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

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1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011 Slide 2 2 Contents Noise Review Photodiode Review Photodiode Noise Theory Bandwidth and Stability OPA827 Hand Calculation OPA827 Tina Spice Analysis OPA827 Measurement Example Slide 3 3 Slide 4 4 Intrinsic Noise Error Source Generated by circuit itself (not pickup) Calculate, Simulate, and Measure Slide 5 5 Time Domain White noise normal distribution Slide 6 6 What is Spectral Density? (Noise per unit frequency) Slide 7 7 Convert Spectral Density to RMS Convert RMS to Peak-to-Peak Slide 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 Slide 9 9 What can we do with the photodiode knowledge? TI Our new friends from National. The competition is in trouble! Slide 10 10 Slide 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 Slide 12 12 Basic Photodiode Physics Slide 13 13 Photodiode Basics Figure1.3 Photodiode Equivalent Circuit Figure1.4 Current VS. Voltage Characteristics Slide 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 Slide 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. Slide 16 16 Slide 17 17 Photo-Diode Amp Noise Model Slide 18 18 Photodiode noise Slide 19 19 Noise Gain Gain seen by the noise voltage source. Simplify the model to compute Noise Gain Slide 20 20 Noise Gain Slide 21 21 Noise Gain Slide 22 22 Noise Gain Slide 23 23 Simulating Noise gain and noise bandwidth Break the loop to measure Aol, 1/B, and I to V Gain Slide 24 24 Voltage Noise eni, eno and Eno Region 1Region 2Region 3Region 4Region 5 E no1 E no2 E no3 E no4 E no5 Slide 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: Slide 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 Slide 27 27 Resistor Noise and Current Noise PolesKn 11.57 21.22 31.16 Current noise and resistor noise are limited by the transimpedance (I-V gain) bandwidth Slide 28 28 Slide 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) Slide 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 Slide 31 31 Feedback Capacitance Required for Stability Applying a Step Input shows instability at output Slide 32 32 Choosing a Minimum Cf for Stability Slide 33 33 Slide 34 34 Noise Model for Simple Transimpedance Amp Slide 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 Slide 36 36 Calculate Diode Current Noise Slide 37 37 OPA827 Noise Hand Calculation (key numbers) Slide 38 38 OPA827 Noise Hand Calculation Slide 39 39 Poles and Zeros in Noise Gain Curve Slide 40 40 1/f (flicker) Noise Corner Slide 41 41 Output Noise from OPA Noise Voltage Slide 42 42 Thermal (Resistor) Noise at Output Slide 43 43 Current Noise to Voltage Noise at Output Slide 44 44 The Final Total Noise Slide 45 45 Reducing Noise (Higher Cf = Lower BW & Noise) Slide 46 46 Flux Capacitor Advantage Reduces noise to zero Available on E-Bay Disadvantage Requires 1.21 gigawatts Size ADS1118 Quarter Slide 47 47 Slide 48 48 OPA827 test the model The noise performance is the same as datasheet. Low Noise : 4nV/Hz at 1kHz Low Offset Voltage: 150V max JFET Input: I B = 15pA typ Wide Bandwidth: 22MHz Slide 49 49 Simulated Spectral Density and Total Noise Output Noise Density Total Output Noise Calculated (rms) Simulated (rms) 116.7uV109.1uV Slide 50 50 Slide 51 51 Validating Test Equipment Capability Slide 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 Slide 53 53 Agilent 4395A Spectrum Analyzer 1.Frequency Range: 10Hz~500MHz 2.Noise floor: 10nV/rtHz 3.Input Impedance: 50 Reference voltage Noise Floor: 10nV/rtHz Slide 54 54 The Noise Floors are Not Good Enough Slide 55 55 Solution: Use A Post Amp What amp do we Choose? Slide 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. Slide 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 Slide 58 58 Test the Noise Floor Slide 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 Slide 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. Slide 61 61 Shield the Circuit if Possible Power Supply Vcc,Vss,GND Input & Output BNC connectors Shield Slide 62 62 Divide by Gain for OPA827 Output Noise Slide 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 Slide 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 Slide 65 65 OPA827-Noise Density 4395A Spectrum Analyzer Slide 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. Slide 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? Slide 68 68 Contact Info: [email protected] 520 750-2150 (phone) 520 208-1595 (cell)

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