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INA Noise Analysis

INA Noise Analysis. Created By: Arthur Kay , Texas Instruments Senior Applications Engineer. Introductions. Art Kay, Applications Engineering Texas Instruments Mixed Signal “System-on-a-Chip” Bridge Sensor Signal Conditioning Evaluation Modules (hardware / software) Noise

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INA Noise Analysis

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  1. INA Noise Analysis Created By: Arthur Kay, Texas Instruments Senior Applications Engineer

  2. Introductions • Art Kay, Applications Engineering Texas Instruments • Mixed Signal “System-on-a-Chip” • Bridge Sensor Signal Conditioning • Evaluation Modules (hardware / software) • Noise • Northrop Grumman, Burr-Brown (Test Engineering) • Cleveland State, Georgia Tech Grad.

  3. Summary • Short Review of INA’s • Noise model for INA’s • Hand analysis of INA’s • Simulation • Application Example – Averaging • Calculation, simulation, measurement

  4. We will terminate the noise sources.

  5. Short Review of 3 Amp INA

  6. Three OPA INA

  7. Real World Input to Mathematical Model

  8. Analyze the Input and Output Separately

  9. Split Input Stage in Half

  10. Use Superposition on Output Amp

  11. Gain For Three Amp IA

  12. Quick Noise Review

  13. Review of Key Noise Concepts • Noise From 2 Independent Sources is Called UNCORRELATED noise • Uncorrelated Noise Sources Add By RSS (Root Sum of Squares) • Good Approximations Greatly Simplify Noise Analysis • Noise Given in Typical Values and can vary 10-20%

  14. Key Noise Equations 1/f Or “Flicker” Noise Broadband Current Noise Thermal Noise in Resistor Total Voltage Noise

  15. Noise Model of 3 Amp INA

  16. Complex Noise Model Noise Sources are Voltage Noise, Current Noise, and Thermal Noise From Resistors

  17. The Complex Model is Simplified Input Stage Noise Modeled as Current and Voltage noise; Output stage lumps all noise sources into 1 Vn_out Source

  18. The Input amplifier dominates at High Gain

  19. Two Ways to represent INA Spectral Density Calculated using graphs and formula Taken directly from the graph

  20. Hand Analysis of 3 Amp INA

  21. Find the total RMS Noise Voltage at the Output Application example with bridge sensor and ½ supply reference buffer

  22. Look at Noise Sources: Bridge, INA333, Reference Buffer Each Piece of the Application Contributes Noise to the Output Voltage

  23. Noise Equivalent Model for Reference Pin Buffer

  24. Reference buffer

  25. The reference voltage directly adds to the output noise Reference buffer noise adds directly to diff amp noise stage

  26. The bridge generates: thermal noise, in x R_bridge

  27. Noise From Bridge / Current Sources RSS noise contributions from the resistor bridge and the effect of INA333 current noise on the resistor values

  28. Combine all the noise sources

  29. Rule of 3x in Noise Analysis

  30. Bandwidth from Data Sheet For G = 100 20dB/decade 1st order Kn = 1.57 “Noise Bandwidth” (BWn) approximates the bandwidth over which the noise spectral density contributes to the total noise

  31. Calculate RMS Output Noise for INA333 From Voltage Noise

  32. To recap… In high gains we can Terminate the noise contribution of the sensor, reference buffer, and output stage with little effect on the total output noise

  33. Simulation of 3 Amp INA

  34. Simulate the Circuit DC operation first ensures a good result Noise Analysis in TINA can only be performed if AC source is present

  35. Using Tina Spice Output Noise = Noise Spectral Density Total Noise = RMS output Noise

  36. Noise Spectral Density at the Output

  37. Total RMS Noise at the Output

  38. Why doesn’t calculation match simulation exactly? Bandwidth from Data Sheet and simulated bandwidth is different. The roll-off was approximated as first order in the calculations. Simulation shows that it is not first order.

  39. Reduce Noise Using Averaging Circuit

  40. Say “Hasta la vista, baby” to nano-volts of noise with averaging!

  41. Averaging Circuit • Inputs V1-Vn assumed are noise sources • R1-Rn assumed to be of EQUAL value • Feedback Resistor Rf scaled based on the # of equal-valued input resistors

  42. Noise in Averaging Circuit

  43. Averaging Circuit with INA333 • Acts as a Single INA333 • R4-6 selected to limit output current to design-specific value

  44. Experiment with 20 Parallel INA333 20 INA333 amps in parallel (jumper selectable) Socketed Gain Set Resistors OPA333 Averaging Circuit

  45. Standard Noise Measurement Precautions Linear Power Source Steel Paint Can for Shielding

  46. Total Output Noise vs Number of Amplifiers Being Averaged

  47. Measured vs simulated spectral density

  48. References • [1] Hann, Gina. "Selecting the right op amp - Electronic Products." Electronic Products Magazine – Component and Technology News. 21 Nov. 2008. Web. 09 Dec. 2009. <http://www2.electronicproducts.com/Selecting_the_right_op_amp-article-facntexas_nov2008-html.aspx>. • Henry W. Ott, Noise Reduction Techniques in Electronics Systems, John Wiley and Sons Acknowledgments: • R. Burt, Technique for Computing Noise based on Data Sheet Curves, General Noise Information • T. Green, General Information • B. Trump, General Information • Matt Hann, General INA information and review Noise Article Series (www.en-genius.net) http://www.en-genius.net/site/zones/audiovideoZONE/technical_notes/avt_022508

  49. I’ll be back. Next year with more exciting noise … Or something else… Noise = 1 trick pony.

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