Lecture Notes on Linear Optocouplers | ECE 5030, Exams of Biomedical Engineering

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APPLICATION NOTE AN-

Linear Optocouplers

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Introduction

This application note describes isolation amplifier design principles for the LOC Series linear optocoupler devices. It describes the circuit operation in photoconductive and photovoltaic modes and provides some examples of applications in different industry segments. The LOC product is intended to give the designer an alternative to bulky transformers and “non-linear” optocouplers for many applications. Galvanic isolation is required for many circuits found in Telecommunication, Industrial, Medical and Instrumentation systems. This has been traditionally accomplished by means of transformers and optocou- plers with transformers being used to couple AC signals and optocouplers used primarily for DC signal cou- pling. Unlike standard optocouplers, the LOC operates in a servo mode configuration which compensates for the LED’s non-linear time and temperature characteristics. In addition, the LOC can couple both AC and DC signals. The following are examples where galvanic isolation is required:

• Telecommunications:Telecom products such as modems require isolation and signal coupling from the telephone line to the modem data pump.
• Industrial Control: Products such as temperature sensors and controllers. Temperature sensors are often remotely located from the controller and reside in hazardous environments near high voltage lines. Isolation provides the required signal coupling while insuring safety to personnel working with the controller.
• Medical: EEG and ECG machines have sensors that attach to the patient. The sensors are galvanically isolated to provide a high voltage isolation barrier between patient and machine.
• Instrumentation: Often use isolated switching supplies where it is required to sense the output voltage and feedback a portion of the signal to the controller for voltage regulation while not compromising power supply isolation.

Description

The LOC Series (LOC110, LOC111 and LOC112 with one optocoupler per package, and LOC210 and LOC211P with two per package) are linear optocouplers designed to be used in applications where galvanic isolation is required for AC and DC signal coupling and linearity from input to output must be accurately pre- served. The device consists of an infra-red LED optically coupled with two phototransistors. One phototran- sistor is typically used in a servo feedback mechanism to control the LED drive current which has the effect of compensating for the LED’s non-linear time and temperature characteristics. The other output phototran- sistor is used to provide the galvanic isolation between the input and output circuit. A typical isolating amplifi- er is shown in figure 1. Curcuit Operation Utilizing the LOC

Photoconductive Operation

With VIN at 0V and IF at 0mA, U1 has large open loop gain. As VIN begins to increase, the output of U1 begins to go to the VCC1 rail. As it does, IF current begins to flow and the LED begins to turn on. As the LED turns on, the incident optical flux on the servo phototransistor causes a current I 1 to flow. As I 1 flows through R1, a volt- age is developed on the inverting input of the op amp VA such that the output of the amplifier will begin to go to the negative supply rail (ground in this case). When the voltage on VA is equal to VIN, IF will no longer increase and the circuit is now in a stable closed loop condition. If VIN is modulated, VA will track VIN. The flux generated by the LED is also incident on the output phototransistor and generates a current I 2 which is pro- portional to the LED flux and LED current; this current closely tracks I 1. The output voltage of the amplifier is the product of the output photocurrent I 2 and resistor R2. The equations and definitions of the circuit are list- ed below (including figure 1). Servo Gain - K Defined as the ratio of the servo photocurrent I 1 to the LED forward current IF: K1 = I 1 /IF. For the LOC110, K1 is typically 0.007 for an IF of 10mA and a VCC of 15V.

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The following derivation ties the example and definitions to one equation relating all the parameters for this cir- cuit: Solving equation #3 for VIN:

7. V (^) IN = I (^) F • K1 • R

Combining equation #4 and #5 and solving for VOUT:

8. VOUT = I (^) F • K2 • R

Dividing equation #7 by equation #8 and solving for VOUT gives the final equation:

and since the definition of K3 is K3 = K2/K1 we can further simplify by writing:

IF was canceled out of equation #10. This is due to the fact that both servo and output photocurrents originate from the same LED source. Since K3 is the ratio K2/K1, in our example K1 = K2 = 0.004, and K3 = 1.

