What’s the Difference Between a Photocoupler and an

What’s the Difference Between a Photocoupler and an Isolation Amplifier?

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What’s the Difference Between a Photocoupler and an Isolation Amplifier? Isolation amplifiers provide a means of inserting galvanic isolation into an analogue signal path, to protect sensitive circuitry or human operators, or eliminating interference. Devices are available with differential-analogue or digital output, and can be used in current-sensing or voltage-sensing applications.

Introduction Optical isolation provides a convenient means of implementing electrical separation between sections of a system, such as a sensor and associated circuitry receiving the data. Galvanically isolating parts of a system in this way may be desirable to protect sensitive components from potentially damaging high-energy signals, which may be present during normal operation or if a fault occurs. On the other hand, isolation is an effective technique for eliminating interference or signal grounding problems. Perhaps more importantly, to humans, isolation provides effective protection for operators or others who may come into contact with the equipment. Optical isolators are widely used for coupling circuits that are at different potentials, such as in the feedback circuitry of inverters in equipment like industrial motor drives and servos, photovoltaic systems, uninterruptible power supplies, and inverter-based motor controls in air conditioners and domestic appliances.

Isolation Devices: Photoouplers and Isolation Amplifiers At the heart of any optical isolation device is an optical emitter, such as an LED, and a photodetector tuned to respond to the emitted wavelength (figure 1). In a basic photocoupler, or optocoupler, the signal to be transmitted across the isolation barrier turns the LED on; the photodetector responds by relaying the signal to the receiving-side circuitry. Optical isolators can have bandwidths of tens of megahertz, and are ideal for transmitting pulsed or digital signals.

Figure 1. General principle of the optocoupler

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To provide isolation of an analogue signal, an ordinary optical isolator would require an analogue-to-digital converter (ADC) in front of the emitting LED. If an analogue signal was required on the other side of the isolation barrier, then a digital-to-analogue converter (DAC) would be required at the output of the receiver signal.

Isolation amplifiers integrate the necessary conversion circuitry to allow an analogue signal to be input to the emitter. Broadly speaking, two types of isolation amplifiers are available, that give designers the choice of an analogue output that is directly equivalent to the input signal, or an output that is a digital representation of the input signal.

Isolation Amplifiers in Detail Toshiba has a line-up of four isolation amplifiers that use the latest technology to deliver high linearity and common-mode transient immunity, and which give engineers the choice of single-phase differential analogue output or 1-bit digital output in either an SO8L or DIP8 package. Each isolation amplifier integrates a delta-sigma ADC on the input side. The delta-sigma converter ensures high transmission accuracy and requires minimal analogue circuitry. The output of the converter is then digitally encoded before being used to activate the LED driver. In the TLP7820/7920 analogue isolation amplifiers, a transimpedance amplifier (TIA) conditions the output of the photodetector. The signal is then decoded, converted through a 1-bit digital-to-analogue converter and passed through a low-pass filter, as shown in figure 2.

Figure 2. Isolation amplifier functional blocks.

The TLP7820/7920 generates a differential analogue output at the VOUT+ and VOUT- pins. Figure 3 illustrates the amplifier’s response to an input voltage ramping from -300mV to +300mV.

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Figure 3. Isolation amplifier input-output characteristic.

In the TLP7830/7930 digital devices, often referred to simply as delta-sigma modulators, the signal received by the photodetector is amplified and decoded and then passed to a buffer that outputs a clock and data signal as a digital equivalent of the analogue input, as shown in figure 4.

Figure 4. delta-sigma modulator functional blocks.

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Figure 5 shows the clock and digital outputs corresponding to an input voltage ramping from -320mV to +320mV.

Figure 5. TLP7830/7930 input-output characteristic.

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Application 1. Using Isolation Amplifiers for Current Sensing

The TLP7820/7920 isolation amplifier can be used for motor-current sensing as shown in figure 6.

