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 Current Noise and Source Resistance

The PGA855 input current noise density interacts with the source resistance generating voltage noise. Consider an example where a bridge sensor is measured at the inputs of the amplifier. The resistive bridge sensor has a thermal noise contribution, and the combined resistance of the filter and bridge sensor scales with the PGA input current noise density.

Figure 7-13 shows the derivation of the equivalent input source resistance in the circuit:

GUID-20240226-SS0I-5HMK-C2PH-J9VD068RGJFK-low.svgFigure 4-4 PGA855 Current Noise and Source Resistance

The circuit at the right of Figure 7-13 shows the simplified PGA855 noise model, where the equivalent input resistance (REQ) has a thermal noise density (eNR) contribution, and the PGA input current noise density (IN_P, IN_N) interacts with REQ.

REQ is a function of the bridge sensor resistance (RSEN) and the input filter resistance (RIN_FIL). Equation 12 solves for the equivalent input resistance at each input terminal of the PGA855:

Equation 12. REQ= RSEN | | RSEN+RIN_FIL =RSEN2 +RIN_FIL 

Equation 13 provides the resistor thermal noise spectral density at each input terminal of the PGA855, where T is the absolute temperature in degrees Kelvin, and k is Boltzman's constant, 1.3807 x 10-23 Joule/°K:

Equation 13. eN_REQ= 4×k×T×REQ 

The equivalent source resistance interacts with the PGA855 current noise density, producing a voltage noise at the input of the instrumentation amplifier. Equation 14 calculates the resulting noise at each input terminal of the PGA855:

Equation 14. eiN= iN ×REQ 

The total current and resistor noise is computed by combining the resistor thermal noise and current noise components at each amplifier input terminal, using the root-sum-of-squares. Equation 15 provides the total current and resistor noise density:

Equation 15. eiN_R= 2×eiN2+2×eN_REQ2 

Equation 16 calculates the RTI current and resistor noise as a function of the effective noise bandwidth in µVRMS:

Equation 16. EiN_R=  eiN_R ×ENBW

Table 7-6 shows the RTI current and source resistance noise of the PGA circuit, for RSEN =2 kΩ, RIN_FIL = 100Ω, and ENBW = 45.5kHz. The resistor and current noise contribution at the input remains constant with PGA gain. When referring this input noise contribution to the output, the input noise needs to be multiplied by the PGA gain.

Table 4-3 PGA855 Input-Referred Current and Source Resistance Noise
iN
(pA / √Hz)
REQ
(Ω)
eN_REQ
(nV / √Hz)
eiN
(nV / √Hz)
EiN_R(RTI)
(µVRMS)
0.31.1 k4.260.331.29