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

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Presentation on theme: "1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011."— Presentation transcript:

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

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

3 3

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

5 5 Time Domain – White noise normal distribution

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

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

8 8 Why Photodiode Noise? 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

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

10 10

11 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 12 Basic Photodiode Physics

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

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

15 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 16

17 17 Photo-Diode Amp Noise Model

18 18 Photodiode noise

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

20 20 Noise Gain

21 21 Noise Gain

22 22 Noise Gain

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

24 24 Voltage Noise eni, eno and Eno Region 1Region 2Region 3Region 4Region 5 E no1 E no2 E no3 E no4 E no5

25 25 Voltage Noise e ni, e no and E no Region 1 noise: Region 2 noise: Region 3 noise: Region 4 noise: Region 5 noise: Total voltage noise:

26 26 Voltage Noise e ni, e no and E no e no Log scale e no Linear scale E no^2 Linear scale R.1 R.2R.3 R.4 R.5

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

28 28

29 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 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 31 Feedback Capacitance Required for Stability Applying a Step Input shows instability at output

32 32 Choosing a Minimum Cf for Stability

33 33

34 34 Noise Model for Simple Transimpedance Amp

35 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 36 Calculate Diode Current Noise

37 37 OPA827 Noise Hand Calculation (key numbers)

38 38 OPA827 Noise Hand Calculation

39 39 Poles and Zeros in Noise Gain Curve

40 40 1/f (flicker) Noise Corner

41 41 Output Noise from OPA Noise Voltage

42 42 Thermal (Resistor) Noise at Output

43 43 Current Noise to Voltage Noise at Output

44 44 The Final Total Noise

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

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

47 47

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

49 49 Simulated Spectral Density and Total Noise Output Noise Density Total Output Noise Calculated (rms) Simulated (rms) 116.7uV109.1uV

50 50

51 51 Validating Test Equipment Capability

52 52 Tektronix DPO 4034 Oscilloscope STDEV: 48uV (same as RMS) P-P: 6.6*STDEV=319uV 40s P-P: 320uV 1)Set DC couple, 20MHz bandwidth limit 2)Short input channel to measure noise floor

53 53 Agilent 4395A Spectrum Analyzer 1.Frequency Range: 10Hz~500MHz 2.Noise floor: 10nV/rtHz 3.Input Impedance: 50Ω Reference voltage Noise Floor: 10nV/rtHz

54 54 The Noise Floors are Not Good Enough

55 55 Solution: Use A Post Amp What amp do we Choose?

56 Use OPA847 as post amplifier Wideband, Ultra-Low Noise, Voltage Feedback, Operational Amplifier At the gain of 150, the bandwidth is 26MHz 0.85nV/ Hz Input Voltage Noise 2.5pA/ Hz Input Current Noise ±100uV Input Offset Voltage (Typical)) 575uVrms post amplifier noise in 20MHz bandwidth.

57 57 Post Amplifier Relatively small error! Vn827 = 109.4uV rms Vnpost = 16.03mV rms Vnpost/Gain = 16.03mV/150 = 106.8uV rms Gain=150

58 58 Test the Noise Floor

59 59 Test The Noise Floor – Post Amp Noise Scope Simulated (rms) Measured (rms) 575uV518uV STDEV: 518uV P-P: 6.6*STDEV=3.4mV 40s P-P: 3.88mV

60 60 Hardware Connections 1.PDB-C158-ND photodiode 2.70pF junction capacitance at Vr=0 V 3.100dB I-V gain 4.4pF compensation capacitor 5.±5V power supply.

61 61 Shield the Circuit if Possible Power Supply Vcc,Vss,GND Input & Output BNC connectors Shield

62 62 Divide by Gain for OPA827 Output Noise

63 63 Measured Total Output Noise (DPO 4034 Scope) OPA847 Measured at Scope: STDEV: 21.7mV P-P: 6.6*STDEV=143mV 40s P-P: 170mV At OPA827 (DUT) Output: Divide by gain Vn827 = 21.7mV/150 = 144.6uV Calculated (rms) Simulated (rms) Measured (rms) 116.7uV109.1uV144.6uV

64 64 Measured Spectral Density 4395A Spectrum Analyzer Linear Scale 1.Agilent 4395A Spectrum Analyzer test 1Hz~20MHz span, 3uV/div, REF=24uV. 2.The tested noise density curve shape is the same as simulation. Measurement 4395A Linear Scale Tina Spice Linear Scale Tina Spice Log Scale

65 65 OPA827-Noise Density 4395A Spectrum Analyzer

66 66 Thanks Bryan Zhao ( ) Matt Hann Collin Wells, Peter Semig, Curtis Mayberry References Jerald Graeme Art Kay HAMAMATSU Tim Green Before Questions, I must ask….

67 67 You will move to a new topic my young apprentice More noise presentations have we must Do I move over to the dark side?

68 68 Contact Info: (phone) (cell)

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