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

1 PGA855 and ADS127L21, 24-Bit, Delta-Sigma ADC Driver Circuit

Analog signal acquisition in industrial systems is a challenging problem. Sensor output signals often present full-scale signals in the millivolt range, requiring resolutions in the microvolt range or even nanovolt range. The system designer's challenge is optimizing the sensor amplifier front-end to achieve the best signal to noise performance. Hence, understanding and minimizing these sources of noise is essential.

The instrumentation amplifier (INA) and the programmable gain instrumentation amplifier (PGA) are essential in industrial acquisition systems. These components offer high accuracy, low noise signal-conditioning and level shifting, and versatile gain programmability. INAs and PGAs offer a high-impedance front-end and have evolved into excellent ADC drivers in the back-end, offering a complete integrated signal acquisition design. Applications include industrial analog input modules measuring a wide variety of bridge, pressure, and temperature sensors; data acquisition cards; surgical equipment; vibration analysis, and power metering/battery testing systems.

Figure 7-13 shows a circuit example for the PGA855 driving the ADS127Lx1, a fully-differential input, high resolution, wide-bandwidth, delta-sigma ADC.

GUID-20231219-SS0I-DKDX-WQ5K-XBPTN32R4KBQ-low.svgFigure 1-1 PGA855 and ADS127L21, 24-Bit, Delta-Sigma ADC Circuit

The PGA accepts single-ended or fully-differential input signals while driving the differential ADC inputs. Pin-controlled gains scale the input signal to the ADC input range. The super-beta input transistors offer a low input bias current, providing a very low input current noise density of 0.3pA/√Hz, making the PGA855 a versatile choice for many sensor types.

The PGA855 offers independent input and output power supplies. In this example, ±15V input power supplies are used for the PGA input section, allowing a wide voltage input range. The output stage is powered with the ADC 5V power supply. The 5V output stage supply operation prevents overloading the ADC inputs during PGA overdrive conditions. The VCM output pin of the ADC drives the PGA855's VOCM pin setting the common mode voltage of the PGA outputs.

The goal of the circuit on Figure 7-13 is to provide a high level of SNR and total harmonic distortion (THD) performance for a given circuit bandwidth requirement. The effective bandwidth of the circuit is affected by the PGA855 analog front end bandwidth as well as the ADC digital filter.