Effects of vdd ang gnd bonding wire L on a differential ckt?

[gilbertomaldito]

Hi

I wanna ask if how the parasitic inductance of a bonding wire in the Vdd and Gnd pins affect the performance of a differential circuit structure especially on the differential output. I actually simulated a differential transmitter (LVDS) and tried to put parasitic L on vdd and gnd rails. I observed that the jitter at the output worsens. Im just confused, since it is a differential structure, how come it is affected ? Also Iwant to ask, how can I check the PSRR of my LVDS transmitter?

Thank you

andrew

 

[erikl]

Re: Effects of vdd ang gnd bonding wire L on a differential

... how can I check the PSRR of my LVDS transmitter?

Same as for any other circuit: Inject a disturbance (e.g. 1V sinus) in series with your VDD (or GND) supply, run an AC or XF analysis over the frequency range of your interest and plot PSRR over frequency. Differences between VDD & GND disturbance injection should provide a hint about possible asymmetries.

 

[dick_freebird]

Bond wire L, with the C )int & ext) will impose ripples on your
transitions and these may add rather than subtract, since the
differential outputs "kick" them in opposite directions. Especially
problematic is the case where the ring happens to oppose the
transition right as it passes across threshold. This is going
to be Cload dependent to great degree, which is ugly from the
perspective of someone trying to make a clean eye across a wide
/ unknown application-space.

It may result in some effprt / constraints passed along to the
board designer & users.

 

[gilbertomaldito]

Re: Effects of vdd ang gnd bonding wire L on a differential

Hi Mr Freebird

Thank you for your explanation, however, I still cant explain based on my results.
So I attached my results below. I simulated 2 cases, first one with less Vdd and Gnd parasitic inductance, and the othe rone with more parasatic L.

On the common mode signal, ringing can be observed, however, on the differential, they are almost the same that means the ringing caused by the inductors is a common mode signal. However I cant understand why it still causes jitter.

Then on the eye, the jitter for the signal with larger L is worst than the other one.

--Andrew