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

7 PGA855 and ADS127Lx1 FFT Measured Performance

Figure 7-13 shows a block diagram for the PGA855-ADS127L21 circuit as tested on the bench.

A 1-kHz sine-wave test signal generates the SNR and THD data. The signal source must provide better noise performance and lower distortion than the circuit under test. The bench set up uses a low-pass filter between the signal source and PGA855 circuit to help reduce extrinsic noise injected into the circuit. The amplitude is adjusted to provide a –0.2-dBFS output from the ADC.

The bench set up uses an external low-jitter clock source of 25.6MHz as shown on Figure 7-13 to obtain a 200 kSPS data rate.

The PGA855 circuit is configured as shown in Figure 7-13. An equivalent resistance of 1kΩ is added in series between the signal source and PGA855 circuit, to account for the noise contribution of the 2kΩ bridge sensor resistance. The ADS127Lx1 GUI is configured with the ADC input buffers enabled and high-reference range selected.

GUID-20240306-SS0I-WCVB-4F1S-S0W68MHSKCHN-low.png Figure 7-1 PGA855-ADS127L21 Bench Test

Table 7-6 lists the ADC settings, reference voltage on the PGA855-ADS127L21 bench measurements.

Table 7-1 PGA855-ADS127L21 Measurement Parameters
PARAMETER VALUE
Input voltages (VPP, differential) 20V, 16V, 8V, 4V, 2V, 1V, 0.5V, 0.25V
ADC reference voltage 4.096
Data rate 200.0 kSPS, OSR = 64
Test frequency 1 kHz
THD (gain = 1) < –120dB

typical
SNR (gain = 1, wideband filter) > 108dB

typical
SNR (gain = 1, sinc4 filter) > 110dB

typical

Table 7-2 shows the SNR measurement of the PGA855 driving the ADS127L21 delta-sigma ADC using a sinc4 or wideband filter at 200kSPS. At gain = 1, the design achieves a 108.1-dB SNR for the wideband filter and 110.1-dB SNR for the sinc4 filter.

Table 7-2 PGA855 and ADS127L21 Calculated and Measured Performance Summary
PGA GAIN (V/V) INPUT (VPEAK) WIDEBAND SNR (dB) SINC4 SNR (dB)
Calculated Measured Calculated Measured
0.125 20 108.5 106.0(1) 110.5 107.6(1)
0.25 16 108.4 108.4 110.4 110.4
0.5 8 108.4 108.4 110.4 110.4
1 4 108.3 108.1 110.3 110.1
2 2 107.5 107.2 109.6 109.3
4 1 105.5 105.0 107.9 107.1
8 0.5 101.2 100.6 103.8 103.0
16 0.25 95.7 95.0 98.5 97.2
(1) The input signal amplitude is limited by the allowed input linear range of the PGA855 circuit at gain of 0.125, affecting the measured SNR.

Figure 7-7 and Figure 7-8 show the respective 1-kHz, full-scale FFT plots for the wideband and sinc4 filters at gain = 1. Because of the frequency roll-off of the sinc4 filter, SNR performance improves by an average of 2dB compared to the wideband filter. The filters provide identical THD results at –120-dB typical.

GUID-20240308-SS0I-FSMT-XDTW-9CVWN15KP30K-low.svg
G = 1V/V, fIN = 1kHz
Figure 7-2 Wideband Filter Performance
GUID-20240308-SS0I-396Z-CZ90-KZ32QD8MS10C-low.svg
G = 1V/V, fIN = 1kHz
Figure 7-3 Sinc4 Filter Performance