SBOA585 March   2024 ADS127L11 , ADS127L11 , ADS127L21 , ADS127L21 , PGA855 , PGA855

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1PGA855 and ADS127L21, 24-Bit, Delta-Sigma ADC Driver Circuit
  5. 2PGA855 Analog Front-End Filters
  6. 3ADS127Lx1 Delta-Sigma ADC and Digital Filter
  7. 4Approximate PGA855 Intrinsic Noise Analysis
    1. 4.1 Simplified Noise Model for the PGA855
    2. 4.2 PGA855 Spectral Noise Density vs Frequency
    3. 4.3 PGA855 Effective Noise Bandwidth
    4. 4.4 PGA855 Low Frequency (1/f) Noise Calculation
    5. 4.5 PGA855 Voltage Broadband Noise
    6. 4.6 PGA855 Current Noise and Source Resistance
    7. 4.7 PGA855 Total Noise
  8. 5PGA855 and ADS127Lx1 System Noise
  9. 6PGA855 and ADS127Lx1 SNR and Noise Calculator
  10. 7PGA855 and ADS127Lx1 FFT Measured Performance
  11. 8Summary
  12. 9References

4.2 PGA855 Spectral Noise Density vs Frequency

Figure 7-13 shows the input referred voltage noise spectral density curve for the PGA855 for each gain setting.

GUID-20231220-SS0I-PPGB-7GVM-PKWGVH8XW6NX-low.svgFigure 4-3 Voltage Noise Spectral Density of PGA855

The spectral noise density plot has two regions: the 1/f noise region and the broadband noise region. The left side of the spectral noise density plot, shows the 1/f noise, also known as flicker noise. 1/f noise is a category of noise in the low frequency range, typically at frequencies lower than 1kHz for most devices, and only showing at lower frequencies than 10Hz for the PGA855. The broadband noise region, at the right side of the curve, has a flat spectral density.

To convert the voltage spectral noise density (nV/√Hz) to RMS voltage noise, one must square the voltage spectral noise density, integrate across the desired bandwidth, and calculate the square root of the result. Equation 5 shows the calculation to convert the spectral noise into RMS noise.

Equation 5. En Vrms  =∫fLfHen (V/√Hz)2df

The following sections detail how to calculate the total PGA RMS noise associated with the 1/f noise region and the broadband noise region.