Campbell Scientific CR3000 CR3000 Micrologger - Page 326

Section 8. Operation Input filters, however, 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 (listed in table Pulse-Input Channels and Measurements (p. 40) ). To avoid over attenuation, sensor output voltage must be increased at higher frequencies. As an example, table Filter Attenuation of Frequency Signals (p. 326) lists low-level ac frequencies and the voltages required to overcome filter attenuation. For pulse-input channels P1 - P4, an RC input filter with an approximate 1.2-μs time constant precedes the inverting CMOS input buffer. The resulting amplitude reduction is illustrated in figure Amplitude Reduction of Pulse-Count Waveform (p. 327). For a 0- to 5-Vdc square wave applied to a pulse channel, the maximum frequency that can be counted in high-frequency mode is approximately 250 kHz. Table 76. Time Constants (τ) Measurement Pulse channel, high-frequency mode Pulse channel, switch-closure mode Pulse channel, low-level ac mode Digital I/O, high-frequency mode Digital I/O, switch-closure mode τ 1.2 3300 See table Filter Attenuation of Frequency Signals (p. 326) footnote 0.025 0.025 Table 77. Filter Attenuation of Frequency Signals. As shown for low-level ac inputs, increasing voltage is required at increasing frequencies to overcome filter attenuation on pulse-input channels*. ac mV (RMS) Maximum Frequency 20 200 2000 5000 20 200 10,000 20,000 *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. 326