Because low-frequency input signals require the internal capacitors to assume prohibitively large values, these signals are pulse-width modulated with the carrier frequency of an internal oscillator, thus creating a high-frequency signal that can pass through the capacitive barrier. As the input is modulated, a low-pass filter (LPF) is needed to remove the high-frequency carrier from the actual data before passing it on to the output multiplexer. Figures 5 and 6 present the high- and low-frequency channels and representative waveforms.
HF-Channel Operations
The single-ended input signal is split into the differential signal components A and /A. Each signal component is then differentiated into the transients B and /B. The following comparators compare the differential transients to another. As long as the positive input of a comparator is on higher potential than its negative input, the comparator output will present a logical high, thus converting an input transient into a short output pulse.
The output pulses set and reset a NOR-gate flip-flop. From the truth table, we see that the NOR-gate configuration presents an inverting flip-flop, meaning that a high at input C sets output /D to high, and a high at /C sets D to high. Because the comparator output pulses are of short duration, there will be times where both outputs are low. During this time, the flip-flop stores its previous output condition. Since the signal at /D is identical in shape and phase with the input signal, /D becomes the output of the high-speed channel and is connected to the output multiplexer.
LF-Channel Operation
Slow input signals are pulse-width modulated with a high-frequency carrier so that a signal high yields a 90:10 duty cycle and a low yields a 10:90 duty cycle at location A. From there on, signal processing is identical with asymmetrical signal processing in the high-speed channel. The only exception is that the high-frequency content of the low-speed channel (/D) is filtered by an R-C low-pass before being passed on to the output multiplexer (E).
The successful proof of concept through the single isolator's capability of transmitting wideband data (from dc to above 100 MHz) inspired isolator manufacturers to fabricate unidirectional and bidirectional devices in dual-, triple- and quad versions. These accommodate the most common digital interfaces encountered in industrial applications.
Applications
When isolating industrial interfaces, we need to distinguish between process control and factory automation applications. That's because their differences will impact the isolation efforts of the digital interface design.
Process control typically involves the detection of various physical quantities, (i.e., pressure AND temperature) of some equipment, system, or process. Each physical quantity uses a specific type of sensor or transducer whose output signal requires specific signal conditioning. Consequently, a variety of different sensors requires different parametric settings, such as in-gain, sampling rate, measurement repetition, or impedance buffering. ADCs supporting a wide range of settings provide multiple interface control lines, all of which require isolation in addition to the standard serial interface lines.
In Figure 7, a number of sensors with different sensitivities (mV/K) measure different process parameters, i.e., temperature, pressure, and current. Various gain settings are required to maximize the input dynamic range of the ADC for each sensor. A possible switch between sampling rates (speed) might be required if one or more channels are expected to show faster input variations than others. The power-down features, used to save power consumption after measuring, allow the controller to perform other system functions. This high versatility requires many control channels to be isolated via two quad isolators.
In contrast to process control, factory automation is usually concerned with the monitoring of a single physical quantity (i.e., temperature OR pressure) of multiple devices or equipment. These systems, therefore, employ multiple sensors of the same type, exhibiting uniform characteristics in sensitivity and response time.
Figure 8 presents such a circuit using four thermocouples of the same type for temperature measurements of different equipment. This application uses the same ADC as the circuit in Figure 7. Due to the uniform sensor characteristics, however, the settings for gain and sample rate are fixed by connecting the associated control pins, (Gain1, Gain2, and Speed), to the appropriate supply rails (VDD or GND). Many autonomous systems in factory automation measure their inputs continuously, which necessitates the connection of the /PWDN-pin to the positive supply rail.
This system configuration drastically simplifies the interface down to the isolation of the data, clock, and address lines. Thus, it requires only one 3:1 quad isolator.
In the previous examples, interface isolation occurred between the ADC and the system controller. This approach works well for input modules with channel counts requiring only one or a maximum of two ADCs per module. Above that, isolating each data converter becomes uneconomical. Hence, implementation of a local controller is recommended. In such a case, each ADC communicates with the local controller via a GPIO bus interface. The actual isolation, however, occurs at the local-to-system controller interface.
In conclusion, it's safe to say that isolation amplifiers are out - and digital isolators are in. Understand your system requirements before deciding what type of isolator to use, and where to place it in the system.
For more information on Texas Instruments interface solutions, visit www.ti.com/interface.