[SI-LIST] Re: DDR-II SDRAM (SSTL-18 class-II): AC-test load

Hello CKW,

the DDR1 spec to my opinion was not very lucky with respect to =
AC-testload:
1) the Ron of the OCD was not specified tightly and the discrete =
30pF(?) of
the AC-testload made a significant RC-delay that goes into tAC. Thus =
the
measured tAC did strongly depend on an uncertain Ron.
In a real system you have no real 25R but the OCD current transients =
were
not that much different into 25R compared to a model of a real DDR2 bus =
(if
you disregard the different static currents that will be present once =
the
transient is over).
Of course there was an equivalent circuit available that did describe =
the
current transients better than the simple 25R but it would have been =
hard to
test with this load circuit in the lab in order to do a proper DLL
alignment. You always have short TLs between DUT and testload and this
easily skews up the measured transients asn thus the alignment.
2) It was hard to test this DDR1 spec (50R || 30pF) since usually you =
have a
Dual Transmission Line setup for testing (~=3D25R).
This way it was necessary to correlate between the spec AC-load and the
production environment and again the Ron uncertainty went into tAC.

It was a good compromise to have a 25R (no Cap) AC-testload compared to =
some
discrete Cap which probably came in when the transients were "long" =
compared
to the TL-length and thus modelling the TL with a discrete "C" was not
inappropriate.

Regards
Mathias

-----Urspr=FCngliche Nachricht-----
Von: ckw1972 [mailto:ckw1972@xxxxxxxxxxx]=20
Gesendet: Freitag, 22. August 2003 02:49
An: si-list@xxxxxxxxxxxxx
Betreff: (no subject)


Hi,

I've a question on the DDR-II SDRAM (SSTL-18 class-II) spec as below,=20
can someone help me to understand this correctly? Thanks in advance.

1. Why they is no clear capacitance load spec specify on the memory=20
spec?
    For example, samsung 512M 200Mhz DDR-II SSTL-18 class-II device=20
datasheet
    only specify their AC load reference is just connect DQ pin to=20
Vtt thru one 25ohm
    resistor. My understanding on this is with Cload=3D5pF & =
Cload=3D0pF=20
will impact on
    the 200Mhz performance quite significantly.


Thanks
Regards,
CKW

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