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Noise In Photodiode Applications

Noise In Photodiode Applications. By Art Kay and Bryan Zhao ( 赵 伟 ) June 1, 2011. Noise Review Photodiode Review Photodiode Noise Theory Bandwidth and Stability OPA827 Hand Calculation OPA827 Tina Spice Analysis OPA827 Measurement Example. Contents. Noise Review. Intrinsic Noise

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Noise In Photodiode Applications

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  1. Noise In Photodiode Applications By Art Kay and Bryan Zhao ( 赵 伟 ) June 1, 2011

  2. Noise Review Photodiode Review Photodiode Noise Theory Bandwidth and Stability OPA827 Hand Calculation OPA827 Tina Spice Analysis OPA827 Measurement Example Contents

  3. Noise Review

  4. Intrinsic Noise • Error Source • Generated by circuit itself (not pickup) • Calculate, Simulate, and Measure

  5. Time Domain – White noise normal distribution

  6. What is Spectral Density? (Noise per unit frequency)

  7. Convert Spectral Density to RMS Convert RMS to Peak-to-Peak

  8. Noise is a key parameter in photodiode design Wide bandwidth (integrate more noise) Low signal levels (noise more critical) Photodiode amplifier noise is more complex Parasitic capacitance and sensor capacitance Poles and zeros Gain peaking Why Photodiode Noise?

  9. What can we do with the photodiode knowledge? The competition is in trouble! Our new friends from National. TI

  10. Photodiode Review

  11. Photodiode Basics • Introduction • Photodiodes convert light into current or voltage. • Photodiode type • PN photodiode – more wavelength selective • PIN photodiode – wide spectral range (less selective) • APD (Avalanche photodiode) – sensitive to low light, fast

  12. Basic Photodiode Physics

  13. Figure1.3 Photodiode Equivalent Circuit Figure1.4 Current VS. Voltage Characteristics Photodiode Basics

  14. Figure1.4 Photodiode Equivalent Circuit Figure1.5 Current VS. Voltage Characteristics Photodiode Basic Use left equivalent circuit, the output current is given as : The open circuit voltage Voc is the output voltage when Io equals 0. Thus Voc becomes:

  15. Photodiode and Control Source TINA model Light exciting source: 1) Use VG1 and VG2 voltage sources to simulate light power wave. 2) Use R1 and C1 shape light signal 3) The Voltage Control Current Source (VCCS1) simulates photodiode sensitivity. Photodiode Equivalent Circuit: 1), Current Source Id simulates Dark current 2), Diode is a ideal diode 3), Cd and Rd simulate photodiode's junction capacitor and dark Resistance. 4), Rs is series resistor, which is far smaller than Rd.

  16. Photodiode Noise Theory

  17. Photo-Diode Amp Noise Model

  18. Photodiode noise

  19. Noise Gain Simplify the model to compute Noise Gain Gain seen by the noise voltage source.

  20. Noise Gain

  21. Noise Gain

  22. Noise Gain

  23. Simulating Noise gain and noise bandwidth • Break the loop to measure Aol, 1/B, and I to V Gain

  24. Voltage Noise eni , eno and Eno Region 1 Region 2 Region 3 Region 4 Region 5 Eno4 Eno5 Eno3 Eno1 Eno2

  25. Voltage Noise eni , eno and Eno Region 1 noise: Region 2 noise: Region 3 noise: Region 4 noise: Region 5 noise: Total voltage noise:

  26. Voltage Noise eni , eno and Eno eno Log scale R.1 R.4 R.2 R.3 R.5 Eno^2 Linear scale eno Linear scale

  27. Resistor Noise and Current Noise Current noise and resistor noise are limited by the transimpedance (I-V gain) bandwidth

  28. Bandwidth and Stability

  29. Parasitic Capacitance Limits the Bandwidth • Max bandwidth with Min Cf • Low Cf may be unstable • Wide BW increases noise • As shown Cf=Cs (stray cap)

  30. Feedback Capacitance Required for Stability • Noise Gain is key to stability • Also called 1/Beta (in stability analysis) • ROC = Rate of Closure • ROC = (Aol slope) – (1/Beta slope) • Unstable when ROC > 20dB/decade

  31. Feedback Capacitance Required for Stability Applying a Step Input shows instability at output

  32. Choosing a Minimum Cf for Stability

  33. OPA827 Hand Calculation

  34. Noise Model for Simple Transimpedance Amp

  35. Example Photodiode: PDB-C158 Unfortunately Cj is not specified at Vr=0V. We called the manufacturer for this info Cj=70pF for Vr=0V

  36. Calculate Diode Current Noise

  37. OPA827 Noise Hand Calculation (key numbers)

  38. OPA827 Noise Hand Calculation

  39. Poles and Zeros in Noise Gain Curve

  40. 1/f (flicker) Noise Corner

  41. Output Noise from OPA Noise Voltage

  42. Thermal (Resistor) Noise at Output

  43. Current Noise to Voltage Noise at Output

  44. The Final Total Noise

  45. Reducing Noise (Higher Cf = Lower BW & Noise)

  46. Advantage Reduces noise to zero Available on E-Bay Disadvantage Requires 1.21 gigawatts Size Flux Capacitor ADS1118 Quarter

  47. OPA827 Tina Spice Analysis

  48. OPA827 – test the model • Low Noise : 4nV/√Hz at 1kHz • Low Offset Voltage: 150µV max • JFET Input: IB= 15pA typ • Wide Bandwidth: 22MHz The noise performance is the same as datasheet.

  49. Simulated Spectral Density and Total Noise Output Noise Density Total Output Noise

  50. OPA827 Measurement Example

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