Therefore, VOUT is directly proportional to the ratio of R2/R1.

The circuit in figure 1 is configured with the phototransistor collector to base reverse biased. This is operation in the photoconductive mode. When an application requires amplifier bandwidth of up to 200kHz, the photo- conductive configuration should be used. This mode has linearity and drift characterisitcs comparable to a 8- bit D/A converter with ±1 bit linearity error.

Photovoltaic Mode Using the LOC product in the photovoltaic mode achieves the best linearity, lowest noise and drift perform- ance. It is possible to achieve up to 14-bit D/A linearity in this mode. The tradeoff with this topology is that band- width is limited to about 40kHz. A typical isolation amplifier in the photovoltaic configuration is shown in figure

In the photovoltaic mode, the LOC phototransistors act as current generators. Since all photogenerators dis- play some voltage dependence on linearity, maintaining a 0V bias on the phototransistor eliminates this prob- lem and improves linearity. If the phototransistor is connected across a small resistance, the output current is linear with increases in incident LED flux. To accomplish this, the phototransistors are connected across the op amp inputs. As VIN increases, the current through the LED increases and so does the optical flux. The LED flux is incident on the servo phototransistor which starts current I 1 to flow from the op amp inverting input through the phototransistor. This servo photocurrent is linearly proportional to VIN, I 1 = VIN/R1 and keeps the voltage on the inverting input equal to zero.

V V K R OUT K R = IN(^ ¥^ ) ( ¥ )

2 2 1 1

V V K R OUT IN R = ¥ 3 ¥ 2 1

Figure 2. VIN vs. VOUTt

(^00)

1

2

3

4

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 V (^) IN(Volts)

V^

(Volts) OUT

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Figure 2A. Photoconductive Amplitude Response

(^0 5 10 15 )

2 1 0

• V – Reverse Voltage

Percent Difference %

R

0.1 1 10 100 1000

0

Input Frequency (kHz)

Phase Response (degrees)

VOUT

VIN

VCC

R R I (^1)

I (^) F

I (^2)

I ' 2

LOC110, LOC or LOC 1 2

4

8 7 6 5

3

Figure 3. Isolation amplifier (Photovoltaic Operation)

Figure 2B. Photoconductive Phase Response

The flux from the LED is also incident on the output phototransistor which causes a current I 2 to flow from the inverting input of the output op amp through the phototransistor. As I 2 is pulled from the inverting node, the out- put of the amplifier begins to go high until a current equal in magnitude to I 2 is injected into the inverting node of the amplifier. Since this current, I 2 ´, flows through R2, an output voltage is developed such that VOUT = I 2 ´ • R2. Since I 2 = I 2 ´, VOUT = I 2 • R2. The composite equation describing the operation of this circuit is the same as in the photoconductive mode, that is: VOUT = VIN • K3 • R2/R1. The frequency and phase response for this circuit is shown in figures 4 and 5 respectively. This circuit has a bandwidth of approximately 40kHz.

Using the LOC210P or LOC211P in a Modem Data Access Arrangement (DAA) Circuit.

Background

In the past, the only way to couple signals from the telephone line and provide the isolation necessary has been to use a transformer. With the advent of pocket and PCMCIA (Personal Computer Memory Card International Association) modems, however, the transformer has become a liability in terms of the size, weight and PCB real estate it occupies. Today, PCMCIA modems demand rugged on-board DAA circuits. The LOC eliminates the transformer problem with no performance sacrifice and improved manufacturability and reliability. With Total Harmonic Distortion typically at -87dB and servo non-linearity less than 0.01%, the LOC210P is well suited for high speed modem applications.

