Analog signal through a digital CMOS switch

[MM]

Hi all,

I tried asking a more specific question about using a LAN mux/demux but
nobody replied. So, here is a very basic question. Is it OK to have biasing
on one side of the switch only while having AC coupling capacitor on the
other side? I don't care about transition time, switching happens only once
before any operation starts, so "clicks" or "pops" are not an issue here.


Thanks,
/Mikhail

 

[[email protected]]

Hi all,

I tried asking a more specific question about using a LAN mux/demux but
nobody replied. So, here is a very basic question. Is it OK to have biasing
on one side of the switch only while having AC coupling capacitor on the
other side? I don't care about transition time, switching happens only once
before any operation starts, so "clicks" or "pops" are not an issue here.

Thanks,
/Mikhail

This isn't the clearest post, but the cap coupling should not be an
issue IF you someone establish a path to DC (generally ground).
Floating nodes are not a good idea.

LAN? Er, as in local area network?

 

[MM]

This isn't the clearest post, but the cap coupling should not be an
issue IF you someone establish a path to DC (generally ground).
Floating nodes are not a good idea.
LAN? Er, as in local area network?

Please read my other post titled "Applying Pericom PI3L301D-A (10/100/1000
BaseT LAN Mux/DeMux)" from a day before.

The purpose of the circuit is to mux/demux PHY between two LANs. Usually a
transformer is used between a PHY and a cable. Some older PHY's required the
center tap of the transformer to be connected to a power source, however the
PHY I am using (VSC8201 by Vitesse) has fully differential active drivers
that can both source and sink current, so when a transformer is used with
this PHY, the center tap is AC coupled to the ground. The PHY outputs are
biased to about 1.5V. Now, imagine that we insert a switch between the PHY's
pins and the transformer (or AC coupling capacitors if they are used instead
of the transformer). The side of the switch connected to the PHY will have
biasing from the PHY, while the other side will not. The question is whether
it is a problem from the switch's point of view (The switch can only pass
positive voltages, hence the biasing is required)? As far as I understand
when the switch closes I should see the same DC offset at both sides, so I
am safe, right? Or not?


Thanks,
/Mikhail

 

[MM]

No, you are not right. No one mux's the PHY lines like that...no need for
it whatsoever.

Well, whether there is need or not is another question isn't it? If there
was no need TI and Pericom wouldn't have parts designed specifically for
this purpose.


/Mikhail

 

[MM]

That Pericom part is intended for control & data portion of the layer,
that means it goes on the side of the PHY transceiver opposite to the PHY
lines, it is an interface between the control logic and transceiver.

Nope. Check the datasheet and the Pericom appnotes (TI doesn't seem to have
any)... It is intended for the PHY lines themselves. The part numbers I am
particularly interested in are TI TS3L301 and Pericom PI3L301D-A, but there
are more...

/Mikhail

 

[MM]

I did check the datasheet and it goes where I said it goes.

http://www.pericom.com/pdf/applications/AB006.pdf
http://www.pericom.com/pdf/applications/AB005.pdf
http://www.pericom.com/pdf/applications/AB041.pdf

Enough?

/Mikhail

 

[MM]

All of those apps have a DC path between (+) and (-) of the differential
outputs through the transformer, it makes no difference that the center
tap is floating at DC, there will be DC on both sides of the switch
because of the transformer path from the complementary side switch. This
will not be the case with capacitor coupling.

So, in your opinion, is it a concern for the mux or for the PHY? The
particular PHY I am using is a voltage mode and it doesn't require DC path
through the transformer. It can happily work through capacitors. I just have
to make sure the mux is happy too...

Thanks,
/Mikhail

 

[MM]

I don't find any apps using capacitor coupling, but if they exist, they
have either a high impedance choke and/or resistor across (+)/(-)...

If you send me your email address I will forward to you some appnotes
showing capacitor coupling. One obvious application is LAN over a backplane
as in PICMG 2.16.

/Mikhail