LVPECL and floating inputs

[samiam]

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

 

[Jim Thompson]

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

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

 

[John Larkin]

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

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

 

[samiam]

What's the receiver device?

SY100EP15V Mux

 

[John Larkin]

SY100EP15V Mux

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