Reference voltage clarification needed

Hello IBIS folks,

As I am working along on our IBIS model maker tool, I discovered another item
that I believe needs clarification in the IBIS specification.

We have four keywords in the IBIS spec which define the reference voltages of
the pulldown, pullup, GNDclamp and POWERclamp. At the time these were conceived
they might have been obvious, but reading the IBIS spec now I am confused as to
how these must be interpreted. After talking to Bob Ross, I understand how we
intended them to be used, but this is not clear from the spec alone.

Examples:

Connect a 5 volt CMOS device to a FLOATING supply with 5 volts between its (-)
and (+) terminals.

If I measure the pulldown I-V curves so that the GND connection of the curve
tracer is connected to the (-) terminal of the floating supply, my Pulldown
Reference is 0V. Similarly, if I connect the GND terminal of the curve tracer
to the (+) terminal of the FLOATING(!) supply and measure the I-V curves of the
pullup structure, my Pullup Reference is 5V. In other words, in this manner the
refence voltages are derived with respect to the (-) terminal of the floating
power supply.

In another situation I am measuring an RS232 driver. The device is connected to
three floating supplies so that it gets the necessary voltages of -10, 0, +5 and
+10 volts.

Since I know that the pulldown is associated with the -10 volt terminal, and the
pullup is associated with the +10 volt terminal, I will connect the GND terminal
of the curve tracer to -10V and +10 to obtain the pulldown and pullup I-V
curves, respectively. The Pulldown Reference is -10V and the Pullup Reference
is +10V in this case. Not that these numbers are all with respect to the
internal ground, 0V. If I did not know what that was, I would end up using a
different set of reference numbers (such as 0V and +20V), but the difference
between Pullup and Pulldown References will still give me the Voltage Range
(=Power Supply) voltage of the output stage of this driver. (This is why the
Voltage Range keyword is somewhat redundant and is not required if all four
Reference keywords are present).

However, the reference keywords could be interpreted another way. Let's say
someone wants to curve trace a pullup structure in a ground-relative (and not
Vcc relative) way. In that case we have a device in high state, connected to a
5 volt supply and the GND terminal of the curve tracer is connected to 0V. So
the Pullup Reference = 0V in this case. At the same time, the Pulldown
Reference is also 0V, because the pulldown structure of the device was also done
in a ground-relative manner. Even though the device was powered up with 5V, the
difference between the two Reference voltages is 0, and value for the Voltage
Range is lost if it is not explicitly defined.

From this, I conclude that 1) the probing device must always be connected to the
same voltage to which the measured structure is connected to (this might not
always be known, though), or 2) we cannot get rid of the Voltage Range keyword
even if all four Reference keywords are defined.

There are many fine details and implications associated with this subject which
I cannot explain here. The main thing is that the spec is not clear on which
interpretation is the correct one regarding the Reference keywords. Or did I
miss out on something?

Arpad
Received on Thu Feb 15 11:23:39 1996