Campbell Scientific CR3000 Micrologger - Manual (Page 328)

Section 8. Operation
328
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.