THS4551: TINA model open loop differential output impedance shift in model update

Hello Michael,

Not sure what is going on for the phase margin of the MFB filters, but in regards to the Zo plot of the models:

The lower THS4551 ouput impedance of below 1-ohm is due to the addition of the 600fF feedback caps. While it may be true that this is needed for the pre-2019 model, the internal feedback capacitance isn't added externally in the GWL version but rather as a part of the curves. You can see the difference in the curves below:

To avoid the issue of having to worry about the capacitance with either model, you can bias the input pins to ground as shown below:

Best,

Hasan Babiker

Not seeing any plots and note that external C is -0.6pF

Hey Michael,

Sorry about that, plots didn't come through correctly when I had initially posted but should be fixed now. Just checked and the same effect occurs with -0.6pF caps.

Best,

Hasan Babiker

You are correct Hasan, overlaying those Zol plots, this is what I actually recall seeing some weeks back - the new model tracks nicely midrange but has much lower output resonance up at higher F - that would be a big phase difference driving the feedback C's.

That sharper resonance comes from using inductors in the Zol lumped element model, I could not find that for the THS4551, but here is THS4561, 

Here is the test of TINA model to Cadence Zol impedance mag/phase - pretty good I would say, I don't think the GWL model uses inductors, RC only. Red is the TINA model, blue cadence full transistor. 

And then here is the Zol phase forTHS4551  original vs 2019 update, that steeper falloff around 150Mhz may be my issue. And actually it may not be, might just be the Aol curve is different. 

Hello Michael,

Probably a mixture of both, the older model has a steeper high frequency fall-off for Aol phase as well.

Hasan Babiker

Before I close these files down again, here was some model testing on the original THS4551 model for Aol gain and phase. The 100ohm loaded tests are of course bringing in the Zol effects, left is zoomed in where the tuning work was going on, right is broadband match

Phase,