In the previous part of this series, we started looking at some methods for doing signal isolation that have not been discussed (or not discussed very much) in other blogs or technical resources. We started looking at magnetic coupling techniques. Let's continue.
A variation of the magnetic approach is the Hall effect transducer, which is normally used to measure current. These devices are typically relatively inaccurate and suffer from thermal drift, but some will work with both DC and AC (others are limited to AC). In addition, they don't need power on the primary side.
Honeywell makes a number of such products, but they mostly require that the current carrying conductor is a wire that you feed through the device's aperture. LEM has a range of similar products. The best of the bunch is from Allegro. You can actually bring the current on the PCB track through a SOIC and handle up to 30 A. I have used the ACS712 extensively, and the device works well.
Digital isolation is cheap, so if you can find a technique to convert the analog input to digital, this becomes a viable approach. It is possible simply to use an ADC or even a second microcomputer to do the analog conversion and transmit it across the isolation barrier. The number of isolators needed depends on the data transfer method.
If you use a micro, you could communicate asynchronously, and with SPI, you may need bidirectional communications. I2 C may be a little more complicated in that the two lines are bidirectional, but that shouldn't be too much of an obstacle. NXP has an app note on just this, as does TI. Linear Tech produces a module for this eventuality.
Several devices now include a number of isolation channels to facilitate data coming and going. Examples include NVE's IsoLoop products and the ADuM family from Analog Devices. And if you are already committed to communicating, Maxim, ADI, and Linear Tech make isolated communications drivers/receivers for RS232 and RS485 and sometimes even provide power for the isolated side of the interface.
An older technique (from before digital protocols invaded everything) is the use of a voltage to frequency converter (VFC) on the input and a frequency to voltage Converter (FVC) on the other side of the barrier. Of course, the frequency can be fed directly into a micro and measured there, replacing the FVC. This setup needs only one isolator, but obviously the time it takes to measure the frequency impacts the conversion time.
A more modern variation is to use a PWM signal to pass the information. This works well as long as the rise and fall times of the edge are sharp enough — or at least small in comparison to the mark/space times — in order to minimize the error. Also, if you are converting back to analog, the low pass filter will place a limitation on response times. (See: Signal Chain Basics #72: Isolating analog signals using a digital isolator.)
We will finish this series with a quick look at the use of a delta-sigma ADC to facilitate the passing of analog signals across a galvanic isolation barrier. Then we will take a look at analog opto-isolators.