Vinod,
By definition, an ideal current source is constant current
at any voltage, which implies infinite impedance.
A current driver tries to imitate this by an I-V curve for
the output transistor that is as horizontal as possible, i.e.
the transistor is operated in the saturation region. They
can usually come pretty close to this, which means the impedance
can be fairly high (at least in the kOhm if not higher range).
However, the output stage of a current mode driver may not only
consist of an open-drain/collector transistor by itself. They
would put a resistor between the output pad and the supply rail
(most often 50 Ohms) and if this resistor is present, the output
impedance will become the parallel combination of the resistor
and transistor, i.e. the value of the resistor.
To get the buffer's impedance from the IBIS model, you just
have to look at the I-V curve and calculate its slope. But
don't forget to add the two clamp I-V curves to the pulldown
or pullup I-V curve, since they are all present in the simulation
and often times there are on-die termination characteristics
in the clamp I-V curves. And yes, if the pullup and pulldown
I-V curves are both present, of if there is on-die termination
in one or both of the clamp curves, the impedance you get may
be different for the logic '0' or '1' or 3-state states of
the buffer.
I hope this answers your questions,
Arpad
===================================================================
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of vinod ah
Sent: Sunday, October 04, 2015 7:38 AM
To: SI-LIST
Subject: [SI-LIST] [SI-LIST]: Output impedance of Open source Current Driver
Hi All,
I am trying to simulate HCSL open source current driver.
IBIS file of the driver has falling and rising waveform table both defined
with R_Fixture set to 50 Ohms and V_Fixture set to 0V.
Since it is current driver, can we compute output impedance from IBIS file.
Will there be impedance for rise and fall states?
Regards
Vinod A H
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