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Designing Linear Amplifiers Using the IL300 Optocoupler Application Note

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Designing Linear Amplifiers Using the IL300 Optocoupler Application Note Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler INTRODUCTION linearizes the LED’s output flux and eliminates the LED’s This application note presents isolation amplifier circuit time and temperature. The galvanic isolation between the designs useful in industrial, instrumentation, medical, and input and the output is provided by a second PIN photodiode communication systems. It covers the IL300’s coupling (pins 5, 6) located on the output side of the coupler. The specifications, and circuit topologies for photovoltaic and output current, IP2, from this photodiode accurately tracks the photoconductive amplifier design. Specific designs include photocurrent generated by the servo photodiode. unipolar and bipolar responding amplifiers. Both single Figure 1 shows the package footprint and electrical ended and differential amplifier configurations are discussed. schematic of the IL300. The following sections discuss the Also included is a brief tutorial on the operation of key operating characteristics of the IL300. The IL300 photodetectors and their characteristics. performance characteristics are specified with the Galvanic isolation is desirable and often essential in many photodiodes operating in the photoconductive mode. measurement systems. Applications requiring galvanic isolation include industrial sensors, medical transducers, and IL300 mains powered switchmode power supplies. Operator safety 1 8 and signal quality are insured with isolated interconnections. These isolated interconnections commonly use isolation 2 K2 7 amplifiers. K1 Industrial sensors include thermocouples, strain gauges, and pressure transducers. They provide monitoring signals to a 3 6 process control system. Their low level DC and AC signal must be accurately measured in the presence of high 4 5 common-mode noise. The IL300’s 130 dB common mode IP1 IP2 rejection (CMR), ± 50 ppm/°C stability, and ± 0.01 % linearity provide a quality link from the sensor to the controller input. 17752 Safety is an important factor in instrumentation for medical Fig. 1 - IL300 Schematic patient monitoring. EEG, ECG, and similar systems demand SERVO GAIN - K1 high insulation safety for the patient under evaluation. The IL300’s 7500 V withstand test voltage (WTV) insulation, DC The typical servo photocurrent, IP1, as a function of LED response, and high CMR are features which assure safety current, is shown in figure 2. This graph shows the typical for the patient and accuracy of the transducer signals. non-servo LED-photodiode linearity is ± 1 % over an LED The aforementioned applications require isolated signal drive current range of 1 to 30 mA. This curve also shows that processing. Current designs rely on A to D or V to F the non-servo photocurrent is affected by ambient converters to provide input/output insulation and noise temperature. The photocurrent typically decreases by isolation. Such designs use transformers or high-speed - 0.5 % per °C. The LED’s nonlinearity and temperature optocouplers which often result in complicated and costly characteristics are minimized when the IL300 is used as a solutions. The IL300 eliminates the complexity of these servo linear amplifier. isolated amplifier designs without sacrificing accuracy or stability. 300 The IL300’s 200 kHz bandwidth and gain stability make it an excellent candidate for subscriber and data phone 250 interfaces. Present OEM switch mode power supplies are 0 °C 25 °C approaching 1 MHz switching frequencies. Such supplies 200 rrent (µA) 50 °C need output monitoring feedback networks with wide u 75 °C bandwidth and flat phase response. The IL300 satisfies 150 these needs with simple support circuits. o Photoc v 100 OPERATION OF THE IL300 50 The IL300 consists of a high-efficiency AlGaAs LED emitter IP1 - Ser coupled to two independent PIN photodiodes. The servo 0 photodiode (pins 3, 4) provides a feedback signal which 0 5 10 15 20 25 30 17753 controls the current to the LED emitter (pins 1, 2). This IF - LED Current - mA photodiode provides a photocurrent, IP1, that is directly proportional to the LED’s incident flux. This servo operation Fig. 2 - Servo Photocurrent vs. LED Current www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1004 Rev. 1.4, 27-Jun-08 Application Note 50 Designing Linear Amplifiers Using the Vishay Semiconductors IL300 Optocoupler The servo gain is defined as the ratio of the servo conditions can be determined by using the minimum value photocurrent, IP1, to the LED drive current, IF. It is called K1, for K1 (0.005) and the normalization factor from figure 4. The and is described in equation 1. example is to