SBOA585 March 2024 ADS127L11 , ADS127L11 , ADS127L21 , ADS127L21 , PGA855 , PGA855
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 20 converts RMS noise to effective resolution.
Where FSR is the full-scale 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.
Table 5-1 shows the PGA855 and ADS127L21 total system noise calculations vs PGA gain.
| 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) |
|---|---|---|---|---|---|---|
| 0.125 | 4.49 | 5.53 | 5.00 | 8.70 | 19.84 | 110.45 |
| 0.25 | 4.50 | 5.53 | 5.00 | 8.71 | 19.84 | 110.44 |
| 0.5 | 4.53 | 5.53 | 5.00 | 8.72 | 19.84 | 110.42 |
| 1 | 4.79 | 5.53 | 5.00 | 8.86 | 19.82 | 110.29 |
| 2 | 5.96 | 5.53 | 5.00 | 9.55 | 19.71 | 109.64 |
| 4 | 8.97 | 5.53 | 5.00 | 11.66 | 19.42 | 107.90 |
| 8 | 17.11 | 5.53 | 5.00 | 18.66 | 18.74 | 103.82 |
| 16 | 33.67 | 5.53 | 5.00 | 34.49 | 17.86 | 98.48 |