1 / 81

SAR - Amplifier Design TI Precision Labs – SAR

Learn about SAR amplifier design in this TI Precision Labs course by Tim Green, Art Kay, Luis Chioye, and Dale Li. Topics include ADC input types, linear operation of amplifier and ADC, common front ends, error sources, input impedance, single-ended inputs, and instrumentation amplifiers.

jhelm
Download Presentation

SAR - Amplifier Design TI Precision Labs – SAR

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. SAR - Amplifier Design TI Precision Labs – SAR by Tim Green, Art Kay, Luis Chioye, Dale Li

  2. Art Kay, MGTS Bio:Applications Engineering Manager, SAR Ref Mux • Career • Formally Op Amp’s Apps Manager • Developed Op Amp Precision Labs • Published a book on Amplifier Noise & Pocket Reference • Formally at Burr-Brown Corp., and Northrop Grumman Corp • M.S.E.E. Georgia Institute of Technology • Expertise • Focus on low noise, precision signal chain design • Sensor signal conditioning

  3. Tim Green, MGTS Bio:Applications Engineering Manager, Precision Op Amp • Career • 32 years of Analog and Mixed Signal Experience • 16 years in Board/System Level Design: • Brushless Motor Control, Jet Engine Control, Missile Systems, Data Acquisition Systems, CCD Cameras • 16 years in Analog/Mixed Signal Semiconductor: • Power Op Amps, Instrumentation Amplifiers, 4-20mA, Difference Amplifiers, Small Signal Op Amps, Programmable Gain Amplifiers • B.S.E.E., University of Arizona 1981 • Expertise • Op Amp Stability • SPICE Op Amp Macro-modeling • Publishing definition-by-example articles on op amp stability • Troubleshooting complex board/system level problems

  4. Agenda ADC Input types • Single ended, pseudo-differential, fully-differential, and true-differential • Switched capacitor vs. buffered Linear operation of amplifier and ADC • Rail-to-Rail amplifiers and crossover distortion • Inverting configuration Common Front Ends • Instrumentation amplifier: Selecting gain and common mode range • Fully Differential Amplifiers: Single Ended to Differential Error Sources: • Statistics: Worst Case vs. Typical • Offset and Gain Error • Calibration • Drift and Non-linearity • Noise AC Specifications and the FFT Aliasing

  5. Pseudo-differential Single Ended

  6. ADC Input Swing and Common Mode Vdif= AIN_P - AIN_M • Differential of ±5V (10Vpp) • Single ended equivalent 0V to 5V (5Vpp) • Double the range of single ended • Negative differential output can occur when absolute output voltage of each output is positive (unipolar single supply) • In this example common mode is always 2.5V AIN_P + AIN_M Vcm = 2

  7. Fully Differential Input Vcm must be constant at Vref/2

  8. True Differential Input Vcm has a wide voltage range.

  9. Summary Examples of ADC input types

  10. Input Impedance: Resistive vs. Switched Capacitor Switched Capacitor Input Resistive, High Voltage, PGA input • Wide bandwidth external amp required • Dynamic input impedance • Input voltage range set by reference • External amplifier bandwidth not critical • Internal PGA, ADC driver, and reference • High voltage input (±12.288V) with 3V & 5V supplies

  11. Agenda ADC Input types • Single ended, pseudo-differential, fully-differential, and true-differential • Switched capacitor vs. buffered Linear operation of amplifier and ADC • Rail-to-Rail amplifiers and crossover distortion • Inverting configuration Common Front Ends • Instrumentation amplifier: Selecting gain and common mode range • Fully Differential Amplifiers: Single Ended to Differential Error Sources: • Statistics: Worst Case vs. Typical • Offset and Gain Error • Calibration • Drift and Non-linearity • Noise AC Specifications and the FFT Aliasing

  12. Single Ended Input: ADC Input Range Considerations AVDD+0.3 = 3.3V + 0.3V = 3.6V

  13. Single Ended Input: OPA320 Linear Range

  14. Single Ended Input: Extending the Op Amp Range Low Noise Negative Bias Generator • Regulated Output Voltage −0.232 V • Output Voltage Tolerance 5% • Output Voltage Ripple 4 mVPP • Maximum Output Current 26 mA

