Campbell Scientific CR3000 CR3000 Micrologger - Page 282

Section 8. Operation is reduced to ±2.5 Vdc, whereas input limits are always ±5 Vdc. Hence for nonnegligible DIFF signals, "input limits" is more descriptive than "common-mode range." Note Two sets of numbers indicate analog channel assignments. When differential channels are identified, analog channels are numbered 1 - 14. Each differential channel has two inputs: high (H) and low (L). Single-ended channels are identified by the number set 1-28. Caution Sustained voltages in excess of ±8.6 V input to the analog input channels can temporarily corrupt all analog measurements. Warning Sustained voltages in excess of ±16 V input to the analog channels will damage CR3000 circuitry. 8.1.2.2 Reducing Error Read More! Consult the following white papers at www.campbellsci.com for indepth treatment of the advantages of differential and single-ended measurements: Preventing and Attacking Measurement Noise Problems, Benefits of Input Reversal and Excitation Reversal for Voltage Measurements, and Voltage Measurement Accuracy, Self-Calibration, and Ratiometric Measurements. Deciding whether a differential or single-ended measurement is appropriate for a particular sensor requires sorting through trade-offs of accuracy and precision, available measurement hardware, and fiscal constraints. In broad terms, analog voltage is best measured differentially because these measurements include noise reduction features, listed below, that are not included in single-ended measurements. • Passive Noise Rejection o No voltage reference offset o Common-mode noise rejection Rejects capacitively coupled noise • Active Noise Rejection o Input reversal Review Input and Excitation Reversal (p. 288) for details Doubles input reversal signal integration time Reasons for using single-ended measurements, however, include: • Sensor is not designed for differential measurement. • Sensor number exceeds available differential channels. Sensors with a high signal-to-noise ratio, such as a relative-humidity sensor with a full-scale output of 0 to 1000 mV, can normally be measured single-ended without a significant reduction in accuracy or precision. 282