filtering caps

[patroclus]

I'm building this circuit:
**broken link removed**

I need caps C1 to C5, 100pF, for filtering, I suppose.
Can I use a higher capacitance caps?? for example, 10nF?? Is there a higher limit??

Thanks

 

[Nigel Goodwin]

I'm building this circuit:
**broken link removed**

I need caps C1 to C5, 100pF, for filtering, I suppose.
Can I use a higher capacitance caps?? for example, 10nF?? Is there a higher limit??

They are just simple low-pass filters, I would suggest you use the correct value, increasing them 100 fold is likely to impact the data transfer. 100pF is a standard value, and freely available, I would just use those!.

 

[patroclus]

Ok, I'll stick to them. It was just to know.
Thank you.

 

[_3iMaJ]

You can change the cap values if you want, but you must change the associated resistances as well.

If they are first order LPF (which they sound like) then 1/(2*pi*R*C) = fc. Where fc is the cutoff frequency of the LPF (or 3db frequency). Simply choose a cap value you like and calculate the associated resistance.

 

[Roff]

You can change the cap values if you want, but you must change the associated resistances as well.

If they are first order LPF (which they sound like) then 1/(2*pi*R*C) = fc. Where fc is the cutoff frequency of the LPF (or 3db frequency). Simply choose a cap value you like and calculate the associated resistance.

This is wrong. The caps work with the output resistance of the 74HC125s, not the 100 ohm resistors. The resistors are there for damping (to minimize ringing on the cable), and also provide a measure of short circuit protection.

 

[_3iMaJ]

No no, C5 works with the 1k resistor as for C1, that may be the case, however its most likely a HF noise cap. So changing it probably wouldn't be a good idea.

C1 is similar to a coupling cap.

 

[Roff]

No no, C5 works with the 1k resistor as for C1, that may be the case, however its most likely a HF noise cap. So changing it probably wouldn't be a good idea.

C1 is similar to a coupling cap.

What you say for C5 is true, but probably only within a limited range. I hadn't looked at it because patroclus mentioned 100pF, and C5 is 0.01uF. What I said holds for C1-C4. Perhaps you hadn't looked at them prior to your first post.

 

[_3iMaJ]

Honestly, I didn't look at the schematic until you replied saying I was wrong

 

[_3iMaJ]

But yes, what you said about C1 - C4 holds because I'd assume the output resistance is very high on those buffers. So it makes sense that the caps would have to be very small.

 

[audioguru]

The output resistance of the 74HC125 buffers is fairly low,except when they go tri-state and the 100pF caps are needed for "sample and hold" (I think). :lol:

 

[Roff]

The output resistance of the 74HC125 buffers is fairly low,except when they go tri-state and the 100pF caps are needed for "sample and hold" (I think). :lol:

I think the 100pF caps are there simply to slow down the edge rates so that ringing on the far end of the cable is minimized. Note that the DATA line has an LED attached to it. That ain't gonna be very kind to a sample and hold.
The 100 ohm resistor helps damp ringing by making the overall source impedance greater than the characteristic impedance of the cable.

 

[instruite]

The capacitors C1-C4 will basically effect the rise and fall time of the output from the buffer IC.
**broken link removed**

if you refer the link above you can see the datasheet for the same IC by two different companies Philips and ST
If you refer to ST datasheet, there is data available for the rise time and fall time with different load capacitance

These capcitors also effect the power dissipation of the IC
refer the philips datasheet for calculations of power dissipation of the IC

 

[patroclus]

What is ringing??
I'm spanish and don't know what this word means (appart from telephone ringing )

 

[JimB]

Ringing, if you have a bell it will just sit there making no noise. If you tap the bell with a hammer the bell will "ring", it will oscillate and eventually the oscillation will die away and the bell will be silent.

The same applies to some electronic circuits, when they recieve a pulse of voltage, they will oscillate for a while. After a few cycles the oscillation will stop.
This is "ringing" in the context of an electronic circuit.

JimB

 

[Roff]

What is ringing??
I'm spanish and don't know what this word means (appart from telephone ringing )

Patroclus, does "damped oscillation" translate a little better?

 

[patroclus]

yes, great. thank you.