Figure 6. Motor-current sensing with the TLP7820/7920. The input side of the isolation amplifier can be powered from the same supply used to power the MOSFET gate drivers controlling the motor. A regulator is needed to step down the voltage so as not to exceed the maximum operating voltage recommended for the TLP7820/7920. As far as the design of the input circuitry is concerned, a 0.01μF capacitor (C3) is inserted at the ADC input to ensure accuracy of the input in relation to the ADC clock. C3 also forms part of the low-pass anti-aliasing filter used to block aliasing noise from the ADC, operating in conjunction with the resistor R1. The filtering frequency range of the anti-aliasing filter should be between the signal bandwidth and the Nyquist frequency, usually in the range 400kHz to 1MHz. The current-sensing resistor, Rshunt, should be set to ensure adequate sensing accuracy while minimising power dissipation. Although a smaller resistance value will minimise dissipation, a maximum voltage amplitude of ±200mV is needed to ensure sensing accuracy minimising effects such as noise and non-linearity. A good compromise between accuracy and power dissipation can be achieved using the graph shown in figure 7. In this graph, a point at the intersection of the solid line (relationship between shunt resistance and power dissipation) with the dotted line (input voltage amplitude auxiliary line) gives a typical resistance value.

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Figure 7. Choosing shunt resistance to optimise accuracy and power loss.

For example, if a circuit is required to sense current of 20A, then Rshunt should be 10mΩ to ensure Vin = 200mV. In this case, the power dissipation at the shunt resistor, Pshunt, is 4W. Alternatively, to design for Pshunt of 1W, Rshunt should be set to 2.5mΩ. In this case, the input signal amplitude is 50mV.

As far as the design of the output circuitry is concerned, a high-accuracy external post amplifier having precision offset, response characteristic and temperature dependence can be added to convert the output to a single-phase representation of the input, and to provide noise filtering. Short track lengths should be used to connect the output of the TLP7820/7920 to the external circuitry.

If a TLP7830/7930 digital isolation amplifier (delta-sigma modulator) is to be used, note that the TLP7830/7930 implements a second-order delta-sigma ADC. In terms of the out-of-band noise suppression, the use of a third-order digital filter is recommended. An FPGA or ASIC with third-order SINC (SINC3) filter function can be used to perform conversion of the output bitstream, and for high-frequency noise reduction.

Table 1 shows the relationship between the decimation rate and design parameters of the SINC3 filter. The design is required to balance responsiveness (response time, bandwidth) against the signal-to-noise performance.

The TLP7830/7930 guarantees characteristics at a decimation rate of 256 and 16-bit resolution.

Table 1. SINC3 filter -relationship between decimation rate and design parameters.

Note that the TLP7820/7830/7920/7930 have a test mode for checking internal LED operation, which is entered when either VIN+ or VIN- or both are equal to or greater than VDD1 - 2V (for example if VDD1 = 5V when VIN+ and/or VIN- are 3V or higher). Therefore, VIN+ and VIN- should be kept below this threshold to keep the device in functional mode.

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Application 2. Voltage Sensing with Isolation Amplifiers

Figure 8 shows how the TLP7820/7920 is used for sensing the bus voltage of an inverter.

Figure 8. Inverter bus-voltage sensing with the TLP7820/7920

A high bus voltage can be sensed using a voltage divider. The resistor values R1 and R2 can be calculated as follows: If the input resistance of the coupler is 80kΩ and sensing error is lower than 0.05 %, then The ratio R1 : [R1 x 80kΩ / (R1 + 80kΩ)] = 1 : 0.9995 It can be shown that R1 = 40Ω If the bus voltage is 20V and sensing voltage is 200mV: 20V : 200mV = (R2 + 40Ω) : 40Ω Hence R2 = 3.96kΩ R1, R2 and C1 form a low-pass filter that determines the bandwidth of the sensor, and hence its responsiveness. The responsiveness can be improved by using smaller values of R1, R2 and C1. However, the value should not be set so low that the current flow at the divider becomes high enough to reduce sensing accuracy. Application 3. Isolation Amplifiers in Inverter-Control In the control circuitry used to manage an inverter in a motor driver, or in other DC-AC-conversion applications, isolation amplifiers are ideal for sensing motor phase current, bus voltage, and over-current detection, as shown in figure 9.

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Figure 9. Multiple roles of isolation amplifiers in inverter control.

Summary Isolation amplifiers do more than the role of the humble optoisolator. They can transmit an analogue signal across an isolation barrier, and integrate all the ADC, DAC, encode/decode and filter circuitry that an engineer would have to implement separately to accomplish all this using ordinary components.

As analogue components, they have their place in even today’s most advanced industrial, medical, smart energy and smart-home equipment.

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What’s the Difference Between a Photocoupler and an