Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 294

Section 8. Operation Other sensors, e.g., LVDTs (linear variable differential transformers), require an ac excitation because they rely on inductive coupling to provide a signal. dc excitation will provide no output. CR800 bridge measurements can reverse excitation polarity to provide ac excitation and avoid ion polarization. Note Sensors requiring ac excitation require techniques to minimize or eliminate ground loops. See Ground Looping in Ionic Measurements (p. 91). 8.1.3.2 Accuracy of Ratiometric-Resistance Measurements The ratiometric-accuracy specification for resistance measurements is: ±(0.04% * V1 + Offset), ‐25° to 50° C, where V1 is the voltage measurement and Offset is equal to one of the following, where the Basic Resolution is the resolution of a single A/D (p. 425) conversion. Note that excitation reversal reduces offsets by a factor of two: • Offset = 1.5 x Basic Resolution + 1.0 µV if the measurement is made on a differential input channel with input reversal • Offset = 3 x Basic Resolution + 2.0 µV if the measurement is made on a differential input channel without input reversal • Offset = 3 x Basic Resolution + 3.0 µV if the measurement is of a singleended input channel • The following table lists basic resolution values. Table 62. Analog Input-Voltage Range and Basic Resolution Range (mV) Basic Resolution (µV) ±5000 ±2500 ±250 ±25 ±7.5 ±2.5 1333 667 66.7 6.7 2.0 0.67 Assumptions that support the ratiometric-accuracy specification include: • Excitation voltages less than 1000 mV are reversed during the excitation phase of the measurement. 294