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Designing Linear Amplifiers Using the IL300 Optocoupler VISHAY SEMICONDUCTORS www.vishay.com Optocouplers Application Note 50 Designing Linear Amplifiers Using the IL300 Optocoupler By Deniz Görk and Achim M. Kruck INTRODUCTION OPERATION OF THE IL300 This application note presents isolation amplifier circuit The IL300 consists of a high efficiency AlGaAs LED emitter designs useful in industrial test and measurement systems, coupled to two independent PIN photodiodes. The servo instrumentation, and communication systems. It covers the photodiode (pins 3, 4) provides a feedback signal which IL300’s coupling specifications, and circuit topologies for controls the current to the LED emitter (pins 1, 2). This photovoltaic and photoconductive amplifier design. Specific photodiode provides a photocurrent, IP1, that is directly designs include unipolar and bipolar responding amplifiers. proportional to the LED’s incident flux. This servo operation Both single ended and differential amplifier configurations linearizes the LED’s output flux and eliminates the LED’s are discussed. Also included is a brief tutorial on the time and temperature dependancy. The galvanic isolation operation of photodetectors and their characteristics. between the input and the output is provided by a second Galvanic isolation is desirable and often essential in many PIN photodiode (pins 5, 6) located on the output side of the measurement systems. Applications requiring galvanic coupler. The output current, IP2, from this photodiode isolation include industrial sensors, medical transducers, accurately tracks the photocurrent generated by the servo and mains powered switchmode power supplies. Operator photodiode. safety and signal quality are insured with isolated Fig. 1 shows the package footprint and electrical schematic interconnections. These isolated interconnections of the IL300. The following sections discuss the key commonly use isolation amplifiers. operating characteristics of the IL300. The IL300 Industrial sensors include thermocouples, strain gauges, performance characteristics are specified with the and pressure transducers. They provide monitoring signals photodiodes operating in the photoconductive mode. to a process control system. Their low level DC and AC signal must be accurately measured in the presence of high common-mode noise. The IL300’s 130 dB common mode IL300 rejection (CMR), high gain stability ± 0.005 %/°C (typ.) and 1 8 ± 0.01 % linearity provide a quality link from the sensor to the controller input. 2 7 The aforementioned applications require isolated signal K1 K2 processing. Current designs rely on A / D or V / F converters to provide input / output insulation and noise isolation. Such 3 6 designs use transformers or high speed optocouplers which often result in complicated and costly solutions. The IL300 4 5 eliminates the complexity of these isolated amplifier designs IP1 IP2 without sacrificing accuracy or stability. APPLICATION NOTE The IL300’s 200 kHz bandwidth and gain stability make it an Fig. 1 - IL300 Schematic excellent candidate for subscriber and data phone interfaces. Present switch mode power supplies are approaching 1 MHz switching frequencies. Such supplies need output monitoring feedback networks with wide bandwidth and flat phase response. The IL300 satisfies these needs with simple support circuits. Rev. 1.7, 27-Nov-17 1 Document Number: 83708 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note 50 www.vishay.com Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler SERVO GAIN - K1 photodiode. The package construction and the insulation The servo gain is defined as the ratio of the servo material guarantee the coupler to have a transient overvoltage of 8000 V peak. photocurrent, IP1, to the LED drive current, IF . It is called K1, and is described in equation 1. K2, the output (forward) gain is defined as the ratio of the K1= I ⁄ I (1) output photodiode current, IP2, to the LED current, IF . K2 is P1 F shown in equation 5. ° ⁄ The IL300 is specified with an IF = 10 mA, TA = 25 C, K2= IP2 IF (5) and VD = -15 V. This condition generates a typical servo The forward gain, K2, has the same characteristics of the photocurrent of IP1 = 120 μA. This results in a typical servo gain, K1. The normalized current and temperature K1 = 0.012. performance of each detector is identical. This results from The servo gain, K1, is guaranteed to be between 0.006 min. using matched PIN photodiodes in the IL300’s construction. to 0.017 max. of an IF = 10 mA, TA = 25 °C, and VD = 15 