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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.
The plot starts with choosing the correct chart type for the data. Selecting the wrong plot type can easily happen when it comes to line charts and scatter plots. They look very similar, especially when a scatter plot is displayed with connecting lines. There is a big difference in the way each of these chart types present data along the horizontal and vertical axes.
In a line chart, the values are displayed as two separate data points that are evenly distributed along the horizontal axis. This is because in a line chart, the vertical axis is a value axis and the horizontal axis is a category axis. Instead of displaying values, a category axis shows evenly spaced groupings (categories) of data. Because the data has only values and no categories, Microsoft Excel automatically generates numbered categories along the category axis, using numbers 1 through 9 on the x axis. The category axis can be changed and a different number set can be used on the x axis, but it is still a category grouping.
Conversely, a scatter chart has two value axes – it displays values rather than categories on the horizontal axis. Therefore, a scatter plot displays the data values as x values on the horizontal axis, and the chosen reference values as y values on the vertical axis. Because this chart type has two value axes, it can display a single data point at the intersection of each x and y value.
The first column selected is the x axis, while the second data column is the y axis. The goal here is to know what ADC voltage value is needed for each temperature value. Displaying the temperature (y axis) and intersecting the voltage (x axis) in Figure 2-1 allows calculation of the temperature from the known voltage using a polynomial equation based on the curve of the plot.
This section shows how to create the scatter plot using Microsoft Excel.
These steps yield a plot similar to Figure 3-1 with the temperature data on the left in the y axis and the voltage data on the bottom in the x axis.