Hello, this is Michael Steffes, Senior Applications Manager for High-Speed Signal Path at Renesas. Today we'll continue our discussion on this active balun approach of using a wide band for differential amplifier, the ISL55210.
And so what we are going to do now is we are going to look at detail at the frequency response and the measured noise figure for a couple of configurations of this very flexible approach to getting a input impedence match with broadband single to differential gain with no actual magnetics in the circuit.
So what we show on the screen here is the configuration of the circuit as the board would be delivered of our active balun evaluation board, would be delivered in this configuration on the screen. We actually have three different output interface options. Here we are showing a transmission line balun, which actually gives quite a bit broader frequency response flatness then a more typical flux couple balun but that option is also available on the board.
So if we look at this circuit, we configured it for a 50ohm input match. We've got a 16.4dB gain to the output pans. And we're going to take a 6dB loss in the matching and we are estimating about a .2dB to .4dB insertion loss in the balun. Now that’s not modeled here but we will see it on the measurement of the board.
So let’s run the simulation and we will measure the output all the way through to the final 50ohm load. We should see a response shape that looks something like this. It does simulate at about 10.4dB. Let’s look closely at where we would see about a .5dB roll off from 10.4. So that is going to be 9.9. We should expect to see a .5dB flatness with the higher frequencies, up through about 470MHz is what the simulations would predict. So this is what we would expect to see.
Let’s go over to the network analyzer and see what we really get. We've got our evaluation board configured to measure the response shape. The marker is currently at 10MHz and we see about a 10.2dB measured gain to the max load. And let’s go up in frequency and just see where we start to see a deviation and that is going to happen at about 470MHz, very much like our simulations.
Extremely capable circuit. Let’s go back now and look at our noise performance for this type of circuit. We will go to a little higher gain to check the noise. And if we do that we will use this table and we will see that we need about a 4.60ohm input resister to get a 50ohm match. We'll need about a 543ohm feedback resister on each side. And we should be getting about 450MHz bandwidth in that configuration. So if we configure that active balun board and with those resister values, we can go make a measurement of the input noise figure for a couple different configurations. This green curve we are showing on this comparison plot was actually in the standard configuration with a resister to ground, 110ohm that typically you would see in these kind of circuits. And we can see, it gives about a 10dB noise figure when you get up above the 50MHz region.
If we now do the same input impedance and the same gain setting, but eliminate the resister ground. Completely depending on the current mode loop band to get our match. We'll see the noise figure indeed drop significantly. Down into the 7dB noise figure region. That correlates to approximately a .9nV input preferred noise. And since the ISL55210 has already a .85nV device, this essentially shows that the resister noise contribution using this active balun approach are extremely low. We've essentially taken the resister noise out of the performance of the circuit.
If you like to try these circuits for yourself and measure these noise in different configurations, go on the Renesas website and order the active balun evaluation board, where there is also an application note describing the board.