SBOA585 March   2024 ADS127L11 , ADS127L11 , ADS127L21 , ADS127L21 , PGA849 , PGA849 , 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

PGA855 and ADS127Lx1 System Noise

Equation 19 computes the total noise in the system, as a function of the PGA855 noise, ADC noise and ADC voltage reference noise. The ADC circuit uses the REF6241 4.096V voltage reference. This reference includes an integrated wide bandwidth buffer that allows the REF6241 to directly drive the switched-capacitor reference input of the ADS127L21. The REF62xx High-Precision Voltage Reference With Integrated ADC Drive Buffer data sheet specifies a total integrated noise of 5 µVRMS when using a 22µF bypass capacitor at frequencies higher than approximately1kHz. The uncorrelated noise sources are combined by computing the root-sum-of-squares:

Equation 19. EnSystem(RTO)= EnPGA855(RTO)2+EnADC2  + EnREF2 

Equation 20 converts RMS noise to effective resolution.

Equation 20. System Effective Resolution Bits=log2 FSR / EnSystem(RTO) 

Where FSR is the full-scale range:

  • FSR = 2 · VREF (1x input range)

Effective resolution is a figure of merit frequently used to specify the performance of delta-sigma ADCs. This specification is different than the effective number of bits (ENOB), which is a function of both noise and THD.

Equation 21 converts RMS noise to SNR.

Equation 21. System SNR dB =20×log FSR VRMSEnSystem(RTO) VRMS

Table 5-1 shows the PGA855 and ADS127L21 total system noise calculations vs PGA gain.

Table 5-1 Total System Noise
PGA GAIN
(V/V)
PGA855 Noise
EnPGA855 (RTO)
(µVRMS)
ADC Noise
EnADC
(µVRMS)
Reference Noise
EnREF
(µVRMS)
System Noise
EnSystem(RTO)
(µVRMS)
System Effective Resolution
(Bits)
System SNR
(dB)
0.1254.495.535.008.7019.84110.45
0.254.505.535.008.7119.84110.44
0.54.535.535.008.7219.84110.42
14.795.535.008.8619.82110.29
25.965.535.009.5519.71109.64
48.975.535.0011.6619.42107.90
817.115.535.0018.6618.74103.82
1633.675.535.0034.4917.8698.48