Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 278

Section 8. Operation 8.1.2.6 Offset Voltage Compensation Analog measurement circuitry in the CR800 may introduce a small offset voltage to a measurement. Depending on the magnitude of the signal, this offset voltage may introduce significant error. For example, an offset of 3 μV on a 2500-mV signal introduces an error of only 0.00012%; however, the same offset on a 0.25mV signal introduces an error of 1.2%. The primary source of offset voltage is the Seebeck effect, which arises at the junctions of differing metals in electronic circuits. Secondary sources of offset voltages are return currents incident to powering external devices through the CR800. Return currents create voltage drop at the ground terminals that may be used as signal references. CR800 measurement instructions incorporate techniques to cancel these unwanted offsets. The table Analog Measurement Instructions and Offset Voltage Compensation Options (p. 278) lists available options. Table 53. 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. 278