[SI-LIST] Re: AC Termination : Capacitor Value

Joe,
I am assuming you mean termination with a 50 Ohm resistor connected to a 
capacitor whose other end goes to ground? I.e.

transmission line --- receiver
                   |
                   |
                   R
                   |
                   C
                   |
                  GND


This type of termination avoids static current flowing when there is no 
activity on the line. The price you pay is that it cannot terminate slow 
(DC) signals. That means this scheme will only work with DC balanced 
signals (usually means an equal number of 1's and 0's in the data stream. 
Examples are 50% duty cycle clock signals, Manchester encoded data, or 
8b/10b encoded). The line will then settle to the average level on the 
line. 

Signal rise/fall times will NOT be increased because of the capacitor - 
for such short time intervals (assuming sufficient capacitance) the 
capacitor looks like a perfect short to ground, leaving you with a matched 
50 Ohm termination. BUT the levels at the receiver will drift around if 
the signal stays low or high for too long. So the important figure of 
merit is the "running disparity", i.e. the maximum number of excessive 1's 
or 0's occuring in your data stream over any subsection of your pattern 
("excessive" meaning not being balanced by preceding bits of the opposite 
polarity). Have a look at Howard Johnsons "Digital Signal Propagation" 
book which takes more about that concept. In a nutshell, your RC time 
constant should be much longer than this time scale. E.g. 8b/10b encoded 
data has a maximum running disparity of +/-2. At e.g. 1 Gb/sec data rate 
that means RxC should be much longer than 2 x 1ns = 2ns. Let's e.g. take a 
factor of 10x, i.e. 20ns. With R=50 Ohms and a line impedance of Zo=50 
Ohms (i.e. effective source impedance of 100 Ohms since R and Zo act in 
series) you'll need at least 0.2nF of capacitance.

Should you use much more to be on the safe side? Depends. First, larger 
capacitors tend to have larger inductive parasitics, which is not good for 
high-speed termination. Even more important, after start of the data 
transmission the line needs time to settle to the average signal level, 
meaning the receiver will need to throw away the first few (or many) data 
bits before it can expect valid data. That's also the reason SerDes 
transmission schemes need to keep sending "idle" patterns when there is no 
data to transmit, to avoid drifting away from the settled levels. The 
settling time is given by the time constant, (Zo+R)xC. So your data 
transmission needs to run sufficient "warmup cycles" to let the levels 
settle out. Make C too large and the system will take "forever" to settle. 
That gives you an upper bound. Summing up:

(R+Zo) x C >> RD x Tbit
(RD = running diparity, Tbit = bit interval)

(R+Zo) x C << maximum tolerable warmup time

So for the present example, maybe 10nF would be a good choice.

Wolfgang







Joe Paul M <joepaulm@xxxxxxxxx> 
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[SI-LIST] AC Termination : Capacitor Value







Is there a thumb rule for finding Capacitor value for AC Parallel 
termination (R-C) for single ended lines ?

I came across people who feel that higher capacitor value for R-C 
termination , will increase capacitive loading on line and hence 
increase rise/fall time . Elementary SI knowledge ,makes me feel that 
this is not true. Am I missing something ?

Joe

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