AD7175-8 Internal Temperature Sensor Conversion

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This is to accompany the equation for a temperature reading in Degrees C for the AD717x family of SD ADCs that include an internal temperature sensor. The AD7175-8 datasheet details the internal temp sensor repeated below. Note, the AD7173-8 datasheet does not include the temperature sensor details. 
 
 From the AD7175-8 Datasheet: The AD7175-8 has an integrated temperature sensor. The 
 temperature sensor can be used as a guide for the ambient 
 temperature at which the device is operating. This can be used 
 for diagnostic purposes or as an indicator of when the applica- 
 tion circuit needs to rerun a calibration routine to take into 
 account a shift in operating temperature. The temperature 
 sensor is selected using the crosspoint multiplexer and is 
 selected in the same way as an analog input channel. The 
 temperature sensor requires that the analog input buffers be 
 enabled on both analog inputs. If the buffers are not enabled, 
 selecting the temperature sensor as an input forces the buffers 
 to be enabled during the conversion. 
 
 To use the temperature sensor, the first step is to calibrate the 
 device in a known temperature (25°C) and take a conversion as 
 a reference point. The temperature sensor has a nominal 
 sensitivity of 470 μV/K; use the difference in this ideal slope and 
 the slope measured to calibrate the temperature sensor. The 
 temperature sensor is specified with a ±2°C typical accuracy 
 after calibration at 25°C. The temperature can be calculated as 
 follows: 
 
 To use the internal Temp sensor on this family of ADCs, the Internal Reference and Reference Buffer must be enabled. 
 Since the internal reference is enabled, the Reference voltage VREF = 2.5V. The analog input buffer is also required and will automatically be enabled when converting the temp sensor. Note since the 470uV/C sensitivity is a typical value, the sensor should not be used as a thermometer unless endpoint calibration has been performed and adjusted in the host processor noted above. 
 In Unipolar Mode: The Conversion result in V = ADC_Code * LSB The LSB size = VREF/2^24 = 2.5V/2^24 = 149nV 
 Instead of calculating the LSB and Conversion result in V, we can simplify this to: T = (ADC_Code * LSB/470E-6) - 273.15 or (ADC_Code * LSB * 2127.7) – 273.15 or T = (ADC_Code * 3.17E-4) – 273.15 C 
 In Bipolar Mode: 
 A 1/2 scale code shift needs to take place since the output coding is Offset Binary. The Conversion result in V = Code – 2^23 * LSB; note the offset of ½ the range to shift out the offset. The LSB size = 2*Vref/2^24 = 298nV As in the same case above, making the math a bit easier for T is: T = (Code – 2^23) * LSB * 2127.7 or T = (Code – 2^23) * 6.34-4) – 273.15 C To verify this, the AD717x Eval+ GUI was used without an eval board connected. 
 The GUI was used with the INT REF and BUF enabled with the following results: 
 Unipolar Mode T = (954422 * 3.17E-4) – 273.15 = 29.4 C. Bipolar Mode T = (8865818 – 2^23 * 6.34E-4) – 273.15 = 29.40

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AD7175-8: FAQ