[SI-LIST] DDR2 Crosstalk/SSN - how much is too much
- From: Ravinder.Ajmani@xxxxxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 7 May 2009 10:00:25 -0700
Hi Experts,
For a new PCB design using two 16-bit DDR2 chips running at 250 MHz, the
trace length increased from 1.5" in previous design to 2.0". Also,
because of reduced card space, the traces had to be routed closer. This
resulted in increase in crosstalk. The worst case crosstalk increased to
about 220 mV, about twice compared to previous design. The design still
seems to work fine. I made worst case crosstalk/noise measurements on the
worst bit (31 bits switching High/Low, with this one bit quiet), which
showed an amplitude of about 400 mV. Next I measured the noise on another
bit, which had zero simulated crosstalk. This measurement showed about
170 mV of noise. The worst crosstalk happens when DRAMs are driving the
bus.
Can I assume that 170mV noise is entirely due to SSN/voltage ripple. In
the next card revision, I have managed to get additional card space for
placing series terminations and also spreading the traces. So now worst
simulated crosstalk is about 100 mV.
The ASIC manufacturer says that the crosstalk amplitude of up to 350 mV
should not cause any problem. I am curious as to how much total noise
will be acceptable in a DDR2 interface.
Thanks for your help.
Regards,
Ravinder Ajmani
Server PCB Development
Hitachi Global Storage Technologies
Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
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