Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 286
Signal Settling Time
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Section 8. Operation Table 56. ac Noise Rejection on Large Signals 2. During A/D, CR1000 turns off excitation for ≈170 µs. 3. Excitation is switched on again for one-half cycle, then the second measurement is made. Restated, when the CR1000 is programmed to use the half-cycle 50-Hz or 60-Hz rejection techniques, a sensor does not see a continuous excitation of the length entered as the settling time before the second measurement if the settling time entered is greater than one-half cycle. This causes a truncated second excitation. Depending on the sensor used, a truncated second excitation may cause measurement errors. 8.1.2.8 Signal Settling Time When the CR1000 switches to an analog input channel or activates excitation for a bridge measurement, a settling time is required for the measured voltage to settle to its true value before being measured. The rate at which the signal settles is determined by the input settling time constant, which is a function of both the source resistance and fixed input capacitance (3.3 nfd) of the CR1000. Rise and decay waveforms are exponential. Figure Input Voltage Rise and Transient Decay (p. 286) shows rising and decaying waveforms settling to the true signal level, Vso. Figure 89: Input voltage rise and transient decay The CR1000 delays after switching to a channel to allow the input to settle before initiating the measurement. The SettlingTime parameter of the associated measurement instruction is provided to allow the user to tailor measurement instruction settling times with 100 µs resolution up to 50000 µs. Default settling times are listed in table CRBasic Measurement Settling Times (p. 287), and are meant to provide sufficient signal settling in most cases. Additional settling time may be required when measuring high-resistance (high-impedance) sensors and / or sensors connected to the datalogger by long leads. Measurement time of a given instruction increases with increasing settling time. For example, a 1 ms increase in settling time for a bridge instruction with input reversal and excitation reversal results in a 4 ms increase in time for the CR1000 to perform the instruction. 286