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

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

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

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5 Time Domain – White noise normal distribution

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

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

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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

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

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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

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12 Basic Photodiode Physics

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

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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

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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.

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17 Photo-Diode Amp Noise Model

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18 Photodiode noise

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

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20 Noise Gain

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21 Noise Gain

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22 Noise Gain

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

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

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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:

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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

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

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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)

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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

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

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32 Choosing a Minimum Cf for Stability

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34 Noise Model for Simple Transimpedance Amp

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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

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36 Calculate Diode Current Noise

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37 OPA827 Noise Hand Calculation (key numbers)

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38 OPA827 Noise Hand Calculation

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39 Poles and Zeros in Noise Gain Curve

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40 1/f (flicker) Noise Corner

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41 Output Noise from OPA Noise Voltage

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42 Thermal (Resistor) Noise at Output

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43 Current Noise to Voltage Noise at Output

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44 The Final Total Noise

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

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

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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

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

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51 Validating Test Equipment Capability

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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

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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

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54 The Noise Floors are Not Good Enough

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

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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.

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

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58 Test the Noise Floor

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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

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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.

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

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62 Divide by Gain for OPA827 Output Noise

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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

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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

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65 OPA827-Noise Density 4395A Spectrum Analyzer

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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….

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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?

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68 Contact Info: (phone) (cell)

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