Signal chain basics: Isolating analogue signals

The growing trend of implementing galvanic isolation into industrial interfaces mainly focuses on digital signals. Applications like LED lighting, brushless motors, power monitoring and many other direct offline power systems require isolated control electronics that utilise analogue voltage control for speed and intensity adjustments.

Historically, high-precision isolation amplifiers have been used to accommodate this task. However, their price range has become prohibitive for high-volume applications. In comparison, a digital isolator is significantly lower in cost but necessitates the signal conversion through an accompanying analogue-to-digital converter (ADC). Data converters can never rely on a single channel isolator. Instead, they require multiple isolation channels in order to isolate data, control, and address lines. This of course increases isolator cost and becomes counteractive for reducing the overall system cost.

In order to circumvent the design-versus-cost dilemma, this article suggests a cost-optimised solution for analogue isolation using a class-D amplifier in combination with a single-channel digital isolator (figure 1).

Here the class-D amplifier uses as a low-cost analogue-to-pulse-width modulation (PWM) converter, has a 20kHz bandwidth and allows for ac- and dc-coupling. While the device provides differential inputs and outputs, the conversion to single-ended mode is accomplished simply by biasing one input with a reference voltage that is half the device supply, and using the other one as signal input. Consequently, only one output is used as PWM source for the subsequent isolator.

Simplified, the amplifier's internal PWM stage comprises a 250kHz triangle waveform generator whose output is compared with the analogue input signal. When the analogue input is greater than the triangle voltage, the non-inverting output is high. When the input is lower than the triangle voltage, the non-inverting output is low. Figure 2 shows that the time span of the input signal exceeding the triangle voltage, determines the pulse-width of the PWM output.

The amplifier outputs a 50 per cent duty cycle when the analogue input at IN+ equals the reference voltage at IN-. The duty cycle is greater than 50 per cent for VAIN > VREF, and less than 50 per cent for VAIN

The digital isolator is simply a logic buffer with a capacitive isolation barrier. It separates the analogue input stage from a second grounding system to prevent the flow of ground-loop currents due to ground potential differences. Because the isolator has a propagation delay of only 20 ns and supports data rates of up to 150Mbit/s, it is transparent to the PWM signal path.

In order to retrieve the analogue signal from the PWM stream, a simple R-C low-pass filter is applied to the isolator output that filters the PWM carrier. Figure 3 shows the final isolator circuit with the necessary isolated power supply, and figure 4 shows the associated input and output waveforms.

References
1. ISO721EVM Users Guide (SLLU091), Texas Instruments, January 2006.
2. TPA2006D1 Audio Power Amplifier EVM Users Guide (SLOU187), Texas Instruments, October 2006.

About the author

Thomas Kugelstadt is a senior systems engineer with Texas Instruments. He is responsible for defining new, high-performance analogue products and developing complete system solutions for industrial interfaces with robust transient protection. Kugelstadt is a Graduate Engineer from the Frankfurt University of Applied Science.

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