Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 283

Section 8. Operation When the CR1000 reverses differential inputs or excitation polarity, it delays the same settling time after the reversal as it does before the first measurement. So, there are two delays per channel when either RevDiff or RevEx is used. If both RevDiff and RevEx are True, four measurements are performed; positive and negative excitations with the inputs one way and positive and negative excitations with the inputs reversed. To illustrate, 1. the CR1000 switches to the channel 2. sets the excitation, settles, measures, 3. reverses the excitation, settles, measures, 4. reverses the excitation, reverses the inputs, settles, measures, 5. reverses the excitation, settles, measures. There are four delays per channel measured. The CR1000 processes the four submeasurements into a single reported value. In cases of excitation reversal, 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 CR1000 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 CR1000 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 CR1000 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 283