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AN897 Datasheet(PDF) 5 Page - Microchip Technology |
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AN897 Datasheet(HTML) 5 Page - Microchip Technology |
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5 / 16 page  2004 Microchip Technology Inc. DS00897B-page 5 AN897 FIGURE 12: PGA Output Voltage. The gain change points were chosen to make the ADC’s resolution as good as possible (see Figure 13) at a reasonable cost. The number of gains was kept low to minimize the piece-wise linear interpolation table’s size in firmware. The maximum voltage allowed in each range is 300 mV from VDD. This keeps the PGA in its specified output range and allows some headroom for noise. The minimum voltage allowed is well above 300 mV from ground, which keeps the PGA in its most linear region of operation. Random noise can make the PGA’s gain change frequently. Adding hysteresis to the gain-selection algorithm (in firmware) reduces this problem. The hysteresis needs to be large enough to compensate for the PGA’s maximum gain error (±1%). Figure 12 and Table 2 show a hysteresis of 1.7°C and 2.0°C at the lower temperature and higher temperature transistions, respectively. The gain-change points are separated by 6% of VDIV, which is six times larger than the PGA’s maximum gain error; this ensures proper functioning of the gain-change algorithm. The thermistor self-heating error has been corrected in Table 2. TABLE 2: PGA GAIN-CHANGE POINTS WITH HYSTERESIS. Analog Error Analysis Figure 13 displays the ADC’s temperature resolution and Figure 14 shows the expected worst-case analog circuit errors. Both plots are based on these assumptions: • ADC’s DC Error ≤ ±3.5 LSb • PGA’s gain error ≤ ±1% (±0.1% at G = +1) • PGA’s input offset error ≤ ±1 mV (including PSRR and temperature drift) • Specified thermistor accuracy This design achieves an ADC temperature resolution of 0.27°C over the -40°C to +150°C temperature range. The analog circuit accuracy is better than 3.0°C over the same range. Other temperature ranges will have different resolutions and accuracies. FIGURE 13: ADC’s Temperature Resolution. FIGURE 14: Analog Circuit Errors. Digital Design The PIC16F684 microcontroller (Appendix A.2.5 “Signal Analysis PICtail Daughter Board”) handles several important tasks. It communicates with the PGA to set its gain and input channel, can provide averaging to reduce the noise and converts the result into the tem- perature at the thermistor using a piece-wise linear interpolation table. The microcontroller can have either a SPI port built in or the SPI interface can be implemented in software on the microcontroller [7]. Gain (V/V) Gain Change (V/V) ADC Code (LSb) VDIV (V) TTH (°C) 11 → 8 < 113 0.552 50.9 88 → 1 > 960 0.586 49.2 8 → 32 < 226 0.138 94.6 32 32 → 8 > 960 0.146 92.6 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 -50 -25 0 25 50 75 100 125 150 Thermistor Temperature (°C) Hysteresis Design # 2 RA = 28.0 k G = +1 G = +32 G = +8 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 -50 -25 0 25 50 75 100 125 150 Thermistor Temperature (°C) Design # 2 RA = 28.0 kΩ 10-bit ADC DC Error ≤ 3.5 LSb 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 -50 -25 0 25 50 75 100 125 150 Thermistor Temperature (°C) Design # 2 RA = 28.0 k PGA Error ADC Error RA Error |
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