Campbell Scientific CR3000 CR3000 Micrologger - Page 304

Section 8. Operation 304 • Offset = 3 x Basic Resolution + 5.0 µV if the measurement is of a singleended input channel The following table lists basic resolution values. Table 65. Analog Input-Voltage Range and Basic Resolution Range (mV) Basic Resolution (µV) ±5000 167 ±1000 33.3 ±200 6.67 ±50 1.67 ±20 0.67 Assumptions that support the ratiometric-accuracy specification include: • • Excitation voltages less than 500 mV and excitation currents less than 500 µA are reversed during the excitation phase of the measurement. • Effects due to the following are not included in the specification: o Bridge-resistor errors o Sensor noise o Measurement noise The ratiometric-accuracy specification is applied to a three-wire half-bridge measurement that uses the BrHalf() instruction as follows: The relationship defining the BrHalf() instruction is X = V1/Vx, where V1 is the voltage measurement and Vx is the excitation voltage. The estimated accuracy of X is designated as ∆X, where ∆X = ∆V1/Vx. ∆V1 is derived using the following method. The ratiometric-accuracy specification is applied to a four-wire full-bridge measurement that uses the BrFull() instruction as follows: The relationship defining the BrFull() instruction is X = 1000*V1/Vx, where V1 is the voltage measurement and Vx is the excitation voltage. Result X is expressed as mV/V. Estimated accuracy of X is ∆X, where ∆X = 1000*∆V1/Vx. ∆V1 is derived using the following method. ∆V1 is derived using the ratiometric‐accuracy equation. The derivation is illustrated in this example, which is supported by the assumption that the measurement is differential with input reversal, datalogger temperature is between 0° to 40°C, analog‐input range is ±200 mV, V1 = 110 mV, and excitation is reversed during the excitation phase of the measurement. The effect each assumption has on the magnitude of ∆V1 in this example is noted in the following figure. In the case of the Resistance() instruction, sensor resistance is determined from Vs/Ix, where excitation current Ix is measured across a 1000 Ω, ±0.005% @ 25 ˚C, 2 ppm/˚C TCR internal resistor.