SBAA201A March   2013  – March 2023 ADS1120 , ADS1120-Q1 , ADS1147 , ADS1148 , ADS114S06 , ADS114S06B , ADS114S08 , ADS114S08B , ADS1220 , ADS122C04 , ADS1247 , ADS1248 , ADS124S06 , ADS124S08

 

  1.   RTD Ratiometric Measurements and Filtering Using the ADS1148 and ADS1248 Family of Devices
  2.   Trademarks
  3. 1Introduction
  4. 2Low-Pass Filter Design Considerations in Ratiometric Measurements
    1. 2.1 Resistor and Capacitor Component Selection
  5. 3Noise Performance Using the ADS1248 in the Ratiometric Configuration
    1. 3.1 Noise Performance Using Mismatched RC Filters
    2. 3.2 Noise Performance Using The Four-Wire Configuration with Matched RC Filters
    3. 3.3 Noise Measurements Using the Three-Wire Configuration with Matched RC Filters
    4. 3.4 Other Considerations
  6. 4Conclusion
  7. 5References
  8. 6Revision History

3.4 Other Considerations

When selecting the corner frequency of the RC filters, make sure to account for both the noise signals present in the environment, and the timing constraints of the application. In many cases, the signal produced by the RTD may be treated as essentially a dc signal. The analog filter must be allowed to completely settle after activating the current source, but before the sensor measurement takes place. On high-resolution applications, when initially biasing the RTD, the user may have to wait several time constants for the filter to settle. For example, when performing a 20-bit resolution measurement, after initially biasing the sensor, the user must wait up to 14 RC filter time constants for the measurement to settle within ½ an LSB. Table 3-1 shows the required RC filter time constants to settle to ½ LSB resolution.

Table 3-1 Required RC Filter Time Constants to Settle to ½ LSB Resolution
Resolution (Bits)Time Constants to ½ LSB
1611.78
1813.17
2014.56
2215.94
2417.33