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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 APPLICATION 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.
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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 NOTE APPLICATION 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. ⁄ ()⁄ Vin = IF x K1 x R1 (11) Vout Vin = -K3 x R2 R1 (18) ⁄ () IF = Vin K1 x R1 (12) The IL300 isolation amplifier gain stability and offset drift Equation 12 shows that the LED current is related to the depends on the transfer gain characteristics. Fig. 5 shows the consistency of the normalized K3 as a function of LED input voltage Vin. A changing Va causes a modulation in the LED flux. The LED flux will change to a level that generates current and ambient temperature. The transfer gain drift as the necessary servo photocurrent to stabilize the optical a function of temperature is typically ± 0.005 %/°C over a feedback loop. The LED flux will be a linear representation -55 °C to 100 °C range. of the input voltage, Va. The servo photodiode’s linearity Fig. 6 shows the composite isolation amplifier including the controls the linearity of the isolation amplifier. input servo amplifier and the output trans resistance The next step in the analysis is to evaluate the output trans amplifier. This circuit offers the insulation of an optocoupler resistance amplifier. The common inverting trans resistance and the gain stability of a feedback amplifier. NOTE APPLICATION amplifier is shown in Fig. 4. The output photodiode is operated in the photoconductive mode. The photocurrent, IP2, is derived from the same LED that irradiates the servo photodetector. The output signal, Vout, is proportional to the output photocurrent, IP2, times the trans resistance, R2.
Vout = -IP2 x R2 (13)
IP2 = K2 x IF (14) Combining equations 13 and 14 and solving for IF is shown in equation 15. ⁄ () IF = -Vout K2 x R2 (15)
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Designing Linear Amplifiers Using the IL300 Optocoupler
Package assembly variations result in a range of K3. Axis Title 1.04 10000 Because of the importance of K3, Vishay offers the transfer Normalized to: gain sorted into ± 6 % bins. The bin designator is listed on I = 10 mA, T = 25 °C 1.03 F amb the IL300 package. The K3 bin limits are shown in table 1. 1.02 This table is useful when selecting the specific resistor Tamb = 100 °C Tamb = 85 °C 1000 values needed to set the isolation amplifier transfer gain. 1.01
1.00 TABLE 1 - K3 TRANSFER GAIN BINS 1st line Tamb = -55 °C 2nd line 2nd line 0.99 Tamb = 25 °C BIN MIN. MAX. Tamb = 0 °C 100 0.98 A 0.557 0.626 B 0.620 0.696 0.97 C 0.690 0.773 NK3 - Normalized Transfer Gain (K2/K1) 0.96 10 D 0.765 0.859 0 1020304050 E 0.851 0.955 IF - Forward Current (mA) F 0.945 1.061 Fig. 5 - Normalized Transfer Gain vs. Forward Current G 1.051 1.181 An instrumentation engineer often seeks to design an H 1.169 1.311 I 1.297 1.456 isolation amplifier with unity gain of Vout/Vin = 1.0. The IL300’s transfer gain is targeted for: K3 = 1.0. J 1.442 1.618
7 3 IL300 V + + CC 1 8 Va 6 3 7 U1 V 2 7 + in V V IF K1 K2 6 out b – VCC 3 6 U2 2 V 4 CC VCC 2 4 5 – IP1 IP2 4 IP1 R1 IP2 R2
17759
Fig. 6 - Composite Amplifier
ISOLATION AMPLIFIER DESIGN TECHNIQUES The IL300 can be configured as either a photovoltaic or NOTE APPLICATION The previous section discussed the operation of an isolation photoconductive isolation amplifier. The photovoltaic amplifier using the optical servo technique. The following topology offers the best linearity, lowest noise, and drift section will describe the design philosophy used in performance. Isolation amplifiers using these circuit developing isolation amplifiers optimized for input voltage configurations meet or exceed 12 bit A / D performance. range, linearity, and noise rejection. Photoconductive photodiode operation provides the largest coupled frequency bandwidth. The photoconductive
configuration has linearity and drift characteristics
comparable to a 8 to 9 bit A / D converter.
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Designing Linear Amplifiers Using the IL300 Optocoupler
PHOTOVOLTAIC ISOLATION AMPLIFIER When low offset drift and greater than 12 bit linearity is The transfer characteristics of this amplifier are shown in desired, photovoltaic amplifier designs should be Fig. 7. considered. The schematic of a typical positive unipolar photovoltaic isolation amplifier is shown in Fig. 8. The input stage consists of a servo amplifier, U1, which controls the LED drive current. The servo photodiode is The composite amplifier transfer gain (Vo/Vin) is the ratio of operated with zero voltage bias. This is accomplished by two products. The first is the output transfer gain, K2 x R2. connecting the photodiodes anode and cathode directly to The second is the servo transfer gain, K1 x R1. The amplifier U1’s inverting and non-inverting inputs. The characteristics gain is the first divided by the second. See equation 19. of the servo amplifier operation are presented in Fig. 7a and Fig. 7b. The servo photocurrent is linearly proportional to the IP1 I V IF P2 out input voltage, IP1 = Vin/R1. Fig. 7b shows the LED current is inversely proportional to the servo transfer gain, IF = IP1/K1. The servo photocurrent, resulting from the LED emission, 1 1 K1 K2 R2 keeps the voltage at the inverting input of U1 equal to zero. R1 The output photocurrent, IP2, results from the incident flux 0 + V + 0 I + 0 I + 0 I supplied by the LED. Fig. 7c shows that the magnitude of in P1 F P2 a b c d the output current is determined by the output transfer gain, 17760 K2. The output voltage, as shown in Fig. 7d, is proportional Fig. 7 - Positive Unipolar Photovoltaic Isolation to the output photocurrent IP2. The output voltage equals the Amplifier Transfer Characteristics product of the output photocurrent times the output amplifier’s trans resistance, R2.
