To test the accuracy of the acquisition circuit, a series of calibrated high-precision discrete resistors are used as an input to the system. Measurements are taken at TA = 25°C. Figure 114 displays the resistance measurement over an input span from 20 Ω to 400 Ω. Any offset error is generally attributed to the offset of the ADC, and the gain error can be attributed to the accuracy of the RREF resistor and the ADC. The RREF value is also calibrated to reduce the gain error contribution.
Precision temperature measurement applications are typically calibrated to remove the effects of gain and offset errors that generally dominate the total system error. The simplest calibration method is a linear, or two-point calibration that applies an equal and opposite gain and offset term to cancel the measured system gain and offset error. In this particular tested application, the gain and offset error was very small, and did not require additional calibration other than the self offset and gain calibration provided by the device. The resulting measured resistance error is shown in Figure 115.
The results in Figure 115 are converted to temperature accuracy by dividing the results by the RTD sensitivity (α) at the measured resistance. Over the full resistance input range, the maximum total measured error is ±0.00929 Ω. Equation 27 uses the measured resistance error and the RTD sensitivity at 0°C to calculate the measured temperature accuracy.
Figure 116 displays the calculated temperature accuracy of the circuit assuming a linear RTD resistance to temperature response. This figure does not include any linearity compensation of the RTD.