Campbell Scientific CR6 CR6 Measurement and Control System - Page 312
Voltage Measurement Quality
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Section 8. Operation With reference to the figure PGIA with Input-Signal Decomposition (p. 312), the PGIA processes the voltage difference between the V+ and V- inputs while ignoring voltages that are common to both inputs, i.e. common-mode voltage. The figure shows the applied input voltage decomposed into a common-mode voltage (Vcm) and the differential-mode component (Vdm) of a voltage signal. Vcm is the average of the voltages on the V+ and V- inputs. So, Vcm = (V+ + V-)/2 or the voltage remaining on the inputs when Vdm = 0. The total voltage on the V+ and V- inputs is given as V+ = Vcm + Vdm/2, and V- = Vcm - Vdm/2, respectively. The PGIA ignores or rejects common-mode voltages as long as the total voltages on the V+ and V- inputs are within the specified input limits, which for the CR6 is ±5 Vdc relative to ground. Input voltages in which V+ or V- or both are beyond the ±5 Vdc input limits may suffer from undetected measurement errors. The term "common-mode range", defines the valid range of common-mode voltages and will be different than the "Input Limits" when the differential mode voltage in non-negligible. The following relationship is derived from the figure: Common-mode range = ±5 Vdc - |Vdm/2|. The conclusion follows that the common-mode range is not a fixed number, but instead decreases with increasing differential voltage. For differential voltages that are small compared to the input limits, common-mode range is essentially equivalent to "Input Limits". Yet for a 5000 mV differential signal, the commonmode range is reduced to ±2.5 Vdc, whereas Input Limits are always ±5 Vdc. Consequently, the term Input Limits is used to specify the valid voltage range of the V+ and V- inputs into the PGIA. Figure 83. PGIA with Input-Signal Decomposition - 8.1.2.1.3 Voltage Measurement Quality Read More Consult the following white papers at www.campbellsci.com for indepth treatments of several topics addressing voltage measurement quality: • Preventing and Attacking Measurement Noise Problems • Benefits of Input Reversal and Excitation Reversal for Voltage Measurements • Voltage Measurement Accuracy, Self- Calibration, and Ratiometric Measurements • Estimating Measurement Accuracy for Ratiometric Measurement Instructions. 312