VCC 1 k
+ Ω 3 6 IL300 R2 U1 1 8 V R1 5.6 kΩ in 2 IF Ω - 5.6 k 2 7 + Voltage K1 K2 IP1 IP2 - 3 6 2 6 U2 4 5 I I V P1 P2 3 out + 17761
Fig. 8 - Positive Unipolar Photovoltaic Amplifier
Vout K2 x R2
------= ------(19) NOTE APPLICATION Vin K1 x R1 Designing this amplifier is a three step process. First, given Equation 19 shows that the composite amplifier transfer the input signal span and U1’s output current handling gain is independent of the LED forward current. The K2/K1 capability, the input resistor R1 can be determined by using ratio reduces to IL300 transfer gain, K3. This relationship is the circuit found in Fig. 8 and the following typical included in equation 20. This equation shows that the characteristics: composite amplifier gain is equal to the product of the IL300 gain, K3, times the ratio of the output to input resistors. U1 Iout = ± 15 mA IL300 K1 = 0.012 V K3 x R2 ------ou-t = ------(20) K2 = 0.012 Vin R1 K3 = 1.0
Vin = 0 ≥ + 1 V
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Designing Linear Amplifiers Using the IL300 Optocoupler
The second step is to determine servo photocurrent, I , P1 Axis Title resulting from the peak input signal swing. This current is the 2 10000 product of the LED drive current, IF , times the servo transfer gain, K1. For this example the Iout max. is equal to the largest 1 LED current signal swing, i.e., IF = Iout max..
IP1 = K1 x Iout max. 0 1000 IP1 = 0.012 x 15 mA IP1 = 180 μA -1 1st line 2nd line The input resistor, R1, is set by the input voltage range and 2nd line the peak servo photocurrent, I . -2 100
P1 Voltage Gain (dB) Thus R1 is equal to: -3 IF = 10 mA R1 = Vin/IP1 IFmod =±4 mA R = 50 Ω R1 = 1 V/180 μA L Ω -4 10 R1 = 5.6 k 0.01 0.1 1 10 100 1000 f - Frequency (kHz) Fig. 9 - Voltage Gain vs. Frequency
1 kΩ IL300 + 1 8 3 6 IF U1 R2 R1 2 7 5.6 kΩ Vin 2 5.6 kΩ – K1 K2 3 6 - Voltage IP1 IP2 2 – 4 5 I I 6 P1 P2 U2 V 3 out 17764 +
Fig. 10 - Negative Unipolar Photovoltaic Isolation Amplifier
The third step in this design is determining the value of the The modifications of the input amplifier include reversing the trans resistance, R2, of the output amplifier. R2 is set by the polarity of the servo photodiode at U1’s input and composite voltage gain desired, and the IL300’s transfer connecting the LED so that it sinks current from U1’s output. gain, K3. Given K3 = 1.0 and a required Vout/Vin = G = 1.0, The non inverting isolation amplifier response is maintained the value of R2 can be determined. by reversing the IL300’s output photodiodes connection to NOTE APPLICATION R2 = (R1 x G) / K3 the input of the trans resistance amplifier. The modified circuit is shown in Fig. 10. R2 = (5.6 k Ω x 1.0) / 1.0 The negative unipolar photovoltaic isolation amplifier R2 = 5.6 k Ω transfer characteristics are shown in Fig. 11. This amplifier, When the amplifier in Fig. 8 is constructed with OP- 07 as shown in Fig. 10, responds to signals in only one operational amplifiers it will have the frequency response quadrant. If a positive signal is applied to the input of this shown in Fig. 9. This amplifier has a small signal bandwidth amplifier, it will forward bias the photodiode, causing U1 to of 45 kHz. reverse bias the LED. No damage will occur, and the The amplifier in Fig. 8 responds to positive polarity input amplifier will be cut off under this condition. This operation signals. This circuit can be modified to respond to negative is verified by the transfer characteristics shown in Fig. 11. polarity signals.
