PCB Technical - High-speed PCB design guide three: signal isolation technology

Signal isolation prevents digital or analog signals from passing through the barrier between the sending and receiving terminals when they are sent through a galvanic connection. This allows the difference between the ground or reference level outside the transmitting and receiving terminals to be as high as several thousand volts, and prevents loop currents between different ground potentials that may damage the signal. Noise on the signal ground can damage the signal. Isolation can separate the signal to a clean signal subsystem ground. In another application, the electrical connection between the reference levels can create a current path that is unsafe for the operator or patient. The nature of the signal can indicate to the circuit designer those correct ICs that the system can consider.

The first type of isolation device relies on no transmitter and receiver to cross the isolation barrier. This kind of device was used for digital signals, but the linearization problem forced the use of transformers for analog signal isolation, and the modulated carrier was used to make the analog signal cross this barrier. Transformers are always difficult to say, and it is usually impossible to make an IC, so I came up with a capacitor circuit to couple the modulated signal to cross the barrier. The high conversion rate transient voltage acting on the isolation barrier can be used as a signal for a single-capacitor barrier device, so a dual-capacitor differential circuit has been developed to minimize the error. Now capacitive barrier technology has been applied in digital and analog isolation devices.

1. Isolate the serial data stream

There is a wide range of options for isolating digital signals. If the data stream is bit-serial, the options range from simple optocouplers to isolated transceiver ICs. The main design considerations include:

• Required data rate

• Power requirements for the isolated end of the system

• Whether the data channel must be bidirectional

LED-based optocouplers are the first technology used to isolate design issues. Several LED-based ICs are now available with data rates of 10Mbps and above. An important design consideration is that the LED light output decreases over time. Therefore, excessive current must be provided to the LED in the early stage so that sufficient output light intensity can still be provided over time. Since there may be limited power available on the isolated side, the need to provide excessive current is a serious problem. Because the drive current required by the LED can be greater than the current available from the simple logic output stage, a special drive circuit is often required.

For high-speed applications and the reverse transfer of data streams under logic signal control, Burr-Brown's ISO 150 digital coupler can be used. Figure 1 shows the ISO150 bidirectional application circuit. Channel 1 controls the transmission direction of Channel 2 and is configured to transmit from end A to end B. The signal applied to the DIA pin determines the direction of the signal flow. The high level sent to end B puts the end of channel 2 into receiving mode. The low level applied to the Mode pin at the 2A end of the channel puts the channel into the sending mode. The state of the direction signal is on both sides of the isolation barrier. This circuit can work at a data rate of 80MHz.

The second variant of bit-serial communication is the differential bus system device under development. These systems are described by RS-422, RS-485 and CANbus standards. Some systems are fortunate to have a common ground, and many systems have nodes with different potentials. This is especially true when the two nodes are separated by a certain distance. Burr-Brown’s ISO 422 is designed for integrated full-duplex isolated transceivers that can be used in these applications. This transceiver can be configured as half-duplex and full-duplex (see Figure 2). The transmission rate can reach 2.5Mbps. This device even includes a loop (Loop-back) test function, so each node can perform a self-test function. During this mode, data on the bus is ignored.

2. Analog signal isolation

In many systems, analog signals must be isolated. The circuit parameters considered by analog signals are completely different from digital signals. The analog signal usually needs to be considered first:

•Accuracy or linearity

•Frequency response

• Noise considerations

Power requirements, especially for the input stage, should also pay attention to the basic accuracy or linearity of the isolation amplifier cannot be improved by the corresponding application circuits, but these circuits can reduce noise and reduce the power requirements of the input stage.

Burr-Brown’s ISO124 simplifies analog isolation. The input signal is duty cycle modulated and sent digitally across the barrier. The output part receives the modulated signal, converts it back to analog voltage and removes the inherent ripple component in the modulation/demodulation process. Due to the modulation and demodulation of the input signal, some limitations of the sampling data system should be followed. The modulator works at a fundamental frequency of 500kHz, so input signals higher than 250kHz Ngquist frequency present lower frequency components in the output.