0.1 1 10 100

0

• Input Frequency (kHz)

Amplitude Response (dB) 0.1 1 10 100

0

Input Frequency (kHz)

Phase Response (degrees)

Figure 4. Photovoltaic Amplitude Response Figure 5. Photovoltaic Phase Response

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Electronic Inductor

The purpose of the electronic inductor circuit is to sink loop current when the modem goes off-hook thus seiz- ing the phone line. The circuit usually consists of a Darlington transistor, a resistor bias network, and a capac- itor to provide AC rejection. This circuit should be designed to work throughout the range of loop currents per FCC Part 68.3. The circuit also presents a high AC impedance to the line so that signal integrity is not com- promised. The zener diode is installed for protection of the Darlington transistor and other circuitry on the line side. The zener voltage is selected based on the voltage rating of the other components selected. Refer to Appendix 1 for details on the electronic inductor design. Switch Mode Power Supply Application (LOC110, LOC111 or LOC112)

Another useful application for the LOC110, LOC111 or LOC112 is in the feedback control loop of isolated switching power supplies. Typically, the DC output voltage of the supply is monitored and fed back to the con- trol input of the switcher through isolated means in order to regulate the output voltage. The most common way of doing this in the past has been to use an additional winding on the isolation transformer, figure 7A.

Control Signal

Rectifiers/ Filters

DCIN Switcher and FiltersRectifiers OUTDC

Transformer

Figure 7A. DC-to-DC Converter with Feedback Winding This winding would generate an AC signal which then needed to be rectified, filtered, and possibly scaled down with a resistor network before going into the control input of the switcher. Using the LOC110, LOC111 or LOC112 to accomplish the same task is a better solution since the special transformer windings, rectification, and filtering are eliminated. Also, the problem of poor load regulation due to inadequate winding coupling is eliminated. Referring to figures 7B and 7C, the design is almost identical to the basic photoconductive isolat- ed unity gain amplifier discussed previously, however a voltage divider consisting of RA and RB is added.

Figure 7B. DC-to-DC Converter with LOC110, LOC111 or LOC112 (Block Diagram) Figure 7C. DC-to-DC Converter with LOC110, LOC111, or LOC112 (Schematic)

Control Signal

DC IN DC OUT Rectifiers Switcher and Filters

Transformer

LOC110 or LOC Amplifier

VCC

R (^) B

R (^) A

VCC

LOC110, LOC or LOC

DC OUT

Control Signal

5 6

8

4 3 2 1

7

Ð

ShieldedCable

Right Leg

Isolation Amplifier

Guarding

To CardiacMonitor

V VCC

Ð

ECG Signal

ISOL

Figure 8. LOC110 Isolated Amplifier in ECG Application

Common mode voltage V of 3750 V

TransducerPressure

PowerSupply

Isolated V-1Converter

ConditionerSignal 0-10V

4-20mA

IsolatedPower Supply

RecordingInstrument

CM

VCM RMS Figure 9. Isolated Pressure Transmitter

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VCC

V 0-10V from Signal Conditioner

IN

VCC

VS

VS

R

V

900½

S

200μA

0-800μA

Q

12.5k½

VCC

VS I (^) OUT

V Q

R 45½ S

Isolated V (^) S Power Supply

R =525½L

C

R

Ð

+U

Ð +U

1 2

4

8 7 6 5

LOC110, LOC or LOC

3 U

12.5V

Ð

0.1 1 10 100 1000

• 90

Frequency (kHz)

CMRR - Common Mode Rejection Ratio (dB)

Figure 9A. Common Mode Rejection

Cardiac Monitoring Application

Designing equipment to measure Cardiac signals such as the Electrocardiogram (ECG) presents some spe- cial problems. Cardiac signals for adults are approximately 1mV in magnitude while for a fetus can be as low as 50μV. Since the heart signals are low in amplitude, noise such as residual electrode voltages and 50/60Hz power line pickup can easily swamp out the signal. Therefore, it is important to design an isolated amplifier circuit which interfaces to the probe that has high Common Mode Rejection (CMR) ratings to reduce or elim- inate common mode noise while providing amplification for the heart signals. The LOC110, LOC111 or LOC112, with the proper support circuitry, can provide the isolation, amplification, linearity, and high CMRR that is required for this type of application. Referring to the diagram in figure 8, the isolated amplifier block contains the LOC110, LOC111 or LOC112 and high CMRR op amps. The electrodes are connected to the amplifier via shielded cable to provide noise immunity. The shield is connected to the patient’s right leg for best CMR performance. For good performance, proper shielding, PCB layout and ampli- fier, design techniques should be practiced.

