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Signal Chain Basics #72: Isolating analog signals using a digital isolator

The rising 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 utilize analog 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 analog-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-optimized solution for analog isolation using a class-D amplifier in combination with a single-channel digital isolator (Figure 1 ).


Click on image to enlarge.

Figure 1. Signal-chain of a low-cost analog isolator

Here the class-D amplifier uses as a low-cost analog-to-PWM (pulse-width modulation) converter, has a 20 kHz 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 250 kHz triangle waveform generator whose output is compared with the analog input signal. When the analog 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 percent duty cycle when the analog input at IN+ equals the reference voltage at IN-. The duty cycle is greater than 50 percent for VAIN > VREF, and less than 50 percent for VAIN < VREF.


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Figure 2. PWM stage with input and output waveforms

The digital isolator is simply a logic buffer with a capacitive isolation barrier. It separates the analog 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 150 Mbps, it is transparent to the PWM signal path.

In order to retrieve the analog 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.


Click on image to enlarge.

Figure 3. Analog isolator design with isolated power supply


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Figure 4. Analog isolator 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.

Please join us next month when we will discuss the basics of audio metering .

About the author
Thomas Kugelstadt is a senior systems engineer with Texas Instruments. He is responsible for defining new, high-performance analog 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. He can be reached at ti_thomaskugelstadt@list.ti.com .

Past entries for Signal Chain Basics series is here.

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