[IBIS-Users] RE: [IBIS] DC sweep

There are times when obtaining and IV curve with .DC
sweeps is impossible.  If the buffer has flip/flops,
SPICE is not going to know how to find the operating
points in the circuit.  You will have to run a few
clock cycles worth of simulations before the output
will be in the correct state.
 
The good news is that IV curves do not have to be
measured in the .DC mode.  You can do just as well
in .TRAN mode.  Just connect an ideal source to the
node you are trying to get the IV curve from.  Define
a ramping voltage for this ideal source (either PWL,
or PULSE, or whatever), and measure its current.
 
Make sure that the polarity of the current measurement
is correct when you use it for IBIS models, and make
sure that the ramp is slow, in the ms range, not ns
or ps.  If you sweep too fast, your error in the
current reading will increase doe to the parasitic
capacitance in the circuit (I=C*dV/dt).
 
I hope this will help...
 
Arpad
====================================================

________________________________

From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Andrew Ingraham
Sent: Tuesday, July 05, 2005 6:26 AM
To: ibis-users@eda.org; ibis@eda.org
Subject: Re: [IBIS] DC sweep


Sivaram,
 
It sounds like you are confusing the purpose of sweeping the V-I curves.  It has nothing to do with edge-triggered or level-sensitive.
 
The only thing you apply the swept voltage to, is the IC pin (actually, the bonding pad).  If your IC has an output buffer connected to that pin, the swept voltage goes to the output of that buffer, but NOT to its input.  Its input should be connected to the normal things you would normally connect to the input of that buffer; not the swept DC source.
 
The purpose of the DC sweep is to measure the current vs. voltage relationship at the pin.  If the pin has an output or I/O buffer connected to the pin, then that I-V relationship depends very strongly on the state of the output buffer, which might be inactive (hi-Z), driving low, or driving high; and you need to extract the I-V relationships separately in each of those states.  You need to place the output buffer into each of those states (through whatever internal controls the buffer has), one at a time, and then sweep the voltage at the pin and measure the current, each time with the buffer held in one state (i.e., constant voltages applied to its edge-triggered or level-sensitive internal nodes).  Then change those internal nodes to put the buffer into its next state, and sweep the external voltage again.
 
If the pin is just an input buffer, then (unless the buffer has feedback, like a "bus-hold" device, which makes it problematic for IBIS) it doesn't make much difference what the input buffer is doing, whether it is toggling, etc.  Just sweep the input pin's voltage and measure the input current.
 
Hope this helps.
 
Regards,
Andy
 
 

	How can I extract V-I curves when the O/P of my buffer is edge-triggered 
	and not level-sensitive? 

	Basically, I have to do DC sweep to extract these curves.But I cannot 
	connect my inputs to steady voltage values as the O/P is edge-triggered, which 
	needs transient analysis. 

	Would anyone of you please let me know how to go about these buffers to extract 
	VI curves? 

	Thanks in advance, 
	Sivaram


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