LVPECL and floating inputs

May 31, 2007 4 Replies

Rate this thread:

samiam 19 years ago

Software guy here working on a simple project thats becoming complicated

Please imagine the following scenario: device with lvpecl inputs. Possibility that the inputs may float (left unconnected). I reckon I should force the inputs to a default level,

If this were a TTL device, I would tie the input to vcc via a pullup or pull down. But its a differential - so I guess I need a voltage divider to balance the two inputs such that theres a difference of say

100mv between the two

How will this affect things when an input IS connected?

I know what you are thinking - is this dude dense or what? I am going to go down to the basement and fish out my ac/dc analysis book and read up on thevenin,superposition,mesh networks and various ways of analyzing circuits.

Its been awhile

Any pointers would help

Vote

Jim Thompson 19 years ago

Differential? Read the data sheet. Probably tie one input high to keep the current mirrors under it happy, the other input low. But it'll depend on the exact circuit.

...Jim Thompson

Vote

John Larkin 19 years ago

You could thevenin terminate them to slightly different voltages, offset in your preferred direction, both near the low end of the common-mode range. That should work OK.

What's the receiver device?

John

Vote

samiam 19 years ago

SY100EP15V Mux

Vote

John Larkin 19 years ago

That part has weak pullup/pulldowns so that it goes to a known state, equivalent to input low, if the inputs are truly open. But if you add your own terminations they will overpower those pulls, so you should make sure they add a small offset if you want the input to be in a known state when the signal source is gone.

John

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required