Rambus (RMBS) - Eagle or Penguin Message Board

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Technology Stocks : Rambus (RMBS) - Eagle or Penguin

RMBS 97.02

-0.5%

3:59 PM EST

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To: Ali Chen who wrote (81683)	3/25/2002 12:03:33 PM
From: John Walliker	of 93625

Ali, What you wrote below assumes that the driver behaves like a resistor which is switched into circuit to signal one logic state. The Rambus driver does indeed have a high output impedance despite being able to sink between 30 and 90 mA to a voltage of 0.9V. Let me quote from table 10, page 13 of a relevant Samsung data sheet: samsungelectronics.com Rout Dynamic output impedance at Vol = 0.9V Vdd min Tj max = 150 Ohms min So the output impedance is at least 150 Ohms, over 5 times higher than the bus impedance under worst case conditions of bus voltage, temperature and power supply voltage. Typically it will be much higher than this. Second, the Rambus signaling scheme is a 28Ohm terminated to 1.8V. If we generously assume that the Rambus "impedance of hundreds of Ohms" is even 100 Ohm, an open-drain N-MOS structure could not drag the wire down by more than 0.35V, which would not enough even to cross the reference level of 1.4V, not speaking of the specified level of 1V. As a matter of fact, the Rambus signaling scheme specifies 30mA as a constant driver sink, so it must be about only 30 Ohm to drive a 28-Ohm transmission line to 1V from 1.8V. Actually, it seems that the driver must have a variable impedance from 60 Ohm (at 1.7V) down to 30Ohm during the switching phase. John

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