4 Intrinsic Noise Error Source Generated by circuit itself (not pickup) Calculate, Simulate, and Measure
5 Time Domain – White noise normal distributionThis slide shows the time domain waveform for broadband noise. The distribution for broadband noise is also shown; note that it is Gaussian.
6 What is Spectral Density? (Noise per unit frequency)
7 Convert Spectral Density to RMS Convert RMS to Peak-to-Peak
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 complexParasitic capacitance and sensor capacitancePoles and zerosGain peaking
9 What can we do with the photodiode knowledge? The competition is in trouble!Our new friends from National.TI
11 Photodiode Basics Introduction Photodiode type Photodiodes convert light into current or voltage.Photodiode typePN photodiode – more wavelength selectivePIN photodiode – wide spectral range (less selective)APD (Avalanche photodiode) – sensitive to low light, fast
12 Basic Photodiode Physics Principle of Operation:. - The P-layer material at the active surface and the N material at the substrate form a PN junction which operates as a photoelectric converter.- When light strikes a photodiode, the electron within the crystal structure becomes stimulated. If the light energy is greater than the band gap energy Eg, the electrons are pulled up into the conduction band, leaving holes in their place in the valence band.- These electron-hole pairs occur throughout the P-layer, depletion layer and N-layer materials. In the depletion layer the electric field accelerates these electrons toward the N-layer and the holes toward the P-layer.- This results in a positive charge in the P-layer and a negative charge in the N-layer. If an external circuit is connected between the P- and N-layers, electrons will flow away from the N-layer, and holes will flow away from the P-layer toward the opposite respective electrodes.
13 Photodiode Basics Figure1.3 Photodiode Equivalent Circuit Figure1.4 Current VS. Voltage Characteristics
14 Photodiode BasicFigure1.4 Photodiode Equivalent CircuitUse left equivalent circuit, the output current is given as :Figure1.5 Current VS. Voltage CharacteristicsThe 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 signal3) The Voltage Control Current Source (VCCS1) simulates photodiode sensitivity.Photodiode Equivalent Circuit:1), Current Source Id simulates Dark current2), Diode is a ideal diode3), Cd and Rd simulate photodiode's junction capacitor and dark Resistance.4), Rs is series resistor, which is far smaller than Rd.
18 Photodiode noiseThe lower limits of light detection for photodiodes are determined by the noise characteristics of the device. The photodiode noise in is the sum of the thermal noise (or Johnson noise) ij of a resistor which approximates the shunt resistance and the shot noise isD and isL resulting from the dark current and the photocurrent.If IL >> 0.026/Rsh or IL >> ID, the shot noise becomes predominant. The lower limit of light detection for a photodiode is usually expressed as the intensity of incident light required to generate a current equal to the noise current . Essentially this is the noise equivalent power (NEP).
19 Noise Gain Simplify the model to compute Noise Gain Gain seen by the noise voltage source.
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 1Region 2Region 3Region 4Region 5Eno4Eno5Eno3Eno1Eno2
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 Log scaleR.1R.4R.2R.3R.5Eno^2Linear scaleenoLinear scaleTwo left figures obviously present:although 1/f noise density is very large in noise density, but noise gain and bandwidth make region (3,4,5) contribute to output noise (Eno) most.Noise gain effect region is controlled by Ci,Cf,Rf,fc.Compensation design and op amp selection is critical for noise characters.
27 Resistor Noise and Current Noise Current noise and resistor noise are limited by the transimpedance (I-V gain) bandwidthPolesKn11.5721.2231.16
29 Parasitic Capacitance Limits the Bandwidth Max bandwidth with Min CfLow Cf may be unstableWide BW increases noiseAs shown Cf=Cs (stray cap)Higher feedback resistor value potentially make parasitic capacitance bypass dominate the photodiode amplifier’s response and set the circuit bandwidth.Good construction parasitic limit Cs around 0.5pF.
30 Feedback Capacitance Required for Stability Noise Gain is key to stabilityAlso called 1/Beta (in stability analysis)ROC = Rate of ClosureROC = (Aol slope) – (1/Beta slope)Unstable when ROC > 20dB/decade
31 Feedback Capacitance Required for Stability Applying a Step Input shows instability at output
55 Solution: Use A Post Amp What amp do we Choose?
56 Use OPA847 as post amplifier Wideband, Ultra-Low Noise, Voltage Feedback, Operational AmplifierAt the gain of 150, the bandwidth is 26MHz0.85nV/Hz Input Voltage Noise2.5pA/Hz Input Current Noise±100uV Input Offset Voltage (Typical))575uVrms post amplifier noise in 20MHz bandwidth.
57 Post Amplifier Relatively small error! Gain=150Vn827 = 109.4uV rmsVnpost = 16.03mV rmsVnpost/Gain = 16.03mV/150 = 106.8uV rms
63 Measured Total Output Noise (DPO 4034 Scope) OPA847 Measured at Scope:STDEV: mVP-P: *STDEV=143mV40s P-P: 170mVAt OPA827 (DUT) Output:Divide by gainVn827 = 21.7mV/150 = 144.6uVCalculated(rms)SimulatedMeasured116.7uV109.1uV144.6uV
64 Measured Spectral Density 4395A Spectrum Analyzer Measurement 4395ALinear ScaleAgilent 4395A Spectrum Analyzer test 1Hz~20MHz span, 3uV/div, REF=24uV.The tested noise density curve shape is the same as simulation.Tina SpiceLinear ScaleTina SpiceLog ScaleLinear Scale
66 Before Questions, I must ask…. ThanksBryan Zhao ( 赵 伟 )Matt HannCollin Wells, Peter Semig, Curtis MayberryReferencesJerald Graeme <Photodiode Amplifiers>Art Kay < Op-Amp Noise Calculation and Measurement >HAMAMATSU <Photodiode Technical Information>Tim Green <Operational amp stability>Before Questions, I must ask….
67 Do I move over to the dark side? You will move to a new topic my young apprenticeMore noise presentations have we must
68 Thank youfor your Time.Contact Info:(phone)(cell)