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Designing Linear Amplifiers Using the IL300 Optocoupler
I I I I P1 F P2 0 P2 +
-1 1 K2 -R2 R1 K1 - -0V 0 I + 0 I + in P1 F Vout 17765 a b c d Fig. 11 - Negative Unipolar Photovoltaic Isolation Amplifier Transfer Characteristics
+ 3 6 IL300a 1 8 R2 U1 1 kΩ Ω R1 5.6 k 2 2 7 5.6 kΩ – K1a K2a Vin 3 6 – 2 6 4 5 U2 IP1a IP2a Vout 3 + IL300b 1 8
2 7 K1b K2b 3 6
4 5
IP1b IP2b 17766
Fig. 12 - Bipolar Input Photovoltaic Isolation Amplifier
IFa IP2a Vout IP1a + + + +
1 1 NOTE APPLICATION R1 K1a K2a R2
0 I 0 + 0 0 + P2a V I + I + in P1a I Fa IP1b I IP2b 0 + Fb IP2b + + +
-1 1 -R2 R1 K1b K2b – – V 0 0 + 0 + in IP1b I Fb Vout 17767 a b c d Fig. 13 - Bipolar Input Photovoltaic Isolation Amplifier Transfer Characteristics
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Designing Linear Amplifiers Using the IL300 Optocoupler
A bipolar responding photovoltaic amplifier can be limited by crossover distortion resulting from the constructed by combining a positive and negative unipolar photodiode stored charge. With a bipolar signal referenced amplifier into one circuit. This is shown in Fig. 12. This to ground and using a 5 % distortion limit, the typical amplifier uses two IL300s with each detector and LED bandwidth is under 1 kHz. Using matched K3s, the connected in anti parallel. The IL300a responds to positive composite amplifier gain for positive and negative voltage signals while the IL300b is active for the negative signals. will be equal. The operation of the IL300s and the U1 and U2 is shown in Whenever the need to couple bipolar signals arises a pre the transfer characteristics given in Fig. 13. biased photovoltaic isolation amplifier is a good solution. By The operational analysis of this amplifier is similar to the pre biasing the input amplifier the LED and photodetector positive and negative unipolar isolation amplifier. This will operate from a selected quiescent operating point. The simple circuit provides a very low offset drift and relationship between the servo photocurrent and the input exceedingly good linearity. The circuit’s useful bandwidth is voltage is shown in Fig. 14.
IP1 IP1Q 1 R1
- + 17768 Vin Fig. 14 - Transfer Characteristics Pre Biased Photovoltaic Bipolar Amplifier
+ 3 6 2N3906 OP-07 IL300 R1 1 8 R2 2 0.1 µF Ω 5.6 kΩ - 5.6 k VCC 100 Ω 2 7 Input K1 K2 3 6 - 0.1 µF 100 µA 2 6 4 5 OP-07 I I 100 µA P1 P2 3 Output +
R2 GAIN = K3 R1 FS = ± 1 V 100 µA 100 µA current
17770 source NOTE APPLICATION Fig. 15 - Pre Biased Photovoltaic Isolation Amplifier
The quiescent operation point, IP1Q, is determined by the a zero volt equilibrium. The bias source can be as simple as dynamic range of the input signal. This establishes a series resistor connected to VCC. Best stability and maximum LED current requirements. The output current minimum offset drift is achieved when a good quality current capability of the OP- 07 is extended by including a buffer source is used. transistor between the output of U1 and the LED. The buffer Fig. 17 shows the amplifier found in Fig. 15 including two transistor minimizes thermal drift by reducing the OP- 07 modified Howland current sources. The first source pre internal power dissipation if it were to drive the LED directly. biases the servo amplifier, and the second source is This is shown in Fig. 15. The bias is introduced into the connected to U2’s inverting input which matches the input inverting input of the servo amplifier, U1. The bias forces the pre bias. LED to provide photocurrent, IP1, to servo the input back to
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Designing Linear Amplifiers Using the IL300 Optocoupler
I I V P1 F IP2 out + 1 1 R1 I K1 K2 0 + P2
- 0 + 0 + 0 + R2 - IP1 IF Vin 17769 a b c d
Fig. 16 - Pre Biased Photovoltaic Isolation Amplifier Transfer Characteristics
+ 3 6 2N3906 OP-07 IL300 R1 R2 2 1 8 Ω – 100 pF 5.6 kΩ 5.6 k VCC 100 Ω 2 7 Input K1 K2 3 6 – 100 pF 100 µA 2 6 Output 4 5 OP-07 12 kΩ I I P1 P2 3 100 µA +
+ 100 µA current 100 µA current 3 6 source 100 µA source OP-07 Ω 2 R2 12 k – GAIN = K3 LM313 R1 V - CC 1.2 V FS = ± 1 V + 3 6 2N4340 OP-07 2 – LM313 0.01 µF V - CC 1.2 V
2N4340
17771 0.01 µF
Fig. 17 - Pre Biased Photovoltaic Isolation Amplifier APPLICATION NOTE APPLICATION
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Designing Linear Amplifiers Using the IL300 Optocoupler
+ 3 6 2N3906 OP-07 IL300 R2 R1 1 8 2 100 pF 5.6 kΩ – 5.6 kΩ VCC 100 Ω 2 7 Input + K1 K2 3 6 – 100µA 2 100 pF 6 4 5 OP-07 12 kΩ IP1 IP2 100 µA 3
+ 10 k
+ 100 µA current Ω 3 6 source OP-07 2 10 kΩ 10 kΩ – LM313 VCC- 1.2 V + 3 6 2N4340 OP-07 2 – 10 k 0.01µF Output Ω – 2 OP-07 10 kΩ 6 10 k 3 + 10 k 100 µA IL300 Ω Ω Ω 12 k 1 8 + 3 VCC 100 Ω 2 7 OP-07 K1 K2 6 2 3 6 – 100 pF 100 µA Input - R4 4 5 IP1 IP2 R3 5.6 kΩ – 5.6 kΩ 2 100 pF OP-07 6 3 + 2N3906
17772 Fig. 18 - Differential Pre Biased Photovoltaic Isolation Amplifier
The previous circuit offers a DC/AC coupled bipolar isolation PHOTOCONDUCTIVE ISOLATION AMPLIFIER amplifier. The output will be zero volts for an input of zero The photoconductive isolation amplifier operates the volts. This circuit exhibits exceptional stability and linearity. photodiodes with a reverse bias. The operation of the input This circuit has demonstrated compatibility with 12 bit A/D network is covered in the discussion of K3 and as such will converter systems. The circuit’s common mode rejection is not be repeated here. The photoconductive isolation APPLICATION NOTE APPLICATION determined by CMR of the IL300. When higher common amplifier is recommended when maximum signal bandwidth mode rejection is desired one can consider the differential is desired. amplifier shown in Fig. 18. This amplifier is more complex than the circuit shown in Fig. 17. The complexity adds a number of advantages. First the CMR of this isolation amplifier is the product of the IL300 and that of the summing differential amplifier found in the output section. Note also that the need for an offsetting bias source at the output is no longer needed. This is due to differential configuration of the two IL300 couplers. This amplifier is also compatible with instrumentation amplifier designs. It offers a bandwidth of 50 kHz and an extremely good CMR of 140 dB at 10 kHz.
