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

2.1 Resistor and Capacitor Component Selection

Another consideration in the RC filter design is selecting resistor and capacitor components. The ADS1248 incorporates a low-noise, high input impedance PGA. This PGA allows for the use of series filter resistors up to a few kΩ; however, avoid using exceedingly high resistor values.

The differential input bias current of the ADS1248 is typically in the order of 100 pA. Use 1% resistors with resistances below 20 kΩ to make the dc errors due to the differential input bias current negligible. In addition, the thermal noise contribution of the resistors is negligible when the resistor values are kept below 20 kΩ.

Among ceramic surface-mount capacitors, COG (NPO) ceramic capacitors provide the best capacitance precision. The type of dielectric used in COG (NPO) ceramic capacitors provides the most stable electrical properties over voltage, frequency, and temperature changes.