Campbell Scientific CR6 CR6 Measurement and Control System - Page 317
First Notch Frequency
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Section 8. Operation offset = 40 µV. Therefore, accuracy = ±0.315 mV + 40 µV = ±0.315040 mV Reducing the value to the significant digits, accuracy = ±0.32 mV. First Notch Frequency All analog signals have a certain amount of noise that cause error in the measurement. In selecting the first notch frequency, fN1, you can filter specific noise frequencies from the analog signal. The smaller the first notch frequency entered, the longer the measurement takes to complete. Read More See the technical paper Preventing and Attacking Measurement Noise Problems, which is available at www.campbellsci.com. The digital output from the CR6 delta sigma A-to-D (p. 489) converter is passed through a digital filter with the sinc frequency response illustrated in the figure Normalized Sinc Frequency Reponse (p. 318). The sinc filter completely notches out (rejects) signals at several frequencies. The first (lowest) notch frequency is referred to as fN1. fN1 is a parameter in CRBasic analog measurement instructions such as VoltDiff(). For example, 50 Hz or 60 Hz noise is common in areas serviced by 50 Hz or 60 Hz the mains ac power grid. The noise can be rejected by entering the argument fN1 = 50 or fN1 = 60, respectively. The time to complete an A-to-D conversion is proportional to 1/fN1, resulting in a trade-off between noise rejection and measurement speed. Selecting fN1 = 60 results in complete rejection of noise at frequencies of 60 Hz, 120 Hz, 180 Hz, etc. The time to complete the A-to-D conversion with fN1 = 60 is 1/60, or approximately 17 ms. The overall measurement time includes 1/fN1 plus the SettlingTime argument in the same CRBasic measurement instruction. Excitation reversal or differential-input reversal doubles this measurement time. Using both in the same measurement results in four measurements averaged together as the reported value. 317
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