  15. Single Ended Input: OPA625

  16. Inverting amplifier: Eliminate Common mode issue Vcm is constant

  17. Input Crossover Distortion in Rail-to-Rail Inputs

  18. Input Cross-Over distortion vs Zero Cross-Over PMOS NMOS

  19. Optimize bypass for internal charge pump op amps Short direct connections from supply to decoupling

  20. THD vs. Crossover Distortion Not zero input crossover Note that SNR remained constant as THD degraded.

  21. Agenda ADC Input types • Single ended, pseudo-differential, fully-differential, and true-differential • Switched capacitor vs. buffered Linear operation of amplifier and ADC • Rail-to-Rail amplifiers and crossover distortion • Inverting configuration Common Front Ends • Instrumentation amplifier: Selecting gain and common mode range • Fully Differential Amplifiers: Single Ended to Differential Error Sources: • Statistics: Worst Case vs. Typical • Offset and Gain Error • Calibration • Drift and Non-linearity • Noise AC Specifications and the FFT Aliasing

  22. Instrumentation Amplifier (INA): Choose Gain

  23. Common mode and output swing for INAs • INA Common mode • Output swing and common mode of internal amplifiers is a complex relationship. • Different INAs have different topologies • Vref, V+, V-, and gain determine common mode vs output swing relationship. • A software tool simplifies this calculation

  24. Verify INA Common Mode Range (INA826) Output Voltage = VDIF * Gain

  25. Verify INA Common Mode Range 1. Select your instrumentation amplifier 2. Enter the supplies, gain, reference and press “Create Graph”. 3. Enter the common mode voltage. 4. Does the output range work for your system? • Vout range limited to -14.8V to +14.2V • -10V to 10V output inside the range • http://www.ti.com/tool/ina-cmv-calc

  26. INA: Setting the reference input • Gain is calculated same as before • Negative Vref input to shift the signal • Reference input must be driven

  27. Output range limited by input common mode • Vout range limited to ±6.4V • Cannot achieve 0 to 10V output

  28. Two Stage Approach Precision DC Input Wide Bandwidth Driver

  29. Fully Differential Amplifier or FDA • Standard Operational Amplifier • Differential in • Single-ended out • Output Common Mode is Vout • Single feedback paths • Fully-Differential Amplifier • Differential in • Differential out • Output Common-mode set by Vocm • Multiple feedback paths • Double the dynamic range of amplifier • Even order harmonic distortion canceled

  30. Fully Differential Amplifier or FDA

  31. FDA – Single Ended Unipolar to Differential

  32. FDA – Single Ended Bipolar to Differential

  33. Improve Linear Range

  34. Finding standard resistors for unusual gains http://www.ti.com/tool/analog-engineer-calc

  35. Agenda ADC Input types • Single ended, pseudo-differential, fully-differential, and true-differential • Switched capacitor vs. buffered Linear operation of amplifier and ADC • Rail-to-Rail amplifiers and crossover distortion • Inverting configuration Common Front Ends • Instrumentation amplifier: Selecting gain and common mode range • Fully Differential Amplifiers: Single Ended to Differential Error Sources: • Statistics: Worst Case vs. Typical • Offset and Gain Error • Calibration • Drift and Non-linearity • Noise AC Specifications and the FFT Aliasing

  36. Find the worst case offset This result may be statistically unrealistic

  37. Statistics Behind Typical and Maximum

  38. Probability that we are near worst case

  39. Compounding probabilities “near” worst case

  40. A more practical approach: use the typical limit

  41. A more practical approach: use typical Set end system specifications based on risk tolerance

  42. Gain Error Calculation

  43. Offset and Gain Calibration: two test signals Two Precision inputs applied Make sure amplifiers are in linear range Average code read

  44. Calibration Example

  45. Error Sources that are difficult to Calibrate • Temperature Drift • Non-linearity • Long term shift (Aging) • Hysteresis • Noise Integral Non-linearity Long term shift Temperature Drift

  46. SNR of Amplifier + ADC: General Equations

  47. Find the REF6050 Noise 2. Enter the bandwidth and select the diagrams 1. Analysis> Noise Analysis The integrated noise is the “total noise”. Look at the final value VnRef≈ 6.31uV rms

  48. Simulating Amplifier Noise For more information: http://www.ti.com/lsds/ti/amplifiers/op-amps/precision-op-amps-precision-labs

  49. SNR of Amplifier + ADC: Example Calculation

  50. Signal Chain Noise: Analog Engineer’s Calculator 1. Enter the total rms noise from the signal chain. 2. Enter the noise from the ADC data sheet. 3. Press “ok” and the total noise is displayed here. http://www.ti.com/tool/analog-engineer-calc

More Related