determine IP1 (min.) for the condition of K1 at (1) TA = 75 °C, and IF = 6 mA. K1= IP1 ⁄ IF NK1() IF = 6mAT, A = 75 °C = 0.72⋅ NK1() IF ,TA (3) The IL300 is specified with an IF = 10 mA, TA = 25 °C, and K1 MIN() IF ,TA = K1 MIN()() 0.005 ⋅ NK1() 0.72 (4) Vd = - 15 V. This condition generates a typical servo K1 MIN() I ,T = 0.0036 (5) photocurrent of IP1 = 70 µA. This results in a typical F A K1 = 0.007. The relationship of K1 and LED drive current is Using K1(IF, TA) = 0.0036 in equation 1 the minimum IP1 can shown in figure 3. be determined. IP1MIN= K1 MIN()() IF ,TA ⋅ IF (6) IP1MIN= 0.0036⋅ 6 mA (7) 0.010 IP1MIN() IF = 6 mA, TA = 75 °C = 21.6 µA (8) 0° The minimum value IP1 is useful for determining the F 0.008 /I 25° maximum required LED current needed to servo the input P1 50° stage of the isolation amplifier. 0.006 75° 100° OUTPUT FORWARD GAIN - K2 o Gain - I v 0.004 Figure 1 shows that the LED's optical flux is also received by a PIN photodiode located on the output side (pins 5, 6) of the 0.002 K1- Ser coupler package. This detector is surrounded by an optically transparent high-voltage insulation material. The coupler 0 construction spaces the LED 0.4 mm from the output PIN 0.1 1 10 100 photodiode. The package construction and the insulation 17754 IF - L ED Current (mA) material guarantee the coupler to have a withstand test voltage of 7500 V peak. Fig. 3 - Servo Gain vs. LED Current K2, the output (forward) gain is defined as the ratio of the The servo gain, K1, is guaranteed to be between 0.005 output photodiode current, IP2, to the LED current, IF. K2 is minimum to 0.011 maximum of an IF = 10 mA, TA = 25 °C, shown in equation 9. and VD = 15 V. K2= IP1 ⁄ IF (9) The forward gain, K2, has the same characteristics of the servo gain, K1. The normalized current and temperature 1.2 performance of each detector is identical. This results from Normalized to: 0 ° I = 10 mA, 25 ° using matched PIN photodiodes in the IL300’s construction. 1.0 F TA = 25 °C o Gain 50 ° v TRANSFER GAIN - K3 0.8 75 ° 100 ° The current gain, or CTR, of the standard phototransistor 0.6 optocoupler is set by the LED efficiency, transistor gain, and optical coupling. Variation in ambient temperature alters the ormalized Ser 0.4 N LED efficiency and phototransistor gain and results in CTR drift. Isolation amplifiers constructed with standard K1 - 0.2 N phototransistor optocouplers suffer from gain drift due to changing CTR. 0.0 0.1 1 10 100 Isolation amplifiers using the IL300 are not plagued with the 17755 drift problems associated with standard phototransistors. IF - LED Current (mA) The following analysis will show how the servo operation of Fig. 4 - Normalized Servo Gain vs. LED Current the IL300 eliminates the influence of LED efficiency on the Figure 4 presents the normalized servo gain, NK1(IF, TA), as amplifier gain. a function of LED current and temperature. It can be used to The input-output gain of the IL300 is termed transfer gain, determine the minimum or maximum servo photocurrent, IP1, K3. Transfer gain is defined as the output (forward) gain, K2, given LED current and ambient temperature. The actual divided by servo gain, K1, as shown in equation 10. servo gain can be determined from equation 2. K3= K2⁄ K1 (10) (2) The first step in the analysis is to review the simple optical K1() IF ,TA = K1() data sheet limit ⋅ NK1() IF ,TA The minimum servo photocurrent under specific use servo feedback amplifier shown in figure 5. Document Number: 83708 For technical questions, please contact: [email protected] www.vishay.com Rev. 1.4, 27-Jun-08 1005 Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler The circuit consists of an operational amplifier, U1, a resistance amplifier. The common inverting trans resistance feedback resistor R1, and the input section of the IL300. The amplifier is shown in figure 6. The output photodiode is servo photodiode is operating in the photoconductive mode. operated in the photoconductive mode. The photocurrent, The initial conditions are: IP2, is derived from the same LED that irradiates the servo photodetector. The output signal, Vout, is proportional to the Va ==Vb 0 . Initially, a positive voltage is applied to the nonirritating input output photocurrent, IP2, times the trans resistance, R2. (Va) of the op amp. At that time the output of the op amp will Vout = - IP2 ⋅ R2 (17) swing toward the positive Vcc rail, and forward bias the LED. IP2 = K2⋅ IF (18) As the LED current, I , starts to flow, an optical flux will be F Combining equations 17 and 18 and solving for I is shown generated. The optical flux will irradiate the servo photodiode F in equation 19. causing it to generate a photocurrent, IP1. This photocurrent will flow through R1 and develop a positive voltage at the IF = - Vout ⁄ ()K2⋅ R2 (19) inverting input (Vb) of the op amp.
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