Separating Signal from Power: New Automotive Inductors from TDK

Inductors such as the ADL3225V devices are useful when you want to use just one conductor to transfer both signal and power.

I still find myself somewhat intrigued by the way in which electrical signals of different frequency can peacefully coexist. There is something counterintuitive about this characteristic of the physical world, at least until university courses have immersed us in the Fourier transform and compelled us to concede that the frequency domain is just as legitimate and real as the time domain.

But as you surely know, we’re dealing with more than just an intellectual curiosity here. This property of electrical (and wireless) systems is exceedingly useful: it allows us to transmit multiple distinct signals using only one wire. We simply have to ensure that the frequency components of the various signals don’t overlap—this is readily accomplished via modulation techniques—and then the receiver can extract the signals using filters.

Two signals of different frequency can share the same physical interconnection.

The generic name for this technique is multiplexing, and I usually think of multiplexing in the context of data transfer. But there is no reason why we can’t apply the same logic to the transfer of power. Imagine we have a control PCB and a remote device of some kind. The control PCB provides the remote device’s supply voltage, and the remote device must send an analog signal back to the control PCB. Why use two separate conductors when one would suffice?

PoC (Power over Coax)

This is the idea behind PoC (Power over Coax). It may seem like overkill to complicate a circuit simply to eliminate one little conductor, but apparently this is a significant concern for automotive engineers.
The specific issue at hand is the situation in which a camera is located in a rather inconvenient location, such as the rear bumper or thereabouts. The cable between this camera and the control unit (presumably located toward the front of the vehicle) represents cost and weight that can be reduced by eliminating unnecessary interconnects. The PoC approach allows one coaxial cable to carry power (from control unit to camera) and the video signal (from camera to control unit).

The Inductor

A fundamental component in this scheme is an inductor. We have a (very)-low-frequency supply voltage and a rather-high-frequency video signal sharing the same wire. As you know, inductors have high impedance at high frequency and low impedance at low frequency. This means that an inductor will allow the power component to pass freely while presenting significant resistance to the video signal.

Frequency Response

Of course, the details are never so simple. First, we need to ensure that the inductor has adequate impedance at the frequencies of interest. You’ll have to explore the part’s datasheet to be sure, but an easy way to start is to look for a device that is specifically recommended for your application. In this case, we have the ADL3225V series, which is indeed recommended specifically for automotive PoC systems. Here’s a plot of impedance vs. frequency:

Plot taken from the ADL3225V datasheet.

DC Current

In typical analog filter applications, we often don’t have to worry about DC current and how it might affect an inductor. But PoC is a different story. Any power-supply current required by the remote device must pass through the inductor, and consequently we need to be aware of how that current might affect the circuit.
First of all, we need to check the inductor’s DC current rating. If your bumper-mounted device needs 100 mA and your 0402 inductor is rated for 50 mA—well, you might have reliability issues.
But there’s another more subtle thing to consider—the inductor’s “DC bias” situation affects its frequency response. Take a look at this next plot for the ADL3225V devices:

Plot taken from the ADL3225V datasheet.

What you see here is that higher DC current can cause the inductor’s impedance to decrease. In this particular case, the effect is nontrivial at 2 MHz, but almost nonexistent at 700 MHz.
If you want details on PoC design, I recommend that you consult this app note from Texas Instruments. One helpful tip I noticed is the following: What is a designer to do if the remote device needs high current, but an inductor with adequate current rating would be too large? Simple—increase the supply voltage (which in turn will reduce the current), and if necessary include a high-efficiency buck regulator to restore the lower supply voltage for the remote device.

Derate!

The ADL3225V series is qualified to AEC-Q200, so it has demonstrated an ability to survive the rather unfriendly automotive environment. It has an operating temperature range that extends from –40°C to +105°C.
However, that doesn’t mean that it exhibits the same performance under all conditions! DC current characteristics are particularly important in the context of PoC, and the ADL3225V’s DC current rating starts to decrease at 75°C.

Plot taken from the ADL3225V datasheet.

---

Do you have any experience with PoC? Feel free to share your knowledge in the comments.