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Capacitor in parallel to Rx buffer

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shaiko

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Suppose we have a circuit of a Tx buffer connected to an Rx buffer - parallel to the Rx buffer there's a small (200 pF) capacitor shorted to ground.
What is the purpose of this capacitor ?
What effect will it have on the signal ?
 

Depends on the the bit rate and driver output current.
 

Around 10MHz and the buffer is a typical LVCMOS 3.3V
 

"Typical LVCMOS" doesn't define a drive strength, usual values are between 1 and 30 mA, corresponding to output impedances between 10 and several 100 ohm. The capacitor effect varies between small delay and transmission failure.
 
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    shaiko

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Just checked - it's a 24mA buffer.
How does the capacitor change the shape of the signal ?
Do it reduce the slew rate of the signal ?
 

It surely reduces the slew rate. It also causes a large output current of the driver and possibly interferences due to ground bounce. I don't think it's a good method to achieve a reasonable circuit effect. It's typically applied as a workaround, to fix timing problems or filter spikes. A few 10 pF would be O.K., but 200 pF is a rather large value, if a delay in this range is intended, we would better cut the signal line and a add series resistor togther with a lower capacitor value. It should be also checked if the receiver tolerates increased signal rise- and fall times.
 
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