Isolating and filtering: Galvanically isolating USB and analog devices and filtering signals | Part 2 of 2

A must for audio freaks and ambitious gamers: galvanic USB isolation

The so-called galvanic isolation or USB isolation is a concept in which two or more circuit parts are electrically isolated to prevent the unwanted flow of current between them. This is usually achieved using transformers, optocouplers or capacitive coupling. The aim is to protect sensitive electronic devices from voltage spikes, earthing problems and other forms of electrical interference. So much for the theory. But what does this mean in practice?

PCs and other electronic devices, for example, can be damaged by sudden voltage spikes caused by external sources such as lightning strikes or switching operations in the power grid. Galvanic isolation prevents such spikes from reaching the connected devices. This doesn’t sound particularly relevant at first, but it also applies to static charges. I myself have already lost a mainboard because I accidentally discharged the USB.

The so-called ground loops, which we already discussed yesterday within the PC, are not entirely insignificant. Particularly with audio DACs and other peripheral devices that are supplied with power directly via the USB, the interference and rapidly pulsating currents via the ground lines unfortunately play an all too often “audible” role. But it also goes “outside the box” when two devices have different ground potentials, creating an unwanted current flow known as a ground loop. This can lead to data loss, malfunctions or even damage to the devices. Galvanic isolation eliminates this risk by preventing a direct electrical connection and also “smoothing” the power supply branch.

But it also serves to reduce electromagnetic interference (EMI), which can interfere with data transmission in electronic systems and lead to data loss or corruption. USB isolators help to protect transmission paths from such interference by providing an insulating barrier that effectively prevents the transmission of EMI, e.g. from graphics cards. So-called USB isolators, which are available in various price ranges, address all these problems. However, in addition to the undeniable advantages, there are also some disadvantages that you need to be aware of.

How USB isolators work

USB isolators typically use galvanic isolation techniques to achieve complete electrical isolation between the USB host (e.g. a PC) and the connected USB device. This isolation is often achieved through transformer coupling (for the data lines) and optical coupling (for the signal lines), with both methods effectively preventing electrical current from flowing directly between the devices. Transformer coupling is used to isolate the data lines. A transformer transmits signals between primary and secondary windings by magnetic induction, allowing physical separation between the input and output signals without affecting data transmission.

 

Optocouplers (or optical isolators) are used to completely isolate signal lines from each other electrically by means of optical coupling. They transmit signals using light that is emitted by an LED (Light Emitting Diode) and received by a photodetector. This method enables signal transmission via a light barrier without a direct electrical connection. By combining these two isolation techniques, USB isolators enable secure communication between devices without the risk of electrical interference, ground loops or EMI damaging data integrity or hardware. Importantly for us, some USB isolators can also provide an isolated power supply to the connected device, which further increases protection against harmful electrical interference and can also significantly reduce noise from the power supply.

Disadvantages of USB isolators and a workaround

Despite their undisputed usefulness, commercially available USB isolators can have certain disadvantages, such as not recognizing certain USB devices or a reduced current carrying capacity on the 5-volt line. These problems can be caused by the way the isolators work and the requirements of the connected devices. We already know that USB isolators electrically isolate the data and power circuit between the USB device and the host computer and how this is done. But while this isolation provides protection, it can also affect transmission performance, especially for devices that require a higher power supply or fast data transfer rates.

Many USB isolators limit the current that can flow to the USB device to values below the USB specification of up to 500 mA (USB 2.0) or 900 mA (USB 3.0) per port. This limitation can result in devices that require higher power not functioning properly or not functioning at all, or not being recognized as such in the first place, which is probably the most common reason for failure of the intended USB isolation in the case of USB DACs, for example. If you read through the reviews and comments on such products at online retailers, for example, you quickly realize that most buyers have zero understanding of this issue. This is a failure with an announcement, unfortunately.

But there are also other interferences and negative influences that I would at least like to mention. This type of isolation can also affect the data transfer rate. Many (especially cheaper) isolators do not support the full transfer speeds offered by USB 3.0 or higher standards, resulting in slower data transfer. The one I have tested here easily fulfills USB 2.0, which is easily enough for PC audio, the mouse or a keyboard. I would no longer connect external hard disks or even monitors. That would definitely go wrong. Certain USB devices, especially those with special drivers or protocols, can also be incompatible with the isolator because the isolation interferes with the necessary direct communication between the device and the host computer. This also applies to USB dongles if there are also security issues relating to device IDs and addresses. However, this is a bit much and also takes us away from the actual topic of today’s article.

Solution approaches

Overcoming these disadvantages requires a multi-layered approach, with the external power supply being the most important and simplest solution. This is because for devices that require more power than the isolator can supply as a bridge, an external power supply can provide a remedy. This solution can be realized either by a dedicated power supply for the USB device or by a USB hub with its own power supply. This is exactly what I have done and use the USB isolator between the PC and a USB hub. The picture below is of course purely exemplary, because the USB isolators I use are invisible at the back of the PC. But you wouldn’t see anything there. The advantage of the active USB hub shown here with its own power supply is that all seven outputs are electrically isolated from the PC and you only need one isolator.

Of course, some manufacturers also offer USB isolators that are designed for higher current carrying capacities or data transfer rates and often even have a separate power supply connection. However, this is of course a question of cost, as an inexpensive USB isolator and an active USB hub are usually significantly cheaper in total. Choosing an isolator that has been specially developed for the requirements of the connected device can avoid many problems from the outset.

Signal amplification is still an issue with long cables (keyword printer connection), because in situations where the data transfer rate is a problem, the use of USB repeater cables or amplifiers can help to maintain signal integrity over longer distances. However, this is not the subject of today’s article. Nevertheless, I wanted to mention this at least once.

Interim conclusion

Effectively circumventing the drawbacks of USB isolators requires an understanding of both the technical challenges and the specific requirements of USB devices. By selecting suitable isolators, adapting the power supply and considering data transfer rates, as well as potentially adapting hardware and software solutions, many of these problems can be solved. In some cases, collaboration with experts or the development of specific solutions may be required to achieve optimal results. Or simply read articles like today’s.