Figure 9B. 0-10V to 4-20mA Converted

Isolated 0-10V to 4-20mA Converter Application

Industrial controllers and data acquisition equipment frequently require an isolated voltage-to-current loop converter in environments where high common mode noise exists and protection of equipment and person- nel from high voltages are required. The current loop, usually 4-20mA, is used to drive control valves or the input to chart recorders for temperature/pressure monitoring over time for example. Figure 9 shows a simpli- fied block diagram of an isolated pressure transmitter. The LOC110, LOC111 or LOC112, with a typical Common Mode Rejection Ratio of 130dB (see figure 9A) and isolation voltage up to 3750VRMS (E version) is a good choice for this kind of application. The example circuit for this application is shown in figure 9B. The LOC110, LOC111 or LOC112 is in the photoconductive mode which has linearity comparable to an 8 bit D/A converter with ±1 LSB nonlinearity or 0.39% of full scale. For this example, the input to the circuit is 0-10V from the output of the pressure transducer signal condition- er, R1 and R2 are calculated based on the K3 of the LOCs being used and should be selected to achieve unity gain for the amplifier. Note that the isolation amplifier portion of the circuit is very similar to the basic pho- toconductive amplifier discussed earlier. The difference is the addition of pass transistor Q1 in the negative feedback loop of U2. VCC is the non-isolated power supply and VS is the isolated power supply which is 12.5V for this example. This supply does not require strict regulation as U3 maintains current regulation for the loop.

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Industry Segment Application Mode Function

Modem DAA PV Mode for best linearity 0.01% H.V. Isolation, Signal Coupling with — 40KHz bandwidth

PBX Isolated SWPS* for PC Mode for 200KHz bandwidth Isolated voltage sensing for Ring Generator 0.39% linearity SWPS* feedback

Industrial RTD PV or PC depending on desired High CMRR** for noise immunity, (Resistance Temp. Device) linearity and bandwidth HV isolation, signal coupling

Industrial Isolated Pressure Sensing PV or PC depending on desired High CMRR** for noise immunity, linearity and bandwidth HV isolation, signal coupling

Isolated 4-20mA Converters PV or PC depending on desired High CMRR** for noise immunity, linearity and bandwidth HV isolation, signal coupling

Isolated EGG/ECG Amplifier PV or PC depending on desired Couples low level signals from Medical linearity and bandwidth transducers, HV isolation, noise immunity

PH Probe PV Mode Maintains high CMRR** for Instrumentation remote PH probe, provides amplification and HV isolation *SWPS: Switch Mode Power Supply **CMRR:Common Mode Rejection Ratio

Table 1. Typical Applications Using the LOC110/LOC210P

Appendix 1

Electronic Inductor Design

The electronic inductor approximates the operation of a discrete inductor by using a Darlington transistor, three (3) resistors and a capacitor. When used in a modem application, the electronic inductor will present a relatively low impedance to DC currents and a relatively high impedance to AC signals.

Circuit Description

Figure 1 shows the electronic inductor in a typical modem environment. Bridge D2 rectifies current on tip and ring for the electronic inductor only. This ensures line-polarity insensitivity required by most regulatory agen- cies. Diode D1 protects Darlington Q1 from excessive transient voltages when going off-hook. The zener volt- age should be less than the VCEO of the Darlington. R1 and R2 set the biasing point for Q1. C1 is used for AC rejection of signals at the base of Q1. C1 should be a good quality Tantalum rated at a minimum of 10WV. R3 is used to provide negative feedback for Q1 so that Q1 will not go into saturation over the loop current range. The AC signal path is coupled to the modem’s transformer via C2. C2 should have a working voltage of 100V, or 50V if two capacitors are used, one on each lead of the primary (see figure 1).

