filter, the
tighter the filter.
The table Time Constants
lists τ values.
Input filters attenuate the amplitude (voltage) of the signal.
The amount of
attenuation is a function of the frequency passing through the filter.
Higher-
frequency signals are attenuated more.
If a signal is attenuated enough, it may not
pass the state transition thresholds required by the detection device as listed in
table Pulse-Input Terminals and Measurements
).
To avoid over attenuation,
sensor-output voltage must be increased at higher frequencies.
For example, table
Low-Level Ac Amplitude and Maximum Measured Frequency
(p. 346)
shows
increasing voltage is required for low-level ac inputs to overcome filter
attenuation on
U
terminals configured for low-level ac: 8.5 ms time constant filter
(19 Hz 3 dB frequency) for low-
amplitude signals; 1 ms time constant (159 Hz 3
dB frequency) for larger (> 0.7 V) amplitude signals.
For
U
or
C
terminals, an RC input filter with an approximate [TBD
µ
s time
constant precedes the inverting CMOS input buffer.
The resulting amplitude
reduction is illustrated in figure
Amplitude Reduction of Pulse-Count Waveform
(p.
346).
For a 0 to 5 Vdc square wave input to a pulse terminal, the maximum
frequency that can be counted in high-frequency mode is approximately 250 kHz.
Table 81.
Time Constants (τ)
Measurement
τ
U
terminal low-level ac mode
See footnote of the table
Filter Attenuation of
Frequency Signals
(p. 346)
U
terminal high-frequency mode
1.2
U
terminal switch-closure mode
3300
C
terminal high-frequency mode
0.025
C
terminal switch-closure mode
0.025
345