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Designing Linear Amplifiers Using the IL300 Optocoupler
UNIPOLAR ISOLATION AMPLIFIER R2= () R1 x G ⁄ K3 (22) The circuit shown in Fig. 19 is a unipolar photoconductive This amplifier is conditionally stable for given values of R1. amplifier and responds to positive input signals. The As R1 is increased beyond 10 kΩ, it may become necessary gain of this amplifier follows the familiar form of to frequency compensate U1. This is done by placing a ⁄ ()⁄ Vout Vin ==GK3 x R2 R1 . R1 sets the input signal small capacitor from U1’s output to its inverting input. This range in conjunction with the servo gain and the maximum circuit uses a 741 op amp and will easily provide 100 kHz or output current, Io, which U1 can source. Given this, greater bandwidth. I = I . R1 can be determined from equation 21. 0max. Fmax. ⁄ () R1 = Vin K1 x I0 (21) max max The output section of the amplifier is a voltage follower. The output voltage is equal to the voltage created by the output photocurrent times the photodiode load resistor, R2. This resistor is used to set the composite gain of the amplifier as shown in equation 22.
3 7 IL300 V + + CC 1 8 V 7 V a 6 CC U1 2 7 – Vin I 2 6 V F K1 K2 b – U2 VCC 3 6 V 2 CC V 4 V 3 out CC + 4 5
IP1 IP1 IP2 R1 4 R2 IP2
17773 Fig. 19 - Unipolar Photoconductive Isolation Amplifier
+ Vref2 V VCC IL300 in 3 - V + 7 CC 1 8 R1 V Ω 7 CC 6 100 U1 2 7 + 741 3 R2 K1 K2 V V 6 2 CC CC U2 – 4 3 6 APPLICATION NOTE APPLICATION -V 741 CC V 20 pF 2 out 4 5 - 4 IP1 IP2
- VCC R3
17774 R4
-Vref1 Fig. 20 - Bipolar Photoconductive Isolation Amplifier
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Designing Linear Amplifiers Using the IL300 Optocoupler
BIPOLAR ISOLATION AMPLIFIER
Many applications require the isolation amplifier to respond From equation 24, Vref2 is shown to be twice Vref1. Vref2 is to bipolar signals. The generic inverting isolation amplifier easily generated by using two 1N914 diodes in series. shown in Fig. 20 will satisfy this requirement. Bipolar signal This amplifier is simple and relatively stable. When better operation is realized by pre biasing the servo loop. The pre output voltage temperature stability is desired, consider the bias signal, Vref1, is applied to the inverting input through isolation amplifier configuration shown in Fig. 23. This R3. U1 forces sufficient LED current to generate a voltage amplifier is very similar in circuit configuration except that across R3 which satisfies U1’s differential input the bias is provided by a high quality LM313 band gap requirements. The output amplifier, U2, is biased as a trans reference source. resistance amplifier. The bias or offset, Vref2, is provided to This circuit forms a unity gain non-inverting compensate for bias introduced in the servo amplifier. photoconductive isolation amplifier. Along with the Much like the unipolar amplifier, selecting R3 is the first step LM113 references and low offset OP- 07 amplifiers the in the design. The specific resistor value is set by the input circuit replaces the 741 op amps. A 2N2222 buffer transistor voltage range, reference voltage, and the maximum output is used to increase the OP- 07’s LED drive capability. The current, Io, of the op amp. This resistor value also affects the gain stability is set by K3, and the output offset is set by the bandwidth and stability of the servo amplifier. stability of OP-07s and the reference sources. The input network of R1 and R2 form a voltage divider. U2 Fig. 24 shows a novel circuit that minimizes much of the is configured as a inverting amplifier. This bipolar offset drift introduced by using two separate reference photoconductive isolation amplifier has a transfer gain given sources. This is accomplished by using an optically coupled in equation 23. tracking reference technique. The amplifier consists of two V out K3 x R4 x R2 (23) optically coupled signal paths. One IL300 couples the input ------= ------()- Vin R3 x R1+ R2 to the output. The second IL300 couples a reference voltage Equation 24 shows the relationship of the Vref1 to Vref2. generated on the output side to the input servo amplifier. ()⁄ This isolation amplifier uses dual op amps to minimize parts Vref2 = Vref1 x R4 R3 (24) count. Fig. 24 shows the output reference being supplied by Another bipolar photoconductive isolation amplifier is a voltage divider connected to V . The offset drift can be shown in Fig. 22. It is designed to accept an input signal of CC reduced by using a band gap reference source to replace ± 1 V and uses inexpensive signal diodes as reference the voltage divider. sources. The input signal is attenuated by 50 % by a voltage divider formed with R1 and R2. The solution for R3 is given in equation 25.
