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

6 PGA855 and ADS127Lx1 SNR and Noise Calculator

The PGA855 and ADS127Lx1 SNR and Noise Calculator is a spreadsheet developed using Microsoft® Excel® that provides an estimate of the overall intrinsic noise performance of the acquisition system.

The tool calculates the noise contribution of the sensor resistance, PGA855, ADS127Lx1 ADC, REF6241 voltage reference and provides an estimate of the overall system noise bandwidth, SNR and effective resolution based on the data sheet noise specifications.

The delta-sigma ADC provides a trade-off between the output data rate and acquisition system bandwidth, against noise performance. The calculator facilitates selecting the ADC filter settings, and data rate or filter bandwidth, depending on the application required resolution, and sensor resistance.

The circuit designer enters the PGA855 circuit bandwidth, the RMS reference noise contribution at the ENBW, and selects the ADS127Lx1 digital filter settings and data rate. An optional field is available to enter the sensor equivalent resistance at each PGA855 input. Figure 7-13 shows the noise calculator inputs:

GUID-20240303-SS0I-WRNR-PB35-W7QJFNWHLMSC-low.pngFigure 6-1 PGA855 and ADS127Lx1 Noise Calculator - Inputs

The calculator generates a plot of the system SNR and system effective resolution. Figure 7-7 shows the calculated SNR (dB) vs PGA gain in V/V, and Figure 7-8 shows the calculated system effective resolution in bits vs PGA gain in V/V.

GUID-20240222-SS0I-12JB-8CGJ-DRMBHP90M8XB-low.png
High-Speed Mode, Sinc4, OSR = 64, Data Rate 200 kSPS
Figure 6-2 PGA855 and ADS127L21 Noise Calculator - System SNR
GUID-20240222-SS0I-R6PX-MG6N-T41BFMNN2V1N-low.png
High-Speed Mode, Sinc4, OSR = 64, Data Rate 200 kSPS
Figure 6-3 PGA855 and ADS127L21 Noise Calculator - System Effective Resolution

The PGA855 and ADS127Lx1 SNR and Noise Calculator also generates a table with an estimate of the equivalent intrinsic noise of the acquisition system in µVRMS, referred to the input of the PGA855 (RTI) or to the output of the PGA855 (RTO). In addition, it provides a separate table showing the noise contribution of the PGA855 circuit stand-alone, and the source or sensor noise contribution. Figure 6-4 and Figure 6-5 show a table with results as displayed in the tool, when selecting Sinc4 filter at 200-kSPS with REQ = 1.1kΩ.

GUID-20240216-SS0I-MPP2-GJNG-QVLQWCQXVKPS-low.png
High-Speed Mode, Sinc4, OSR = 64, Data Rate 200 kSPS
Figure 6-4 PGA855 and ADS127L21 - Noise Calculator, System Noise and Bandwidth
GUID-20240216-SS0I-PWWM-ZGSL-QPRB5RWXJWRN-low.png
High-Speed Mode, Sinc4, OSR = 64, Data Rate 200 kSPS
Figure 6-5 PGA855 and ADS127L21 Noise Calculator, PGA855 Noise Contribution
Note: The PGA855 calculator only accounts for the PGA855 and ADC intrinsic circuit noise to provide an estimate of noise performance based on the typical data sheet specifications. When measuring the SNR performance of the PGA-ADC acquisition system on the bench, the circuit designer must use a high-performance, very low noise, low distortion source. The bench measurement is sensitive to extrinsic noise sources, source distortion, as well as jitter in the signal.