Hello, this is Michael Steffes, the Senior Applications Manager for Intersil in the high-speed signal path products. And today we're going to continue our discussion on how to get single to differential conversion in a couple different techniques for your circuits.
Showing on the screen behind me is a very capable technique of using an input balun and a fully-differential amplifier, which has been a very useful technique over the past decade. We're going to move forward from this, of course, to show an active balun approach using the ISL55210. But it's this circuit that you would get on the ISL55210 evaluation board available from Intersil's website or your favorite distributor.
This circuit comes with an input transformer and then an output transformer, really just for measurement purposes. On the input side, we use a range of transformers or baluns to get from single to differential into a termination impedance being set here by the gain resistors. So this is a very nice circuit and it certainly works very well as a low-power, single differential circuit. Once we're balanced on the opposite side, this circuit does have some advantages in terms of second harmonic suppression.
Now, if we move on to this new technique enabled by the ISL55210, we eliminate both the balun, and in this case, we eliminate the resistor to ground that you'll see in everybody else's fully-differential amplifier datasheets or application notes. And we're going to try to do our input match and gain setting just with the resistors we have available in the circuit. If we do that and run a sweep of gains, we can step the gain up from 14 dB to 34 dB, and see what happens to the resistor values.
One of the really surprising things for instance, at a gain of 20, let's just look at these values, is the input resistor that we end up driving is an extremely low value, 4.57 ohms. But the circuit will appear as if it's giving you a 50 ohm input match through very broad frequencies. A couple things come out of that, what we're seeing here is that the input match is largely being set by the action of the common-mode loop because that's what's giving us our active termination characteristic. And to hold that match through extremely high frequencies implies you need a very broadband common-mode loop which is one of the unique characteristics of the ISL55210 is it has a common-mode loop band with an exceeding 1.5 GHz, whereas, all other FDAs today have a much lower bandwidth on the common-mode loop.
As we look at these extremely low resistor values, we would expect the noise, or the input-referred noise figure, to step down considerably using these 4.5 ohm kind of input resistors in the active balun circuit. You can go to another Intersil video to see those test results.