Hi all,


I am trying to use s-parameter data in an amplifier design. However, I have encountered a problem. I have attached an image below. The circuit that i'm trying to model with s-parameters is inside the red box. Now suppose I have two different circuits as shown. Both circuits will have the same s-parameters. However, the absolute voltages V1 and V2 wrt ground are different for the two circuits.

I have noticed that in ADS, the circuit is always interpreted as in the second figure, while in Cadence V1 and V2 can be floating (more like the first figure). However, neither interpretation is necessarily correct. So how can I resolve this problem?


thanks,
Aaron

Hi aarron,
i guess you have S-parameters data set, which you are trying to re simulate as a black-box. If so you should use grounded ports means two ports should have 50ohm wrt ground. I didn't understand I have noticed that in ADS, the circuit is always interpreted as in the second figure, while in Cadence V1 and V2 can be floating. According to me both are wrong as you don't have gnd on both sides.

As matter of fact, if you don't have 50ohm wrt gnd then there will some capacitor between node v2 and gnd which alters the results.

Hi Raja.Cedt,


why do you say I "must" have grounded ports. So you are saying all of my s-parameter data must be referenced to the same point (ground)?

In ADS, the s-parameter element only has one reference port, while in cadence, each port has its own reference.


thanks,
Aaron

Interesting ...

The original question is very general:

The two simulators express the ports on an n-port device differently (Cadence nport has +/- terminals per port and ADS only has one terminal per port plus a global reference), so what is the difference (if any) when  each simulator is using the same s-parameter file description?

As you mention, two-ports are general so there is no need to define a global ground to reference the ports to. In the old days, s-parameters were explicitly ground referenced because most circuits and network analyzer measurements were single ended. But now days there are many differential devices where the differential mode and common mode responses are generally different.

So, my question is what is the appropriate way to accurately represent both the common mode and differential mode responses of a multi-port network?

Hi RFICDUDE,


I think for differential circuits, a differential port needs to be represented by two ports, one for the positive terminal and one for the negative, and each one referenced to ground.

The question is then, where is the ground reference in your EM model? In the schematic simulation, all reference ports would be tied to the same point, so ideally in your EM model, you also need to make sure all ports are referenced to the same point. Since this doesn't seem to be possible, the next best thing is to have all ports referenced to an ideal ground plane (low resistance and inductance). However, my problem is that if your circuit doesn't have an ideal ground plane (i.e. its not real), then there might be problems. For example, some of the return current might flow through your ideal ground plane instead of the real return path.

So basically I'm still wondering what is the most accurate approach to EM modeling...

OK I realise it might be difficult to follow my thinking, so if its unintelligible, please let me know and I'll try and draw some figures.


thanks,
Aaron