Application circuit examples of Si photodiode 1. Low noise light-sensitive preamplifier Used in receivers for spatial light transmission and optical remote control. +15 V A reverse bias is applied to the photodiode to improve frequency 10 k 10 µ + 10 µ + response. This circuit outputs an amplified signal from the FET drain, but 1 k 0.1 µ 0.1 µ signals can also be extracted from the source side for interface to the next Vo stage circuit with low input resistance. PD 1000 p FET RL 1 M RS 0.1 µ PD : High-speed PIN photodiodes (S5052, S2506-02, S5971, S5972, S5973, etc.) RL : Determined by sensitivity and time constant of Ct of photodiode RS : Determined by operation point of FET FET: 2SK152, 2SK192A, 2SK362, etc. KPDC0014ED 2. Low-level-light sensor head The whole circuit is housed in a metallic shield box to eliminate external Rf1 S1 +5 V +10 µ 0 Rf2 S2 + EMI (electromagnetic interference). The photodiode window size should 10 µ -5 V be as small as possible. Use of an optical fiber to guide the signal light Cf 2 PD 2 - 6 into the shield box is also effective in collecting light. If dry batteries are - A2 Vo ISC 6 3 + A1 used and housed in the same shield box to supply power to the 3 + 5 1 10-TURN operational amplifier, noise originating from the AC source can be POTENTIOMETER eliminated and the S/N ratio will be further improved. METALIC SHIELDED BOX Bold lines should be within guarded layout or on teflon terminals. A1 :AD549, etc. Rf : 10 GW Max. A2 :OP07, etc. S : Low-leakage reed relay Cf :10 to 100 pF, polystyrene capacitor PD: S1226/S1336/S2386 series, etc. KPDC0016ED 3. Light balance detection circuit Rf The output voltage Vo of this circuit is zero if the amount of light entering D the two photodiodes PD1 and PD2 is equal. The photoelectric sensitivity is D ISC2 SC1 I 2 +15 V 7 determined by the feedback resistance. By placing two diodes D in - 6 PD2 PD1 A Vo + 4 reverse parallel with each other, Vo will be limited to about ±0.5 V 3 (maximum) in an unbalanced state, so that the region around a balanced -15 V state can be detected with high sensitivity. Use of a quadrant photodiode PD: S1226/S1336/S2386 series, etc. allows two-dimensional optical axis alignment. A : LF356, etc. D : ISS270A, etc. Vo=Rf × (Isc2 - Isc1) (V) (Vo < ±0.5 V) When the amount of light entering the two photodiodes is equal, the output voltage Vo will be zero. In unbalanced state, Vo will be ±0.3 to 0.5 V. This circuit can be used for light balance detection between two specific wavelengths using optical filters. KPDC0017EB 4. Luxmeter This is an basic illuminometer circuit using a visual-compensated Si 1 M 10 mV/lx 1 mV/lx photodiode S7686 and an op amp. A maximum of 10000 lx can be measured 100 k 10 k with a voltmeter having a 1 V range. It is necessary to use a low consumption 0.1 mV/lx current type op amp which can operate from a single voltage supply with a 100 p low input bias current. VR * 500 1 k 2 7 - An incandescent lamp of 100 W can be used for approximate calibrations. To 6 3 IC make calibrations, first select the 10 mV/lx range and short the wiper terminal 8 PD + of the variable resistor VR and the output terminal of the op amp. Adjust the 4 1 k V VOLTMETER Isc 006 p distance between the photodiode S7686 and the incandescent lamp so that (9 V) the voltmeter reads 0.45 V. (At this point, illuminance on S7686 surface is about 100 lx.) Then adjust VR so that the voltmeter reads 1.0 V. Calibration IC : ICL7611, TLC271, etc. has now been completed. PD: S7686 (0.45 µA/100 lx) * Meter calibration trimmer potentiometer KPDC0018ED 1 Application circuit examples of Si photodiode 5. Light sensor using high-speed operational amplifier (1) This circuit uses a high-speed photodiode applied at a reverse voltage ISC Rf 10 k PD 51 W +15 V Vo and a current-to-voltage conversion operational amplifier. The time + +15 V response of the circuit greatly depends on the time constant of the 10 µ 0.1 µ 7 2 - 0.1 µ feedback resistance Rf and its parallel stray capacitance. To minimize the A + 6 effect of this time constant, two or more resistors are connected in series 3 14 0.1 µ as the feedback resistance to disperse the parallel stray capacitance. Use -15 V of chip resistors as the feedback resistance will be effective in reducing the stray capacitance. PD: High-speed PIN photodiodes (S5052, S5971, S5972, S5973, etc.) Rf : Two or more resistors are connected in series to eliminate parallel capacitance. A : AD743, LT1360, HA5160, etc. KPDC0020EE 6. Light sensor using high-speed operational amplifier (2) +5 V This light detection circuit uses a high-speed, current-feedback operational 10 k amplifier, and allows direct connection to a coaxial cable. Because this circuit 0.1 µ + 10 µ PD 0.1 µ performs signal amplification after current-to-voltage conversion by load Isc 3 7 + 6 51 W resistance RL, there will be no detrimental effects which result from a phase AA VO RL - shift in the amplifier. 