a HIGH IMPEDANCE SENSORS I Photodiode Preamplifiers I Piezoelectric - PowerPoint PPT Presentation

PRACTICAL DESIGN TECHNIQUES FOR SENSOR SIGNAL CONDITIONING 1 Introduction 2 Bridge Circuits 3 Amplifiers for Signal Conditioning 4 Strain, Force, Pressure, and Flow Measurements I 5 High Impedance Sensors 6 Position and Motion Sensors 7 Temperature Sensors 8 ADCs for Signal Conditioning 9 Smart Sensors 10 Hardware Design Techniques 5.0 a

HIGH IMPEDANCE SENSORS I Photodiode Preamplifiers I Piezoelectric Sensors N Accelerometers N Hydrophones I Humidity Monitors I pH Monitors I Chemical Sensors I Smoke Detectors I Charge Coupled Devices and Contact Image Sensors for Imaging 5.1 a

PHOTODIODE APPLICATIONS I Optical: Light Meters, Auto-Focus, Flash Controls I Medical: CAT Scanners (X-Ray Detection), Blood Particle Analyzers I Automotive: Headlight Dimmers, Twilight Detectors I Communications: Fiber Optic Receivers I Industrial: Bar Code Scanners, Position Sensors, Laser Printers 5.2 a

PHOTODIODE EQUIVALENT CIRCUIT INCIDENT LIGHT PHOTO R SH (T) C J CURRENT IDEAL 100k Ω Ω - DIODE 100G Ω Ω NOTE: R SH HALVES EVERY 10°C TEMPERATURE RISE 5.3 a

PHOTODIODE MODES OF OPERATION – – + + –V BIAS PHOTOVOLTAIC PHOTOCONDUCTIVE I Zero Bias I Reverse Bias I No "Dark" Current I Has "Dark" Current I Linear I Nonlinear I Low Noise (Johnson) I Higher Noise (Johnson + Shot) I Precision Applications I High Speed Applications 5.4 a

PHOTODIODE SPECIFICATIONS Silicon Detector Part Number SD-020-12-001 I Area: 0.2mm 2 I Capacitance: 50pF I Shunt Resistance @ 25°C: 1000M Ω Ω I Maximum Linear Output Current: 40µA I Response Time: 12ns I Photosensitivity: 0.03µA / foot candle (fc) 5.5 a

SHORT CIRCUIT CURRENT VERSUS LIGHT INTENSITY FOR PHOTODIODE (PHOTOVOLTAIC MODE) ENVIRONMENT ILLUMINATION (fc) SHORT CIRCUIT CURRENT Direct Sunlight 1000 30µA Overcast Day 100 3µA Twilight 1 0.03µA Full Moonlit Night 0.1 3000pA Clear Night / No Moon 0.001 30pA 5.6 a

CURRENT-TO-VOLTAGE CONVERTER (SIMPLIFIED) R = 1000M Ω Ω I SC = 30pA (0.001 fc) _ V OUT = 30mV Sensitivity: 1mV / pA + 5.7 a

LOW BIAS CURRENT PRECISION BiFET OP AMPS (ELECTROMETER GRADE) PART # V OS , TC V OS , I B , 0.1Hz TO 10Hz PACKAGE MAX* MAX MAX* NOISE AD549 250µV 5µV/°C 100fA 4µV p-p TO-99 AD645 250µV 1µV/°C 1.5pA 2µV p-p TO-99, DIP AD795 250µV 3µV/°C 1pA 2.5µV p-p SOIC, DIP * 25°C SPECIFICATION 5.8 a

BiFET OP AMP INPUT STAGE +V S _ OFFSET VOLTAGE 2 TRIM RESISTORS REST OF + AMPLIFIER 3 6 V BIAS 1 5 DRIFT TRIM NULL NULL RESISTORS –V S 5.9 a

AD795 BiFET OP AMP KEY SPECIFICATIONS I Offset Voltage: 250µV Max. @ 25°C (K Grade) I Offset Voltage Drift: 3µV / °C Max (K Grade) I Input Bias Current: 1pA Max @ 25°C (K Grade) I 0.1Hz to 10Hz Voltage Noise: 2.5µV p-p I 1/f Corner Frequency: 12Hz I Voltage Noise: 10nV / √ √ Hz @ 100Hz I Current Noise: 0.6fA / √ √ Hz @ 100Hz I 40mW Power Dissipation @ ± ± 15V I 1MHz Gain Bandwidth Product 5.10 a

LEAKAGE CURRENT PATHS C2 R2 +V S _ 7 2 6 3 + 4 –V S 5.11 a

PCB LAYOUT FOR GUARDING DIP PACKAGE INVERTER 8 1 2 GUARD _ AD795 7 INPUT 2 6 AD795 3 "N" 6 GUARD 3 + PACKAGE 4 5 FOLLOWER 1 8 AD795 3 + 6 2 7 GUARD "N" AD795 PACKAGE 2 _ INPUT 3 6 GUARD 4 5 5.12 a

