Section 8. Operation
415
specifications for pulse input terminals to emphasize the need for matching the
proper device to the application.
Three Specifications Differing Between P and C Terminals
P Terminal
C Terminal
High-Frequency
Maximum
250 kHz
400 kHz
Input Voltage
Maximum
20 Vdc
16 Vdc
State Transition
Thresholds
Count upon transition
from
<0.9 Vdc to >2.2 Vdc
Count upon transition
from
<1.2 Vdc to >3.8 Vdc
8.1.3.8.2 Input Filters and Signal Attenuation
P and C terminals configured for pulse input have internal filters that reduce
electronic noise, which can cause false counts. However, input filters attenuate
(reduce) the amplitude (voltage) of the signal. Attenuation is a function of the
frequency of the signal. Higher-frequency signals are attenuated more. If a signal
is attenuated enough, it may not pass the detection thresholds required by the
pulse count circuitry.
The metric for filter effectiveness is τ, the filter time constant. The higher the τ
value, the less noise that gets through the filter. But, the higher the τ value, the
lower the signal frequency must be to pass the detection thresholds.
Detection thresholds, τ values, and low
-level ac pulse input ranges are listed in
TABLE: Time Constants
(p. 415)
A deduction from the specifications is that while a C terminal measured with the
TimerIO() frequency measurement may be superior for clean signals, a P
terminal filter (much higher τ) may be required to get a measurement on an
electronically noisy signal.
SPEC For example, increasing voltage is required for low-level ac inputs to
overcome filter attenuation on P 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 example
, the amplitude reduction that results from τ in high
-frequency pulse
input mode is illustrated in figure FIGURE: Amplitude Reduction of Pulse Count
Waveform
(p. 416).
Summary of Contents for CR3000 Micrologger
Page 2: ......
Page 3: ......
Page 4: ......
Page 6: ......
Page 30: ......
Page 34: ......
Page 36: ......
Page 96: ......
Page 485: ...Section 8 Operation 485 8 11 2 Data Display FIGURE 110 Keyboard and Display Displaying Data ...
Page 487: ...Section 8 Operation 487 FIGURE 112 CR1000KD Real Time Custom ...
Page 491: ...Section 8 Operation 491 FIGURE 116 Keyboard and Display File Edit ...
Page 496: ......
Page 502: ......
Page 564: ...Section 11 Glossary 564 FIGURE 126 Relationships of Accuracy Precision and Resolution ...
Page 566: ......
Page 594: ......
Page 598: ......
Page 600: ......
Page 602: ......
Page 624: ......
Page 642: ......
Page 643: ......