2 4 0.1 µ As with the circuit 5., two or more resistors are used as the feedback R resistance to disperse parallel stray capacitance in the resistors. A ceramic Rf capacitor of 0.1 µF is connected to the power supply pin of the IC, and should -5 V be grounded at a minimum distance. For bandwidths over 100 MHz, use of PD : High-speed PIN photodiodes chip resistors and capacitors is recommended to reduce the entire circuit size (S5052, S5971, S5972, S5973, etc.) and suppress the undesired effects of lead inductance of each component. RL , R, Rf: Determined by recommended conditions of the operational amp Performance can also be improved by using a ground plane structure in which A : AD8001, etc. Vo = Isc × RL × (1 + Rf ) [V] the entire copper foil surface on the PCB is used at the ground potential. R KPDC0015ED 7. Light-to-logarithmic-voltage conversion circuit The output voltage of this circuit is proportional to the logarithmic change D1 +15 V in the detected light level. A base-emitter junction of small signal IB 2 - 7 6 transistors or a diode between the gate-source of junction FETs can be R A Vo + 4 used as the log diode D1. IB is the current source that supplies the log PD 3 Isc diode with a bias current. If IB is not present, the circuit will be unstable or -15 V latched up when Isc by the incident light decreases to zero. D1: Diode of low dark current and low series resistance IB : Current source for setting circuit operation point, IB << Isc R : 1 G to 10 GΩ -15 -12 IO : D1 saturation current, 10 to 10 A A : FET input Op-amp . Isc + IB Vo = . -0.06 log ( + 1 ) [V] IO KPDC0021EA 8. High-speed light detector using PIN photodiode This circuit uses no active components. Since no signal amplification is 50 Ω COAXIAL SMA COAXIAL A CABLE CONNECTOR, ETC. SIGNAL performed, this circuit is mainly used for detection at relatively high PD VOLTAGE light levels. The impedance matching load resistance of 50 W can be directly connected to the 50 W input terminal of an oscilloscope or other K 50 Ω C measurement equipment. R VR High-speed photodiodes designed for the GHz range can be used with - BIAS + this circuit. A chip capacitor should be used as the bypass capacitor C. SUPPLY The photodiode leads and the conductors of the coaxial cable where high- PD: High-speed PIN photodiodes (S5052, S5971, S5972, S5973, etc.) frequency current flows should be made as short as possible. Since the R : 10 kΩ, Voltage drop by photocurrent should be sufficiently smaller than VR. signal current flows instantaneously to the load from the capacitor C in the C : 0.1 µF ceramic capacitor figure, it is necessary to select the capacitance that supplies the The leads of PD and C from coaxial cable should be as short as possible. corresponding electric charge. (Chip components are recommended.) In the figure at the right, if the cable is not terminated, the center conductor is charged up at the power supply potential. If a high reverse KPDC0022ED bias is used, sufficient caution must be taken not to exceed the maximum rating of the input circuit of the measurement equipment, otherwise the equipment may be damaged. 2 Application circuit examples of Si photodiode 9. CT scanner, X-ray monitor In this application, scintillators are used to convert gamma-ray or X-ray Rf into UV or visible light which is detected by the photodiode coupled to the 10 p +15 V scintillator. Reflecting material is used to wrap the scintillator to prevent 2 - 7 0.1 µ 6 background light from entering the photodiode through the scintillator. At X-RAY ISC A Vo + 0.1 µ the same time, the aluminum foil collects the generated light effectively PD 3 4 onto the photodiode. -15 V SCINTILLATOR Beta-ray can also be detected with the same circuit configuration. However, it is necessary to use a light-shielding material which permits PD with scintillator : S8559, S8193, etc. efficient transmission of beta-ray. Rf : 10 M to 100 MΩ A : FET operational amp Scintillator : CsI (Tl), ceramic scintillator, etc.