PCB LAYOUT FOR GUARDING SOIC PACKAGE INVERTER GUARD 1 8 2 _ INPUT 2 7 AD795 6 AD795 3 6 GUARD 3 "R" + PACKAGE 5 4 –V S PINS 1, 5, 8 ARE OPEN ON "R" FOLLOWER PACKAGE 8 1 AD795 3 + 6 GUARD 7 2 "R" AD795 PACKAGE 2 _ INPUT 6 3 4 5 GUARD –V S 5.13 a

INPUT PIN CONNECTED TO "VIRGIN" TEFLON INSULATED STANDOFF BENT INPUT PIN: PIN 2 FOR INVERTER PIN 3 FOR FOLLOWER INPUT SIGNAL LEAD AD795 "N" PACKAGE PC BOARD "VIRGIN" TEFLON INSULATED STANDOFF 5.14 a

AD795 PREAMPLIFIER DC OFFSET ERRORS R2 1000M Ω Ω I B ~ _ OFFSET RTO V OS R1 AD795K I B + DC NOISE GAIN = 1 + R2 R3 R1 I B DOUBLES EVERY 10°C TEMPERATURE RISE R1 = 1000M Ω Ω @ 25°C (DIODE SHUNT RESISTANCE) R1 HALVES EVERY 10°C TEMPERATURE RISE R3 CANCELLATION RESISTOR NOT EFFECTIVE 5.15 a

AD795K PREAMPLIFIER TOTAL OUTPUT OFFSET ERROR 0°C 25°C 50°C 70°C V OS 0.325mV 0.250mV 0.325mV 0.385mV Noise Gain 1.1 2 7 24 V OS Error 0.358mV 0.500mV 2.28mV 9.24mV RTO I B 0.2pA 1.0pA 6.0pA 24pA I B Error 0.2mV 1mV 6.0mV 24mV RTO Total Error 0.558mV 1.50mV 8.28mV 33.24mV RTO 5.16 a

GENERALIZED NOISE GAIN (NG) BODE PLOT Open Loop 100k NG = 1 + R2 ( R1 C1s + 1 ) Gain R1 ( R2 C2s + 1 ) GAIN τ τ 1 s + 1 1 + R2 = C1 C2 τ τ 2 s + 1 R1 10k B τ τ 1 = R1 R2 C1 + C2 R1 R2 R1+R2 _ 1k τ 2 = R2 C2 τ + C2 = 0 A 100 1 + C1 f CL = Closed Loop BW C2 1 f 1 = π τ τ 2 π 1 1 10 1 + R2 f 2 = π τ τ 2 π 2 R1 f U 1 0.1 1 10 100 1k 10k 100k 1M 10M FREQUENCY (Hz) 5.17 a

NOISE GAIN OF AD795 PREAMPLIFIER @ 25°C C1 C2 100k B R1 = 1000M Ω Ω @ +25°C GAIN R2 = 1000M Ω Ω R1 R2 _ C1 = 50pF 10k C2 = 10pF I D + f u = 1MHz A AD795 Open Loop Gain 1 Signal BW = 1k 2 π π R2 C2 100 167kHz = f cl Closed Loop BW 16Hz = Signal BW 10 5.3Hz f u =1MHz NG = 6 NG = 2 1 0.1 1 10 100 1k 10k 100k 1M 10M FREQUENCY (Hz) 5.18 a

VOLTAGE AND CURRENT NOISE OF AD795 VOTAGE NOISE DENSITY CURRENT NOISE DENSITY 1k 100 nV fA Hz Hz 100 10 8 nV/ √ √ Hz 0.6 fA / √ √ Hz 10 1.0 1 / f Corner = 12Hz 0 0.1 1 10 100 1k 10k 100k 1M 1 10 100 1k 10k 100k 1M FREQUENCY (Hz) FREQUENCY (Hz) 5.19 a

AMPLIFIER NOISE MODEL C2 1k Ω Ω @ +25°C has 4nV/ √ √ Hz Noise V N,R2 C1 R2 ∼ B V N,R1 R1 I N– TOTAL NOISE RTO = ∼ ∼ – ∫ V 1 (f) 2 df + ∫ V 2 (f) 2 df + ... V N (f) A V N,R3 R3 V ON I N+ ∼ + NOISE SOURCE RTO INTEGRATION BW V N (f) V N (f)•Noise Gain 1.57•Closed Loop BW I N+ I N+ •R3• Noise Gain 1.57•Closed Loop BW I N – I N – •R2 1.57 •Signal BW R1 V N,R1 •(R2/R1) 1.57 •Signal BW R2 V N,R2 1.57 •Signal BW R3 V N,R3 •Noise Gain 1.57•Closed Loop BW 5.20 a