R3 ()0.5 V V ⁄ ()I x K1 (25) = inmax + ref1 F For this design R3 equals 30 kΩ. The output trans resistance is selected to satisfy the gain requirement of the composite isolation amplifier. With K3 = 1, and a goal of unity transfer gain, the value of R4 is determined by equation 26. R4= [] R3 x G x ()R1+ R2 ⁄ ()K3 x R2 (26) R4 = 60 kΩ APPLICATION NOTE APPLICATION
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Designing Linear Amplifiers Using the IL300 Optocoupler
Non-inverting input Non-inverting output
+Vref2 R5 V 7 -Vcc IL300 in 3 + 1 8 Vcc R6 100 Ω R1 6 2 7 2 + 7 R2 K1 K2 Vcc Vcc 6 2 – -Vcc +Vcc 3 6 4 Vo 20 pF 4 5 3 -V I I – cc P1 P2 4 R3 R4 -Vref1
Inverting input Inverting output
V 7 in 3 + Vcc 100 Ω +Vref2 R1 6 IL300 R2 1 8 +V 2 -V cc 3 7 – cc 2 7 + 4 K1 K2 Vcc 20 pF Vcc 6 3 6 Vout 2 R3 4 5 – -Vcc IP1 IP2 -Vcc 4
+Vref1 R4 iil300_22
Fig. 21 - Non-Inverting and Inverting Amplifiers
OPTOLINEAR AMPLIFIERS AMPLIFIER INPUT OUTPUT GAIN OFFSET
VOUT K3 x R4 x R2 Vref1 x R4 x K3 Inverting Inverting ------= ------()- Vref2 = ------VIN R3 x R1 + R2 R3 Non-inverting () VOUT K3 x R4 x R2 x () R5 + R6 -Vref1 x R4 x R5 + R6 x K3 Non-inverting Non-inverting ------= ------()Vref2 = ------VIN R3 x R5 x R1 + R2 R3 x R6 () VOUT -K3 x R4 x R2 x () R5 + R6 Vref1 x R4 x R5 + R6 x K3 Inverting Non-inverting ------= ------()- Vref2 = ------NOTE APPLICATION VIN R3 x R1 + R2 R3 x R6 Inverting VOUT -K3 x R4 x R2 -Vref1 x R4 x K3 Non-inverting Inverting ------= ------()- Vref2 = ------VIN R3 x R1 + R2 R3
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Designing Linear Amplifiers Using the IL300 Optocoupler
+VCC 13.7 kΩ +
1N914 22 μF
VCC -VCC IL300 Vin 3 7 + 1 8 30 kΩ 100 Ω 6 U1 2 7 7 30 kΩ + VCC 741 K1 K2 3 +V +V 6 Vout 2 CC CC U2 - 4 3 6 20 pF 741 -VCC 4 5 2 -V I I - CC P1 P2 4 30 kΩ 14.3 kΩ R3 60 kΩ
-VCC 20 pF 1N914 22 μF +
17775 Fig. 22 - Bipolar Photoconductive Isolation Amplifier
7 VCC V 3 CC V + Ω in 18 kΩ 1 kΩ 6.8 k 6 OP07 2N2222 1.5 kΩ 2 20 pF - LM313 4 10 µF - VCC 1 kΩ Gain 2 kΩ 1 kΩ V IL300 Ω CC 1 8 2 k R3 18 kΩ 2 6.8 kΩ 10 kΩ 2 7 7 VCC - VCC Ω K1 K2 6 NOTE APPLICATION - V 10 k CC Ω V 2 k CC 3 6 OP07 Offset 0.1 µF LM313 3 + 47 µF 4 5 4 - VCC IP1 IP2
20 kΩ
17776 Fig. 23 - High Stability Bipolar Photoconductive Isolation Amplifier
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Designing Linear Amplifiers Using the IL300 Optocoupler
+VCC 3 7 0.1 V + 470 Ω IL300 90 kΩ 6 1 8 10 kΩ OP77 Gain adjust +V 1 V 2 CC V - 4 2 7 7.5 kΩ 5 kΩ in 900 kΩ K1 K2 +V -V 20 pF CC CC 3 6 10 V -VCC Ω 9 M -V 4 5 4 10 kΩ CC I I 100 V P1 P2 2 - OP77 6 + ± 0 mV to 100 mV 3 7 Output +VCC
-V +V CC ref2 220 pF 4 IP2 IP1 + 5 4 +V 3 ref1 +V CC -VCC 10 kΩ OP77 4.7 kΩ 6 3 4 - 2 6 VCC K2 K1 - 7 2 OP77 7 2 6 V Ω CC +V 470 CC 5 kΩ 8 1 + adjust 7 3 IL300 73.2 kΩ +VCC Zero 1 kΩ
17777 Tracking reference Fig. 24 - Bipolar Photoconductive Isolation Amplifier with Tracking Reference
One of the principal reasons to use an isolation amplifier is Axis Title to reject electrical noise. The circuits presented thus far are 0 10000 of a single ended design. The common mode rejection, CMRR, of these circuits is set by the CMRR of the coupler -20 and the bandwidth of the output amplifier. The typical common mode rejection for the IL300 is shown in Fig. 25. -40 1000 APPLICATION NOTE APPLICATION -60 1st line 2nd line 2nd line -80 100
-100 Common Mode Rejection Ratio (dB) -120 10 0.1 1 10 100 1000 f - Frequency (kHz) Fig. 25 - Common Mode Rejection
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Designing Linear Amplifiers Using the IL300 Optocoupler
