Question on Differential Input Termination

[suria3]

Hi guys,

I have a question overhere regarding the impedance matching. I'm confused over the measurement of the differential input termination. Let say i want to terminate my input differentially (100ohm) or 50ohm single ended, i have to model the impedance as shown in the schematic. But when i measured input impedance differentially thgrough AC response, i'm getting somehow about 48ohm. and if I'm measure by single ended, i'm getting somekind of 1680ohm. So, how actually i can model a circuit to terminate exactly 100ohm differentially or 50ohm single ended. pls advise on this and explain to me on the theory of this differential termination.

thanks,
suria

 

[surianova]

hi!

If u measure differentially at inp and inn, you should get ard 50 ohm and depend
as well the value of r1 and r2 (must be big compare to 50 ohm). The center of 100 ohm resistor will be AC gnd. So, at point inp, the impedence should be
50 ohm||r1||r2 and almost 50 ohm. Any other opinion?

Surianova

 

[Btrend]

to calculate input differential input impedance, a voltage source was applied between inp and inn, and the current drawn by the load was measured.
After some derivation, I got Rdiff=Vdiff/Iin=1/{1/100+1/[2*(R1||R2)]},
so ur Rdiff is not 100.

 

[neoflash]

Your circuit are designed for termination of both common mode and diff mode impedance;

so the resistor between trace+ and trace- should be slightly larger than 100ohm to compensate the parallel resistors, which serve as common mode termination.