Campbell Scientific CR3000 CR3000 Micrologger - Page 289

Section 8. Operation excitation "on time" for each polarity is exactly the same to ensure that ionic sensors do not polarize with repetitive measurements. Read More! A white paper entitled "The Benefits of Input Reversal and Excitation Reversal for Voltage Measurements" is available at www.campbellsci.com. 8.1.2.6.2 Ground Reference Offset Voltage When MeasOff is enabled (= True), the CR3000 measures the offset voltage of the ground reference prior to each VoltSe() or TCSe() measurement. This offset voltage is subtracted from the subsequent measurement. 8.1.2.6.3 Background Calibration If RevDiff, RevEx, or MeasOff is disabled (= False) in a measurement instruction, offset voltage compensation is still performed, albeit less effectively, by using measurements from automatic background calibration. Disabling RevDiff, RevEx, or MeasOff speeds up measurement time; however, the increase in speed comes at the cost of accuracy 1) because RevDiff, RevEx, and MeasOff are more effective techniques, and 2) because background calibrations are performed only periodically, so more time skew occurs between the background calibration offsets and the measurements to which they are applied. Note Disable RevDiff, RevEx, and MeasOff when CR3000 module temperature and return currents are slow to change or when measurement duration must be minimal to maximize measurement frequency. 8.1.2.7 Integration Read More! See White Paper "Preventing and Attacking Measurement Noise Problems" at www.campbellsci.com. The CR3000 incorporates circuitry to perform an analog integration on voltages to be measured prior to the A/D conversion. The magnitude of the frequency response of an analog integrator is a SIN(x) / x shape, which has notches (transmission zeros) occurring at 1 / (integer multiples) of the integration duration. Consequently, noise at 1 / (integer multiples) of the integration duration is effectively rejected by an analog integrator. table CRBasic Measurement Integration Times and Codes (p. 289) lists three integration durations available in the CR3000 and associated CRBasic codes. If reversing the differential inputs or reversing the excitation is specified, there are two separate integrations per measurement; if both reversals are specified, there are four separate integrations. Table 56. CRBasic Measurement Integration Times and Codes Integration Time (ms) CRBasic Code Comments 250 µs 250 Fast integration 16.667 ms _60Hz Filters 60-Hz noise 20 ms _50Hz Filters 50-Hz noise 289