The CMRR of the isolation amplifier can be greatly The offset independent of the operational amplifiers is given enhanced by using the CMRR of the output stage to its in equation 28. fullest extent. This is accomplished by using a differential Is x []R1 x R3 x K3() U2 – R2 x R4 x K3() U5 V = ------(28) amplifier at the output that combines optically coupled offset R1+ R2 differential signals. The circuit shown in Fig. 26 illustrates the circuit. Equation 29 shows that the resistors, when selected to produce equal differential gain, will minimize the offset Op amps U1 and U5 form a differential input network. U4 voltage, Voffset. Fig. 27 illustrates the voltage transfer creates a 100 μA, IS, current sink which is shared by each of characteristics of the prototype amplifier. The data indicates the servo amplifiers. This bias current is divided evenly the offset at the output is -500 μV when using 1 kΩ 1 % between these two servo amplifiers when the input voltage resistors. is equal to zero. This division of current creates a differential signal at the output photodiodes of U2 and U6. The transfer gain, Vout/ Vin, for this amplifier is given in equation 27.
Vout () () R4 xR2 x K3 U5 + R3 x R1 x K3 U2 ------= ------(27) Vin 2 x R1 x R2
7 VCC U2 + 3 IL300 6 Ω U1 2.2 k 1 8 OP-07 2 Ω 2 7 Inverting 10 kΩ – 4 - VCC 470 100 pF 2N3904 K1 K2 V VCC 3 6 CC
4 5 Gain IP1 IP2
1 kΩ 1 % 2 kΩ
7 V 1.2 V CC 6.8 kΩ 7 VCC 2N3904 + 3 2 – 1 kΩ 6 U4 U3 0.01 µF 6 OP-07 OP-07 2 4 - V – 3 Output CC + Common 100 µA current sink 4-VCC LM313 1 kΩ 1 % 12 kΩ
- VCC Zero adjust Ω 7 VCC U6 2 k
3 + 6 IL300 Ω NOTE APPLICATION U5 2.2 k 1 8 OP-07 10 kΩ 2 2 7 Non-inverting Ω – 4 - VCC 470 100 pF 2N3904 K1 K2 V VCC 3 6 CC
4 5 IP1 IP2
17779
Fig. 26 - Differential Photoconductive Isolation Amplifier
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Designing Linear Amplifiers Using the IL300 Optocoupler
Axis Title Axis Title 0.6 10000 5 45 Vout = -0.4657 mV - 5.0017 x Vin Phase response reference Tamb = 25 °C to amplifier gain of -1, 0° = 180° 0.4 0 0 0.2 1000
0 -5 dB -45 1st line 2nd line 2nd line 2nd line 2nd line -0.2 100 - Output Voltage (V)
out -10 -90 V Ø - Phese Response (°C) Amplitude Response (dB) -0.4 Phase
-0.6 10 -15 -135 -0.15 -0.10 -0.05 00.05 0.10 0.15 0.1 1 10 100 1000
17780 Vin - Input Voltage (V) 17781 f - Frequency (kHz) Fig. 27 - Differential Photoconductive Isolation Amplifier Fig. 29 - Transistor Unipolar Photoconductive Isolation Transfer Characteristics Amplifier Frequency and Phase Response
Axis Title CONCLUSION 46 10000 I = 74.216 μA - 6.472 (μA/V) x V P2 in The analog design engineer now has a new circuit element 45 T = 25 °C amb that will make the design of isolation amplifiers easier. The 44 preceding circuits and analysis illustrate the variety of 1000 isolation amplifiers that can be designed. As a guide, when 43 highest stability of gain and offset is needed, consider the 42 photovoltaic amplifier. Widest bandwidth is achieved with 1st line 2nd line the photoconductive amplifier. Lastly, the overall 2nd line 41 100 performance of the isolation amplifier is greatly influenced - Output Current (μA)
P2 40 by the operational amplifier selected. Noise and drift are I directly dependent on the servo amplifier. The IL300 also 39 can be used in the digital environment. The pulse response 38 10 of the IL300 is constant over time and temperature. In digital 4.4 4.6 4.8 5.05.2 5.4 5.6 designs where LED degradation and pulse distortion can
17810 Vin - Input Voltage (V) cause system failure, the IL300 will eliminate this failure
Fig. 28 - Transistor Unipolar Photoconductive Isolation mode. Amplifier Transfer Characteristics
DISCRETE ISOLATION AMPLIFIER
A unipolar photoconductive isolation amplifier can be NOTE APPLICATION constructed using two discrete transistors. Fig. 30 shows such a circuit. The servo node, Va, sums the current from the photodiode and the input signal source. This control loop keeps Va constant. This amplifier was designed as a feedback control element for a DC power supply. The DC and AC transfer characteristics of this amplifier are shown in Fig. 28 and Fig. 29.
