Campbell Scientific CR3000 CR3000 Micrologger - Page 328

Section 8. Operation 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 CR3000 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. 328) 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. Figure 105: Input conditioning circuit for period averaging 8.1.7 SDI-12 Recording Read More! SDI-12 Sensor Support (p. 177) and Serial Input / Output (p. 512). SDI-12 is a communications protocol developed to transmit digital data from smart sensors to data-acquisition units. It is a simple protocol, requiring only a single communication wire. Typically, the data-acquisition unit also supplies power (12 Vdc and ground) to the SDI-12 sensor. The CR3000 is equipped with 4 SDI-12 channels (C1, C3, C5, C7) and an SDI12Recorder() CRBasic instruction. 328