Transceiver Pseudo-CML equivalent circuit?

The other interesting thing that I observe, is that if I change VOD, I change the voltage swing, but not the common mode voltage, ie.,

1. VOD = 3 (default), Vhigh = 910mV, Vlow = 540mV, so Vcm = 725mV and Vpp = 370mV

2. VOD = 0, Vhigh = 800mV, Vlow = 660mV, so Vcm = 730mV and Vpp = 140mV

3. VOD = 7, Vhigh = 1100mV, Vlow = 330mV, so Vcm = 715mV and Vpp =770mV

So the common mode voltage is essentially unchanged.

If the transmitter was really only able to sink current, then as the sink current is increased, the Vlow voltage should get further from the Vhigh voltage, and the common-mode point should move lower. The observations show that changing VOD increases the peak-to-peak voltage, without changing the common mode voltage. Hence, the transmit buffer is not a simple CML source.

Cheers,

Dave