This application note is a simplified version of an earlier global-ground-or-differential-ground appnote,
with a special focus on SMD port calibration in ADS Momentum, and how to use it properly.

It is important to understand that SMD port calibration in Momentum is only applicable to pure series elements, and there must be no other path to global ground or other circuit parts. The current at the (+) terminal of an SMD port is, by definition, exactly the same as the current at the (-) terminal.

Sometimes we see configurations like this

and users wonder why SMD port calibration results are very different from other calibration types. The answer is very simple: with SMD port cal we have a “floating port”  configuration with i(P1)=-i(P2) and all the other circuitry connected there has no effect.

This is not a valid configuration for SMD port calibration!

To connect something else than a series element, this is the wrong port type. We will show this with a simple testcase, a line with SMD port in the middle.

If you look closely, you will notice the hint/warning at the SMD port. Moving the mouse over the warning symbol, you will see this warning text:

If we ignore this information/warning, what does that mean to our circuitry attached to the SMD port? Let’s check that in an extreme way: we try to short the SMD ports to schematic ground.

In the schematic testbench, we compare the lines with SMD port calibration (top, ports 1 and 2) to the same layout with zero length TRL calibration (bottom, ports 3 and 4).

As already explained above, the configuration with with SMD port calibration is invalid here, and we get wrong results: the schematic ground connection has no effect, because the SMD port defines purely series current flow, with no path to any other ground. Wrong setup leads to wrong results.

For comparison, the configuration below with zero length TML port shows the expected short circuit behaviour, because these ports have a path to global ground.

We can also visualize the “problem” with the EM model preview: purely series for the SMD port configuration. No access to global ground, only useful for series connections at that port.

So please, don’t just ignore the information/warning shown for the SMD port calibration. SMD port calibration requires purely differential, floating ports. If you need to connect anything else than a series element, you need to use another port type that has global ground reference.