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Designing Linear Amplifiers Using the IL300 Optocoupler
6.2 kΩ 5 V VCC
IL300 1 8 MPSA10
2 7 K1 K2 Va 3 6 5 V V MPSA10 CC Ω 100 k 4 5 I I P1 P2 V Vin out + 5 V 15 kΩ 1.1 kΩ Ω 200 Ω 10 k
GND1 GND2
17782 Fig. 30 - Unipolar Photoconductive Isolation Amplifier with Discrete Transistors
SUPPLEMENTAL INFORMATION The photocurrent is converted to a voltage by the load resistor RL. Fig. 31 also shows that when the incident flux is PHOTODETECTOR OPERATION TUTORIAL zero (E = 0), a small leakage current or dark current (ID) will flow. PHOTODIODE OPERATION AND CHARACTERISTICS Photoconductive Vd/RL load line The photodiodes in the IL300 are PIN (P-material - Intrinsic Ee-5 material - N-material) diodes. These photodiodes convert RL (small) Ee-4 the LED’s incident optical flux into a photocurrent. The Photovoltaic load line magnitude of the photocurrent is linearly proportional to the Ee-3 incident flux. The photocurrent is the product of the diode’s 2 Ee-2 responsivity, S l , (A / W), the incident flux, Ee (W/mm ), and 2 the detector area AD (mm ). This relationship is shown Ee-2 below: RL (large) Ee-1 IP = SI x Ee x A (1a) Id Forward bias Reverse bias PHOTODIODE I TO V CHARACTERISTICS 17783 Reviewing the photodiode’s current / voltage Fig. 31 - Photodiode I to V Characteristics characteristics aids in understanding the operation of PHOTOVOLTAIC OPERATION the photodiode, when connected to an external load. The I to V characteristics are shown in Fig. 31. The graph Photodiodes, operated in the photovoltaic mode, generate shows that the photodiode will generate photocurrent a load voltage determined by the load resistor, R L, and the in either forward biased (photovoltaic) or reversed photocurrent, IP . The equivalent circuit for the photovoltaic biased (photoconductive) mode. operation is shown in Fig. 32. The photodiode includes a
In the forward biased mode the device functions as a current source (IP), a shunt diode (D), a shunt resistor (RP), a NOTE APPLICATION photovoltaic, voltage generator. If the device is connected series resistor (RS), and a parallel capacitor (CP). The to a small resistance, corresponding to the vertical load line, intrinsic region of the PIN diode offers a high shunt the current output is linear with increases in incident flux. As resistance resulting in a low dark current and reverse leakage current. R L increases, operation becomes nonlinear until the open circuit (load line horizontal) condition is obtained. At this I Anode + point the open circuit voltage is proportional to the logarithm P I R F R S of the incident flux. P I L R In the reverse-biased (photoconductive) mode, the CP L VO photodiode generates a current that is linearly proportional D to the incident flux. Fig. 31 illustrates this point with the Cathode - equally spaced current lines resulting from linear increase of 17784 E e. Fig. 32 - Equivalent Circuit - Photovoltaic Mode
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Designing Linear Amplifiers Using the IL300 Optocoupler
The output voltage, Vo, can be determined through nodal Axis Title analysis. The circuit contains two nodes. The first node, VF , 0.5 10000 50 000 includes the photocurrent generator, IP , the shunt diode, D, 30 000 20 000 shunt resistor (RP), and parallel capacitance, CP . The 10 000 0.4 second node, VO, includes: the series resistor, RS, and the 7000 5000 load resistor, RL. The diode, D, in the VF node is responsible 3000 1000 1000 for the circuit’s nonlinearity. The diode’s current voltage 0.3 700 relationship is given in equation 2a. 500 300
100 1st line []()⁄ 2nd line IF = IS x EXP VF K – 1 (2a) 2nd line 0.2 100 This graphical solution of 2a for the IL300 is shown in - Output Voltage (V) out Fig. 33. V 0.1
Axis Title 0 10 100 10000 050100 150 200
17786 IP - Photocurrent (μA) 10 Fig. 34 - Photovoltaic Output vs. Load Resistance and Photocurrent 1 1000 This curve shows a series of load lines and the output 0.1 voltage, Vo, caused by the photocurrent. Optimum linearity 1st line
2nd line is obtained when the load is zero ohms. Reasonable linearity 2nd line Ω 0.01 100 is obtained with load resistors up to 1000 . For load Ω - Forward Current (μA) resistances greater than 1000 , the output voltage will F I 0.001 respond logarithmically to the photocurrent. This response is due to the nonlinear characteristics of the intrinsic diode, 0.0001 10 D. Photovoltaic operation with a zero ohm load resistor 00.10.20.3 0.4 0.5 0.6 offers the best linearity and the lowest dark current, ID. This
17785 VF - Forward Voltage (V) operating mode also results in the lowest circuit noise. A zero load resistance can be created by connecting the Fig. 33 - Photodiode Forward Voltage vs. Forward Current photodiode between the inverting and non-inverting input of a trans resistance operational amplifier, as shown in Fig. 35. Inserting the diode equation 2a into the two nodal equations gives the following DC solution for the photovoltaic V = RI IL300 out p operation (equation 3a): 1 8 ({}[]()⁄ R 0I= P – IS x EXP VO RS + RL K x RL – 1 2 7 [] () ⁄ () ) (3a) - – VO RS ++RL RP RP x RL K1 K2 Ip 3 6 IF U Typical IL300 values: V 4 5 + out -12 I I IS = 13.94 x 10 P1 P2 R = 50 Ω 17787
S NOTE APPLICATION Ω RP = 15 G Fig. 35 - Photovoltaic Amplifier Configuration K = 0.0288 By inspection, as R approaches zero ohms the diode L PHOTOCONDUCTIVE OPERATION MODE voltage, VF , also drops. This indicates a small diode current. All of the photocurrent will flow through the diode series Isolation amplifier circuit architectures often load the resistor and the external load resistor. Equation 3a was photodiode with resistance greater than 0 Ω. With non-zero solved with a computer program designed to deal with loads, the best linearity is obtained by using the photodiode nonlinear transcendental equations. Fig. 34 illustrates the in the photoconductive or reverse bias mode. Fig. 36 shows solution. the photodiode operating in the photoconductive mode. The output voltage, Vo, is the product of the photocurrent times the load resistor.