DC Characteristics (Figure 2) The electronic inductor should be tailored to meet the following requirements:

• CO (Central Office) Battery (42.5 - 56.5V DC)
• Loop Resistance (400 - 1740Ω)

Maximum allowed DC-resistance of CPE (Customer Premise Equipment) in off-hook mode (200Ω) per FCC 68.314 (c1), (c2). Minimum recommended DC resistance in off-hook mode (90Ω) per EIA-496A, 4.2.2.1.

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The two extremes of operation are as follows:

1. Minimum loop current:

• CO battery drops to 42.5V DC
• Loop resistance is 1740Ω
• Electronic coil has highest DCR of 200Ω resulting in a minimum loop current of 22mA

2. Maximum loop current:

• CO battery is 56.5V DC
• Loop DC resistance is 400Ω
• Electronic coil has the lowest DCR of 90Ω the resulting maximum current is 115mA The circuit should be tested per FCC 68.314 which consists of a battery and variable resistor to simulate prop- er operation at the above stated conditions.

AC Characteristics

For good performance, the electronic inductor should emulate an inductance of between 4-10H. To approxi- mate the value of the inductor: L ≈ R1 • C1 • R3.

V OH

CC

V

T

D

C

L

R

100V

Equivalent Circuit To Modem T 50V if 2 capacitors are used

I Current

15VD

Q

R C1+ R

R

R

Ring

CC

Figure 1. Dry Circuit with Electronic Inductor

VDC

L

R

Loop Resistance

Central Office (Co)Battery 42.5-56.5 (^) I

CENTRAL OFFICE LOOP CUSTOMER PREMISE EQUIPMENT (CPE)

D V

400 Ω 0 Ωmin

0 Ωmin

670 Ωmax

670 Ωmax Electronic Inductor

Figure 2. Central Office to CPE Interconnect

Appendix 2

Photoconductive Description

When the LOC is used in the photoconductive mode, the phototransistors are operated with the collector and base reversed biased as shown in figure 1A. The equivalent circuit model is shown in figure 1B which shows the photocurrent source I, dark current component ID, intrinsic diode D, and junction capacitance CP. The inci- dent flux from the LED on the phototransistor causes a photocurrent (I) to flow from the collector to the base and through the load resistor RL. This photocurrent is linearly proportional to the LED flux. The output voltage VO results from the product of the photocurrent (I) plus a small dark current times the load resistance RL: VO=[I+ID]•RL. The dark currents from both phototransistors track closely and are canceled when used in the servo mode. One of the attributes of the photoconductive mode is a bandwidth of about 200kHz. This is considerably high- er than the photovoltaic mode bandwidth discussed earlier which was around 40kHz. One of the reasons for this is that with the photoconductive mode, since the base-collector junction is reversed biased, the depletion area of the junction is wider than when no bias or forward bias is applied. The wider depletion area of the junc- tion results in a lower junction capacitance (CP in figure 1B) which results in a faster rise time or responsivity:

tR=RL• CP

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The equivalent circuit with RL=0Ω is shown in figure 4. To achieve 0V bias, the configuration shown in figure 5 is implemented. The inverting input of the amplifier is at virtual ground so a 0V bias is obtained. When LED flux is incident on the phototransistor, a current is gen- erated by the phototransistor and pulled from the inverting input. Since by Kirchoff’s law the sum of the cur- rents entering and leaving a node must be zero, the amplifier responds with a current I 1 of equal magnitude to the current leaving the node IG, and is injected into the inverting node via RF which maintains zero volts at this node. The output voltage of the op amp is the current I 1 • RF. The junction capacitance is higher than in the photoconductive configuration due to a zero volt bias which results in a narrower depletion region and a higher junction capacitance which limits the bandwidth to approx- imately 40kHz.

0½ I (^) G Figure 4. Equivalent Circuit with RL = 0 Ω

I (^) G

I

V

1

I (^) G

C (^) P

R

V

F I (^1)

R (^) F

Ð

Ð

Circuit Equivalent Model Figure 5. Implementation of 0V Bias in Photovoltaic Mode

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