1 Capturing the Data

To capture the data needed to create the polynomials, mount the desired part onto a test board. The test board can be an EVM or any UUT (Unit Under Test) board. A data logging method is required to capture the ADC voltage at each preferred specific step. An example is a desired temperature range from –40°C to 125°C. Place the UUT inside a temperature chamber that is capable of sweeping from –40°C to 125°C. A moving sweep is a slow sweep that is always moving in the direction selected. The moving sweep must be programmed with a slope. The temperature slope must be between 2–3 minutes per degrees C, or about 330 minutes for a full sweep. In a temperature chamber enough time must be allowed for the entire circuit board to saturate at a given temperature. A smaller board becomes saturated quickly and is brought to temperature quicker; therefore, the test is faster. If capturing data at 1°C degrees steps, then the temperature must be captured every few seconds to line up the reference temperature with the UUT ADC voltage at that temperature. This usually requires a high-end chamber controller.

The simple method is to set the chamber to the desired temperature and allow time for the UUT to come up to temperature or saturate the UUT. Next, record the ADC voltage at that temperature. Set the temperature for the next data point and repeat the process until the entire range of data points is captured. The standard is 1 C degree steps for an NTC for a total of 165 steps. However, the TI TMP6 parts are very linear across the temperature range, so 5°C degree steps work very well for these parts. With a 5°C degree step, 33 data points are captured at –40°C, –35°C, –30°C, through 115°C, 120°C and 125°C. Align the voltage and temperature in a Microsoft Excel spreadsheet in the order as demonstrated in Figure 1-1. The voltage is on the left and the temperature is on the right. The importance of this setup becomes obvious later in the process.

Figure 1-1 Microsoft® Excel® Data Collected