CycloneIII: LVDS_E_3R TX output voltage

Hi,

I am currently using an EP3C10F256C8 Cyclone III in a project for LVDS transmission/mixing. It contains a 2x5-channel input on I/Obanks 5 + 6

(true LVDS input: I/O standard LVDS) and a same-sized output on

I/Obanks 3 + 4 (emulated LVDS output: I/O standard LVDS_E_3R).

The used LVDS input/output frequencies range from 60 to 75 MHz, the

whole system runs fine from a functional point of view.

However, I am measuring a higher LVDS TX voltage on the LVDS_E_3R

outputs, though following the Device Handbooks recommendations with a

VCCIO = 2.5V and a 2x120 + 170 Ohms resistor network plus a differential

load of 100 Ohms each.

My measurements:

VOS (dc offset) = 1.25 V, VOH = ca. 1.7 V, VOL = ca. 850mV.

So I end up with a VOD (diff. amplitude) of ca. 850mV, higher than

the ANSI standard allows.

I have tried to change pre-emphasis and slow slew rate assignments, but

with no luck/effect. A fix would be to re-calculate the 3R network.

But I was wondering why the recommended settings end in these values

or if I have overlooked anything related.

Thanks for any hint,

EP3Seb.