4 Replies Latest reply on Apr 22, 2011 10:27 AM by weston_beal

    Creating an IBIS model of an input load from a SPICE circuit.

    mvonahnen

      I want to add to my simulation a model of a FET being connected to a signal.  I need to simuate for conditions when the FET is biased on and when it is biased off.

       

      I have seen the tools for generating an IBIS model from a SPICE model, but it is not clear how to do that with a FET circuit.  Has anybody done anything similar to this?

        • 1. Re: Creating an IBIS model of an input load from a SPICE circuit.
          weston_beal

          Assuming that the gate of the FET is connected to the signal, you can start with one of the simple IBIS models in the HyperLynx model library and edit it. You really just need to know the capacitance from the gate pin to the power or ground pins. Put that in the IBIS model C_comp parameter. Make the model_type Terminator if you don't want HyperLynx to automatically measure the signal at the FET pin.

           

          If the pin of the FET connected to your signal is not the gate, then you have a more complicated issue. IBIS may or may not have the syntax to model this, depending on the configuration of the FET connections.

           

          Also, if you have the FET SPICE model, and you have the GHz license, you can probably use the FET model directly in your simulation. Look at the application notes on SupportNet for help on this.

          http://supportnet.mentor.com/reference/technotes/public/technote.cfm?tn=MG56397

           

          Regards,

          Weston

          • 2. Re: Creating an IBIS model of an input load from a SPICE circuit.
            mvonahnen

            Unfortunately, I don't want the signal to drive the gate of the FET, this circuit is used as a switch and I want to verify that when the switch if off, I don't corrupt the signal.

             

            I also do not have the GHz license for my Hyperlynx.

             

            I am attempting to get the SPICE models to do the simulation in that simulator, so if this type of simulation is not supported in Hyperlynx, I will try to do the verification in SPICE.

            • 3. Re: Creating an IBIS model of an input load from a SPICE circuit.
              mvonahnen

              I have now found a part with an IBIS model that does the function I want.  One issue, however.  The part uses a "series_switch" model, which apparently Hyperlynx (at least my version) does not support.  The model (which I can not provde per the confidentiality agreement with the vendor) has an input port model and a single_switch model connected to the input.  I really want to understand what effects this part has on my circuit when the switch is off.

               

              The model for the "Single_switch" has model data for both the OFF state and for the ON state.  I removed the switch part of the model and simulated with just the input buffer model.  Is this sufficient or do I need to somehow connect up the OFF state model to the input buffer to get the true simulation?

               

              If the later is true, is there any easy way to do this?

              • 4. Re: Creating an IBIS model of an input load from a SPICE circuit.
                weston_beal

                The model for the MOSFET in the off state is just a very large resistance, so it's OK to just remove the series switch in this case. The simulation using the input pin model shows the behavior of the net with the effect of having the MOSFET connected to the net, but turned off.

                 

                IBIS Series MOSFET models are supported in HyperLynx starting in version 8.0 with both EXT and GHZ licenses. The User Guide and SupportNet have very good instructions on how to use series MOSFET models in LineSim and BoardSim.