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Designing Linear Amplifiers Using the IL300 Optocoupler
The reverse bias voltage causes a small leakage or dark Axis Title current, ID, to flow through the diode. The output 2 10000 photocurrent and the dark current, sum the load resistor. This is shown in equation 4a. 0 () VL = RL x IP + ID (4a) 1000 -2 I Cathode P ID I F RP C P 1st line 2nd line D + 2nd line -4 100
VD Percent Difference (%) R -6 S Anode VO -8 10 17788 I L 01015205 RL 17790 Vr - Reverse Voltage (V) Fig. 38 - Photoconductive Responsivity vs. Bias Voltage Fig. 36 - Photoconductive Photodiode Model The photodiode operated in the photoconductive mode is The dark current depends on the diode construction, easily connected to an operational amplifier. Fig. 39 shows reverse bias voltage and junction temperature. The dark the diode connected to a trans resistance amplifier. The current can double every 10 °C. The IL300 uses matched transfer function of this circuit is given in equation 5a. () PIN photodiodes that offer extremely small dark currents, Vout = R x IP + ID (5a) typically a few picoamperes. The dark current will usually track one another and their effect will cancel each other when a servo amplifier architecture is used. The typical dark BANDWIDTH CONSIDERATIONS current as a function of temperature and reverse voltage is PIN photodiodes can respond very quickly to changes in shown in Fig. 37. incident flux. The IL300 detectors respond in tens of The responsivity, S, of the photodiode is influenced by the nanoseconds. The slew rate of the output current is related potential of the reverse bias voltage. Fig. 38 shows the to the diodes junction capacitance, Cj, and the load resistor, responsivity percentage change versus bias voltage. This R. The product of these two elements set the graph is normalized to the performance at a reverse bias of photo-response time constant. 15 V. The responsivity is reduced by 4 % when the bias is τ = R x Cj (6a) reduced to 5 V. This time constant can be minimized by reducing the load Axis Title resistor, R, or the photodiode capacitance. This capacitance 100 10000 is reduced by depleting the photodiode’s intrinsic region, I, by applying a reverse bias. Fig. 40 illustrates the effect of T = 70 °C amb photodiode reverse bias on junction capacitance. 10 T = 50 °C amb 1000 NOTE APPLICATION IL300 1 8 1 7 1st line T = 25 °C 2nd line 2 7 + 2nd line amb 3 I K1 K2 Vout 100 F Vcc 6
- Dark Current (nA) U2
d 3 6 I 0.1 2 - 4 5 4 I I P1 P2 R 0.01 10 IP2 0101520355 25 30 17791 17789 Vr - Reverse Bias (V) Fig. 39 - Photoconductive Amplifier Fig. 37 - Dark Current vs. Reverse Bias
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Designing Linear Amplifiers Using the IL300 Optocoupler
Axis Title Axis Title 20 10000 2 10000
1 15 1000 0 1000
10 -1 1st line 1st line 2nd line 2nd line 2nd line 2nd line 100 -2 100 Voltage Gain (dB) 5 - Junction Capacitance (pF) J IF = 10 mA C -3 IFmod =±4 mA RL = 50 Ω 0 10 -4 10 01025305 15 20 0.01 0.1 1 10 100 1000
17792 Vr - Reverse Bias (V) f - Frequency (kHz) Fig. 40 - Photodiode Junction Capacitance vs. Reverse Voltage Fig. 41 - Voltage Gain vs. Frequency
The zero biased photovoltaic amplifier offers a 50 kHz to Axis Title 60 kHz usable bandwidth. When the detector is reverse 2 10000 biased to -15 V, the typical isolation amplifier response increases to 100 kHz to 150 kHz. The phase and frequency 0 response for the IL300 is presented in Fig. 41. When maximum system bandwidth is desired, the reverse biased -2 1000 photoconductive amplifier configuration should be considered. -4 1st line 2nd line 2nd line
Phase(°) Angle -6 100
-8 IF = 10 mA I =±4 mA Fmod RL = 50 Ω -10 10 0.01 0.1 1 10 100 1000 f - Frequency (kHz)
Fig. 42 - Phase Angle vs. Frequency APPLICATION NOTE APPLICATION
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