V. TRANSFER GAIN - K3 Axis Title 8 10000 The current gain, or CTR, of the standard phototransistor Normalized to: optocoupler is set by the LED efficiency, transistor gain, and I = 10 mA, 7 F optical coupling. Variation in ambient temperature alters the VD = -15 V, T = 25 °C 6 amb LED efficiency and phototransistor gain and results in CTR drift. Isolation amplifiers constructed with standard T = 25 °C 1000 5 amb phototransistor optocouplers suffer from gain drift due to changing CTR. 4 Tamb = 0 °C 1st line Isolation amplifiers using the IL300 are not plagued with the 2nd line 2nd line 3 T = -55 °C amb 100 drift problems associated with standard phototransistors. The following analysis will show how the servo operation of 2 T = 85 °C amb the IL300 eliminates the influence of LED efficiency on the - Normalized Photodiode Current 1 P1 amplifier gain. Tamb = 100 °C NI 0 10 The input / output gain of the IL300 is termed transfer gain, 0 1020304050 K3. Transfer gain is defined as the output (forward) gain, K2, IF - Forward Current (mA) divided by servo gain, K1, as shown in equation 6. ⁄ Fig. 2 - Normalized Photodiode Current vs. Forward Current K3= K2 K1 (6) The first step in the analysis is to review the simple optical Fig. 2 presents the normalized servo photocurrent, NIP1 servo feedback amplifier shown in Fig. 3. (IF , TA), as a function of LED current and temperature. It can The circuit consists of an operational amplifier, U1, a be used to determine the servo photocurrent, I P1, given LED feedback resistor R1, and the input section of the IL300. The current and ambient temperature. servo photodiode is operating in the photoconductive The servo photocurrent under specific use conditions can mode. The initial conditions are: be determined by using the typical value for IP1 (120 μA) and Va ==Vb 0 the normalization factor from Fig. 2. The example is to Initially, a positive voltage is applied to the nonirritating input determine IP1 for the condition at TA = 85 °C, and IF = 6 mA. (Va) of the op amp. At that time the output of the op amp will () swing toward the positive Vcc rail, and forward bias the LED. IP1 = IP1, typ x NIP1, typ IF, TA (2) As the LED current, I , starts to flow, an optical flux will be APPLICATION NOTE I = 120 μA x 0.5 (3) F P1 generated. The optical flux will irradiate the servo I = P1 60 μA (4) photodiode causing it to generate a photocurrent, IP1. This The value IP1 is useful for determining the required LED photocurrent will flow through R1 and develop a positive current needed to servo the input stage of the isolation voltage at the inverting input (Vb) of the op amp. The amplifier. amplifier output will start to swing toward the negative supply rail, -VCC. When the magnitude of the Vb is equal to OUTPUT FORWARD GAIN - K2 that of Va, the LED drive current will cease to increase. This Fig. 1 shows that the LED’s optical flux is also received by a condition forces the circuit into a stable closed loop PIN photodiode located on the output side (pins 5, 6) of the condition. coupler package. This detector is surrounded by an optically transparent high voltage insulation material. The coupler construction spaces the LED 0.4 mm from the output PIN Rev. 1.7, 27-Nov-17 2 Document Number: 83708 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note 50 www.vishay.com Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler 7 3 IL300 IL300 + + VCC 1 8 V 7 a 6 U1 2 7 3 + Vin Vout V I F K1 K2 V 6 b - CC U2 V 3 6 2 CC 4 VCC 2 4 5 - IP1 IP2 4 IP1 R1 R2 IP2 17757 17756 Fig. 3 - Optical Servo Amplifier Fig. 4 - Optical Servo Amplifier When Vin is modulated, Vb will track Vin. For this to happen The input / output gain of the isolation amplifier is the photocurrent through R1 must also track the change in determined by combining equations 12 and 15. Va. Recall that the photocurrent results from the change in ⁄ () IF = Vin K1 x R1 (12) LED current times the servo gain, K1. The following I = -V ⁄ ()K2 x R2 (15) equations can be written to describe this activity. F out V ⁄ ()K1 x R1 = -V ⁄ ()K2 x R2 (16) V ==V V =0 (7) in out a b in ⁄ ()⁄ () Vout Vin = - K2 x R2 K1 x R1 (17) IP1 = IF x K1 (8) Note that the LED current, I , is factored out of equation 17. V = I x R1 (9) F b P1 This is possible because the servo and output photodiode The relationship of LED drive to input voltage is shown by currents are generated by the same LED source. This combining equations 7, 8, and 9. equation can be simplified further by replacing the K2/K1 Va = IP1 x R1 (10) ratio with IL300’s transfer gain, K3.
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