Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 307

Section 8. Operation 8.1.5.4.3 Switch Bounce and NAN NAN will be the result of a TimerIO() measurement if one of two conditions occurs: 1. timeout expires 2. a signal on the channel is too fast (> 3 KHz) When the input channel experiences this type of signal, the CR800 operating system disables the interrupt that is capturing the precise time until the next scan is serviced. This is done so that the CR800 does not get bogged down in interrupts. An small RC filter retrofitted to the sensor switch should fix the problem. 8.1.6 Period Averaging The CR800 can measure the period of a signal on any single-ended analog-input channel (SE1 - 6). The specified number of cycles is timed with a resolution of 136 ns, making the resolution of the period measurement 136 ns divided by the number of cycles chosen. Low-level signals are amplified prior to a voltage comparator. The internal voltage comparator is referenced to the user-entered threshold. The threshold parameter allows a user to reference the internal voltage comparator to voltages other than 0 V. For example, a threshold of 2500 mV allows a 0- to 5-Vdc digital signal to be sensed by the internal comparator without the need of any additional input conditioning circuitry. The threshold allows direct connection of standard digital signals, but it is not recommended for small amplitude sensor signals. For sensor amplitudes less than 20 mV peak-to-peak, a dc blocking capacitor is recommended to center the signal at CR800 ground (threshold = 0) because of offset voltage drift along with limited accuracy (±10 mV) and resolution (1.2 mV) of a threshold other than zero. Figure Input Conditioning Circuit for Period Averaging (p. 308) shows an example circuit. The minimum pulse-width requirements increase (maximum frequency decreases) with increasing gain. Signals larger than the specified maximum for a range will saturate the gain stages and prevent operation up to the maximum specified frequency. As shown, back-to-back diodes are recommended to limit large amplitude signals to within the input signal ranges. Caution Noisy signals with slow transitions through the voltage threshold have the potential for extra counts around the comparator switch point. A voltage comparator with 20 mV of hysteresis follows the voltage gain stages. The effective input-referred hysteresis equals 20 mV divided by the selected voltage gain. The effective input referred hysteresis on the ± 25-mV range is 2 mV; consequently, 2 mV of noise on the input signal could cause extraneous counts. For best results, select the largest input range (smallest gain) that meets the minimum input signal requirements. 307