Although the output stage removes most of the carrier frequency in the output signal, there is still a certain amount of carrier signal. Figure 4 shows a combined filtering method to reduce high-frequency noise pollution in the rest of the system. The power supply filter can significantly reduce the noise entering from the power supply pin. The output filter is a two-pole Sallen-key stage with a Q of I and a 3dB frequency of 50kHz. This reduces the output ripple by 5 times.

Another issue with isolation voltage is the power required by the input stage. The output stage is usually based on the chassis or ground, and the input is usually floating on another potential. Therefore, the power supply of the input stage must also be isolated. Usually a single power supply is used instead of the ideal +15V and -15V power supplies.

Figure 5 shows that a single voltage power supply in the ISO124 input stage combined with a 1NA2132 dual differential amplifier can boost the swing to the full range of the input signal level. The only requirement is that the input power supply voltage remains greater than 9V, which is required for the ISO124 input voltage.

The lower half of the INA2132 produces a half of the output voltage of a VS+ power supply. This voltage is used as a pseudo ground for the REF pin of the other half of INA2132 and the GND input of ISO124. The swing of the differential input signal of the INA2132 can be higher or lower than the new reference level. The output of ISO124, like the input, will be completely bipolar.

3. Isolated parallel data bus system

The isolation of parallel digital data buses will increase three more important design parameters:

• The bit width of the bus

• Allowable deviation

• Clock speed requirements

This task can be accomplished with a row of optocouplers, but the supporting circuits may be very complex. The propagation time mismatch between the optocouplers will cause data offset, which will cause data errors at the receiving end. To minimize this problem, the ISO508 isolated digital coupler (Figure 3) supports double-buffered data buffering at the input and output. This configuration will transmit data at a rate of 2MBps.

ISO508 has two working modes. When the CONT pin is set to a low state, under the control of the LE1 signal, data is transmitted across the barrier in a synchronous mode. When LE1 is in the high state, data is transferred from the input pin to the input latch. When LE1 goes low, data bytes begin to travel across the barrier. At this time, the input pin can be used for next-generation data bytes. In this mode, the transferable data rate can reach 2MBps.

When the CONT pin is set to a high state, the data is sent across the barrier under the control of the internal 20MHz clock of the device. Data transmission is asynchronous to the external latch enable signal. Data is strobed from input latch to output latch in serial form. After a byte is transferred, the entire byte is moved into the output latch, and the output latch will offset the transferred data byte. For a complete 8-bit byte, the propagation delay will be less than 1ms.

4. Multifunctional IC for isolation

The new multi-functional data acquisition IC gives designers the opportunity to complete multiple tasks while crossing the isolation screen. A complete data acquisition device can include multiple analog switches, programmable gain instrumentation amplifiers, A/D converters, and one or more digital I/O channels. All these functions are controlled through a serial data port. Burr-Brown's ADS7870 is such a device. ADS7870 works very well with ISO150 and is shown in Figure 6.

In this application, each programmable function of ADS7870 is placed under the control of the main microprocessor, and the control of the microprocessor itself is realized by writing commands to the register through the serial communication port. Control features include:

• Choice of multiplexer

• 4 differential channels or 8 single-ended channels

• Programmable gain setting of instrumentation amplifier, 1～20

• Initialization of 12-bit A/D conversion

The 4 digital I/O lines of this device are also useful and can be individually specified to report the status of digital signals or output digital signals. This allows isolation of certain support functions, such as level or error flag readout via the same ISO150 extension signal multiplexer.

Concluding remarks

There are many devices available for designers to choose and use in designs where the ground potential of the system is very different. Each device is designed for unique system requirements. The high level of performance integration of the new devices enables more complex operations that were previously impossible to achieve across the isolation barrier.