Chapter 4: Analog Design Considerations; Serial I/O Description; Differential Transmitter; Figure 4-1: Differential Amplifier - Xilinx RocketIO X User Manual

R
Analog Design Considerations

Serial I/O Description

The RocketIO X transceiver transmits and receives serial differential signals, using a
nominal supply voltage of 1.5 VDC. A serial differential pair consists of a true (V
complement (V
Thus: V
complementary signals so that no separate reference level is needed.
A graphical representation of this concept is shown in

Differential Transmitter

The RocketIO X transceiver is implemented in Current Mode Logic (CML). A CML
transmitter output consists of transistors configured as shown in
positive supply and offers easy interface requirements. In this configuration, both legs of
the driver, V
complement. The CML output consists of a differential pair with 50Ω source resistors. The
signal swing is created by switching the current in a common-source differential pair. The
differential transmitter specification is shown in

Table 4-1: Differential Transmitter Parameters

Parameter
V Serial output differential peak to peak
OUT
(TXP/TXN)
V Output termination voltage supply
TTX
V Common mode output voltage range
TCM
V Differential output skew
ISKEW
RocketIO™ X Transceiver User Guide
UG035 (v1.5) November 22, 2004
) set of signals. The voltage difference represents the transferred data.
N
– V = V . Differential switching is performed at the crossing of the two
P N DATA

Figure 4-1: Differential Amplifier

and V , sink current, with one leg always sinking more current than its
P N
Min
200
0.9
www.xilinx.com
1-800-255-7778
Figure
V
P
V V
-
P
V
N
CML Output Driver
Table 4-1.
Typ Max Units
1600 mV
1.5 V
1.4 V
TBD ps
Chapter 4
) and a
P
4-1.
V
=
DATA
N
U035_06_091903
Figure 4-1. CML uses a
Conditions
Output differential voltage
is programmable
91