Campbell Scientific CR10 CR10 Measurement and Control - Page 167

SECTION 13. CR1O MEASUREMENTS 13.1 FAST AND SLOW MEASUREMENT SEQUENCE The CR10 makes voltage measurements by integrating the input signal for a fixed time and then holding the integrated value for the analog to digital (A/D) conversion. The A,/D conversion is made with a 13 bit successive approximation technique which resolves the signal voltage to approximately one part in 7500 of the full scale range on a differential measurement (e.9., 1nsOO x 2.5 V = 333 uV). The resolution of a single-ended measurement is one part in 3750. Integrating the signal removes noise that could create an error if the signal were instantaneously sampled and held for the A,/D conversion. There are two integration times which can be specified for voltage measurement instructions, the slow integration (2.72 ms), or the fast integration (250 us). The slow integration time provides a more noise-free reading than the fast integration time. Integration time is specified in the Bange Code of the measurement instruction. Instructions 1 - 14 RANGE codes: Slow (2.72 ms lntegration time) Fast (250 us Integration time) 60 Hz reiection 50 Hz reiection 1 11 21 21222 31323 41424 51525 Full Scale range 31t 2.5 mV 32x 33t 7.5 mV 25 mV 34x 35t 250 mV 2500 mV One of the most common sources of noise is 60 Hz from AC power lines. Where 60 Hz noise is a probfem, range codes 21 - 25 should be used. Two integrations are made spaced 112 cycle apart (Figure'132-2\, which results in the AC noise integrating to 0. lntegration time for the 2500 mV range is 1/10 the integration time for the other gain ranges (2.72 ms). For countries with 50 Hz power Range codes 31 - 35 are used for 50 Hz rejection. There are severalsituations where the fast integration time is preferred. The fast integration time minimizes time skew between measurements and increases the throughput rate. The current drain on the CR10 batteries is lower when the fast integration time is used. The fast integration time should always be used with the AC half bridge (lnstruction 5)when measuring AC resistance or the output of an LVDT. An AC resistive sensor will polarize if a DC voltage is applied, causing erroneous readings and sensordecay. The induced voltage in an LVDT decays with time as current in the primary coil shifts from the inductor to the series resistance; a long integration time would result in most of the integration taking place after the signal had disappeared. 1/2 Cycle -->1 FIGURE 13.1-1. 50 and 60 Hz Noise Reiection 13-1