Isolation is a means of preventing current from flowing between two communicating points. Typically, isolation is used in two situations. The first is where there's the potential for current surges that may damage equipment or harm humans. The second is where interconnections involve different ground potentials and disruptive ground loops must be avoided. In both cases, isolation is used to prevent current flow, yet allow for data or power flow between the two points.
Recent changes in legislation with regard to both the design and use of machinery and equipment require the isolation of almost any type of data-acquisition system in harsh environments. In addition, the trend from historic single-channel isolated systems to applications utilizing multichannel isolation led to the introduction of new isolation strategies. These applications involve high-voltage, high-speed/high-precision communications, or communication over large distances. Common examples include industrial I/O systems, sensor interfaces, power-supply/regulation stems, motor-control/drive systems, and instrumentation.
This article gives an overview of previous isolation methods and their components, then continues with the principles of operation of digital isolators and their applications in multichannel data-acquisition systems.
Early Isolation Techniques In addition to the use of transformers, early designs utilized analog isolation amplifiers (iso amps) to isolate the sensor circuitry on the factory floor from the signal-processing system in the control room. Some of these amplifiers are still in use today in applications with limited channel count and small signal bandwidth. Figure 1 illustrates this kind of isolation in a single-channel temperature measurement.
These isolation amplifiers were precision amplifiers incorporating a novel duty-cycle modulation-demodulation technique to digitally transmit the input signal across a differential capacitive barrier (Fig. 2). With digital modulation, the barrier characteristics don't affect signal integrity, resulting in excellent reliability and good high-frequency transient-immunity across the barrier.
Referring to Figure 2, the input amplifier A1 integrates the difference between the input current, (VIN/RIN), and a switched current source. The integrator ramps in one direction until it exceeds the comparator threshold. The comparator and sense amplifier AS1 force the current source to switch. The resulting signal is a triangular waveform with a 50% duty cycle. The internal oscillator forces the current source to switch at high frequency (i.e., 500 kHz). The resultant capacitor drive is a complementary, duty-cycle-modulated square wave.
At the same time, sense amplifier AS2 detects the signal transitions across the capacitive barrier and drives a switched current source into integrator A2. The output stage balances the duty-cycle-modulated current against the current through the feedback resistor RF, resulting in an average value at the VOUT pin equal to VIN. The sample-and-hold amplifiers in the output feedback loop remove undesired voltage ripples inherent to the demodulation process.
Despite their high reliability and precision, isolation amplifiers were limited in signal bandwidth to 50 kHz. Their older technology requiring a minimum supply of ±4 V doesn't support today's low-voltage applications of 3 V and below. Also, their manufacturing process - which includes the separate fabrication of the input and output sections, laser trimming for exceptional circuitry matching, and mounting both sections with isolating capacitors in between - made these devices rather expensive.
Multichannel Isolation Many data-acquisition systems in industrial automation use multi-input channel analog-to-digital converters (ADC) to capture the input data (measurands) of multiple analog inputs (Fig. 3). Most delta-sigma ADCs feature serial interfaces to reduce package size and board space. The complexity of serial interfaces varies in the number of slow-speed control signals required, such as chip-select, power-down, gain- and speed settings, and multiplexer addressing. Common to all serial interfaces, however, are the high-speed transmission lines for the clock signal and the output data (conversion results).
Because signal capture and conditioning occur within the ADC, the best-suited location to isolate the sensor circuit from the signal-processing circuitry is at the digital interface using digital isolators. As mentioned before, due to interface complexity, the isolators must be able to transmit high-speed ADC conversion results, as well as low-speed control data. The next section explains the internal operation of a digital isolator showing how these devices are capable of high- and low-speed data transmission.
Digital Isolator The isolator in Figure 4 is based on a capacitive isolation barrier technique. The device consists of two data channels, a high-frequency channel (HF) with a bandwidth from 100 kHz up to 150 MHz, and a low-frequency channel (LF) covering the range from 100 kHz down to dc.
In principle, a single-ended input signal entering the HF-channel is split into a differential signal via the inverter gate at the input. The following capacitor-resistor networks differentiate the signal into transients, which are then converted into differential pulses by two comparators. The comparator outputs drive a NOR-gate flip-flop whose output feeds an output multiplexer. A decision logic (DCL) at the driving output of the flip-flop measures the durations between signal transients. If the duration between two consecutive transients exceeds a certain time limit (as in the case of a low-frequency signal), the DCL forces the output multiplexer to switch from the high- to the low-frequency channel.
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