OUTPUT VOLTAGE NOISE COMPONENTS SPECTRAL DENSITIES (nV / √ √ Hz) @ +25°C 10k 16Hz = Signal BW TOTAL AREAS: 4000 R1 : 20µV RMS R1, R2 R2 : 20µV RMS I N– 1k I N– : 3µV RMS 600 nV V N (f ) : 24.6µV RMS Hz TOTAL = 37.6µV RMS V N (f) 100 48 40 16Hz 12Hz 5.3Hz f CL = 167kHz 10 = Closed Loop BW C1 C2 B R1 = 1000M Ω Ω @ +25°C R1 R2 R2 = 1000M Ω Ω _ 1 C1 = 50pF I D C2 = 10pF + A AD795 f u = 1MHz 0.1 0.1 1 10 100 1k 10k 100k 1M 10M FREQUENCY (Hz) 5.21 a

AD795 PHOTODIODE PREAMP WITH OFFSET NULL ADJUSTMENT 10pF GAIN: 1mV / pA 1000M Ω Ω NOISE: NOISE: _ 37.6µV RMS 28.5µV RMS 20Hz AD795K LOWPASS I D FILTER + +15V 100k Ω Ω INPUT OFFSET 1M Ω Ω NULL RANGE: ± ± 1.5mV 100 Ω Ω 0.1µF –15V 5.22 a

AD795 PHOTODIODE CIRCUIT PERFORMANCE SUMMARY I Output Offset Error (0°C to +70°C) : 33mV I Output Sensitivity: 1mV / pA I Output Photosensitivity: 30V / foot-candle I Total Output Noise @ +25°C : 28.5µV RMS I Total Noise RTI @ +25°C : 44fA RMS, or 26.4pA p-p I Range with R2 = 1000M Ω Ω : 0.001 to 0.33 foot-candles I Bandwidth: 16Hz 5.23 a

COMPENSATING FOR INPUT CAPACITANCE IN A CURRENT-TO-VOLTAGE CONVERTER C2 f 2 = SIGNAL BW R2 f u = OP AMP UNITY _ GAIN BW PRODUCT I C1 f u 1 Total Input f 1 = 2 π π R2 C1 Capacitance + –V B 1 f 2 = 2 π π R2 C2 OPEN LOOP GAIN f 2 = f 1 • f u GAIN UNCOMPENSATED C1 NOISE C2 = 2 π π R2 f u GAIN COMPENSATED FOR 45° PHASE MARGIN f 2 f u 1 f f 2 = 2 π π R2 C1 f 1 f u 5.24 a

FET-INPUT OP AMP COMPARISON TABLE FOR WIDE BANDWIDTH PHOTODIODE PREAMPS Input Bias Unity GBW Input f u /C IN Voltage Noise Current Product Capacitance (MHz/pF) @ 10kHz I B (pA) (nV/ √ Hz) fu (MHz) C IN (pF) AD823 16 1.8 8.9 3 16 AD843 34 6 5.7 600 19 AD744 13 5.5 2.4 100 16 AD845 16 8 2 500 18 OP42 10 6 1.6 100 12 AD745* 20 20 1 250 2.9 AD795 1 1 1 1 8 AD820 1.9 2.8 0.7 2 13 AD743 4.5 20 0.2 250 2.9 *Stable for Noise Gains ≥ ≥ 5, Usually the Case, Since High Frequency Noise Gain = 1 + C1/C2, and C1 Usually ≥ ≥ 4C2 5.25 a

HP 5082-4204 PHOTODIODE I Sensitivity: 350µA @ 1mW, 900nm I Maximum Linear Output Current: 100µA I Area: 0.002cm 2 (0.2mm 2 ) I Capacitance: 4pF @ 10V Reverse Bias I Shunt Resistance: 10 11 Ω Ω I Risetime: 10ns I Dark Current: 600pA @ 10V Reverse Bias 5.26 a

2MHz BANDWIDTH PHOTODIODE PREAMP WITH DARK CURRENT COMPENSATION ≈ 0.8pF ≈ C2 C D = 4pF, C IN = 1.8pF 33.2k Ω Ω 33.2k Ω Ω 33.2k Ω Ω C1 = C D + C IN = 5.8pF R2 = 100k Ω Ω +15V _ –10V D1 C1 AD823 5.8pF D2 + D1, D2: HP-5082-4204 –15V 100k Ω Ω 0.1µF LOW LEAKAGE POLYPROPYLENE 5.27 a

EQUIVALENT CIRCUIT FOR OUTPUT NOISE ANALYSIS C2 C1 = 5.8pF R2 C2 = 0.76pF V N = 16nV/ √ √ Hz R2 = 100k Ω Ω ~ _ V N,R2 ~ V N AD823 C1 C1 1 + C2 + V BIAS = –10V NOISE GAIN 1 f 1 f 2 fu 274kHz 2.1MHz 16MHz V N RTO NOISE ≈ ≈ V N 1 + C1 1.57 f 2 = 250µV RMS C2 V N,R2 RTO NOISE ≈ ≈ 4kTR2 • 1.57f 2 = 73µV RMS 250 2 + 73 2 TOTAL RTO NOISE = = 260µV RMS 10V DYNAMIC RANGE = 20 log = 92dB 260µV 5.28 a