Then an envelope detector extracts the original signal for output.ģ. On the receive side of the IC, the signal is conditioned and amplified. The resulting higher-frequency signal is converted to balanced form and passed through the isolation capacitors. The single-ended signal is sent to an AND gate along with an oscillator using spread-spectrum techniques to modulate the input. If the input signal is too low in frequency, a decision logic circuit (DLC) detects the situation and switches the multiplexer to the low-frequency path.įigure 3 shows the OOK method. ![]() A low-pass filter (LPF) filters out the high-frequency PWM and recovers the original signal. The pulse-width-modulation (PWM) signal is converted to a balanced signal and passed through the isolation capacitors, where it’s processed back into pulses. Note that the input signal is also sent to the upper low-speed path, where it pulse-width modulates a higher-frequency oscillator. ![]() Comparators and a flip-flop process these pulses into pulses that are applied to a multiplexer (MUX), which provides the output. In the lower path, the received signal is differentiated into narrow pulses. A single-ended input signal to be transmitted is first converted into a balanced signal that then passes through the isolation capacitors. They’re isolated by the capacitive coupling. (Source: Texas Instruments)Ī typical digital isolator is made up of a transmitter (TX) section and a receiver (RX) section. The edge-based method of isolation uses two paths, one for slow data and the other for high-speed data. One technique is edge-based and the other employs on-off keying (OOK) modulation.Ģ. However, because the capacitance is restricted by the physical limitations of an integrated circuit, special techniques are used to ensure the fast transfer of energy. Today, a newer form of isolator using capacitive connectivity is now available.ĭigital isolators use silicon-dioxide dielectric capacitors as the isolation method. ![]() Their main disadvantage is speed of operation in some digital systems. Optocouplers work well and provide good high-voltage isolation up to 5 to 10 kV. When an input signal is applied, the LED turns on, the base of the phototransistor is illuminated, and it produces the bias that turns the transistor on. With no input signal, the LED is dark and the phototransistor is off, so the external pull-up resistor produces a high output. An optocoupler uses an IR LED and a phototransistor to provide a signal path, but with high-voltage isolation. In some devices, a separate photodiode detector is used (Fig. It’s made up of an infrared LED and phototransistor, usually a BJT with an open collector. One isolation method that’s been around for years is optical isolation, and it involves an integrated circuit called an optocoupler. While transformers can be used for signal transfer, they’re limited in speed, and are bulky and expensive. A transformer is still a key part of isolation methods, and is the approach of choice for dc power isolation. The primary and secondary windings are electrically separate, and power transfer is strictly by way of magnetic induction rather than current flow. One rather obvious method of electrical isolation is to use a transformer. This article introduces isolation methods and a modern integrated approach. A fully integrated signal and power isolation product that’s now available brings a number of benefits to system design, including reduced board space, ease of certification, and simpler design. There’s a need for an efficient, affordable, and compact isolation solution for industrial equipment. Industrial equipment that requires isolation includes programmable logic controllers (PLCs), motor drives, medical equipment, solar inverters, electrical vehicles (EVs), and some special power supplies. Communicating with high-side devices in a motor drive or power-converter systems. ![]() Preventing ground loops in communications networks.Protecting expensive processors and related circuits from high voltage.Protecting industrial operators from high voltage.This kind of isolation is required in a number of instances, such as: This article is part of the TechXchange : Delving into Digital IsolatorsĮlectronic isolation is a means of preventing the transfer of direct current (dc) and unwanted alternating current (ac) between two parts of a system while still enabling signal and power transfer between those two parts.
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