Mutual capacitance CM between adjacent pins of a logic devices can couple voltages on to sensitive inputs.The figure shown depicts one such situation.
The cross talk introduced by pin 1 on pin 2 is given as
Crosstalk = ( Rpin2 x CM)/T10-90
Rpin2 is the impedance seen by pin 2. For the current situation it is 37.5ohms.
T10-90 is the rise time at pin 1, and is given as 5ns.
Therefore crosstalk factor is 0.03 or 3%.
From the equation it is clear that cross talk becomes more serious as rise time get shorter or with higher impedance input connection.
The below figure illustrates high-impedance input problem. Without C1 and C2 the impedance of the R1 and R2 is so high that we expect capacitive crosstalk to be a problem. Using the same equation we get a crosstalk factor of 8. This means practically all the clock signal from pin 1 will appear on pin 2.
The capacitors C1 and C2 are introduced to seal off this crosstalk problem.When dealing with capacitive loads on a receiveing circuit, the percentage of crosstalk is just equal to the ratio of the capacitance: Crosstalk = CM / CI
With CI set to 0.01uF, we get a crosstalk of only 0.0004.
Adjacent lead capacitance of logic packages are given below.
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