Campbell Scientific CR3000 CR3000 Micrologger - Page 288

Section 8. Operation Table 55. Analog Measurements and Offset Voltage Compensation CRBasic Voltage Measurement Instruction Input Reversal (RevDiff = True) Excitation Reversal (RevEx = True) Measure Ground Reference Offset (MeasOff = True) VoltDiff() * VoltSe() * TCDiff() * TCSe() * BrHalf() * BrHalf3W() * Therm107() * Therm108() * Therm109() * BrHalf4W() * * BrFull() * * BrFull6W() * * AM25T() * * Background Calibration (RevDiff = False) (RevEx = False) (MeasOff = False) * 8.1.2.6.1 Input and Excitation Reversal Reversing inputs (differential measurements) or reversing polarity of excitation voltage (bridge measurements) cancels stray voltage offsets. For example, if there is a 3-μV offset in the measurement circuitry, a 5-mV signal is measured as 5.003 mV. When the input or excitation is reversed, the measurement is -4.997 mV. Subtracting the second measurement from the first and dividing by two cancels the offset: 5.003 mV - (-4.997 mV) = 10.000 mV 10.000 mV / 2 = 5.000 mV. When the CR3000 reverses differential inputs or excitation polarity, it delays the same settling time after the reversal as it does before the first measurement. So, there are two delays per channel when either RevDiff or RevEx is used. If both RevDiff and RevEx are True, four measurements are performed; positive and negative excitations with the inputs one way and positive and negative excitations with the inputs reversed. To illustrate, 1. the CR3000 switches to the channel 2. sets the excitation, settles, measures, 3. reverses the excitation, settles, measures, 4. reverses the excitation, reverses the inputs, settles, measures, 5. reverses the excitation, settles, measures. There are four delays per channel measured. The CR3000 processes the four submeasurements into a single reported value. In cases of excitation reversal, 288