Datasheet

ADN8102
Rev. B | Page 27 of 36
APPLICATIONS INFORMATION
OUTPUT COMPLIANCE
In low voltage applications, users must pay careful attention
to both the differential and common-mode signal levels. The
choice of output voltage swing, preemphasis setting, supply
voltages (V
CC
and V
TTO
), and output coupling (ac or dc) affect
peak and settled single-ended voltage swings and the common-
mode shift measured across the output termination resistors.
These choices also affect output current and, consequently,
power consumption. For ac-coupled applications, certain
combinations of supply voltage, output voltage swing, and
preemphasis settings may violate the single-ended absolute output
low voltage, as specified in Tabl e 1. Under these conditions,
the performance is degraded; therefore, these settings are not
recommended. Tabl e 21 includes annotations that identify these
settings. In dc-coupled applications, the far-end termination voltage
should be equal to V
TTO
to allow the full list of output swing and
preemphasis settings listed in Table 17.
TxHeadroom
The TxHeadroom register (Register 0x23) allows configuration
of the individual transmitters for extra headroom at the output
for high current applications. The bits in this register are active
high (default) and are one per output (see Table 2 2 ). Setting a
bit high puts the respective transmitter in a configuration for
extra headroom, and setting a bit low does not provide extra
headroom. The TxHeadroom bits should only be set high when
required for a given output swing as listed in Table 21. Note that
TxHeadroom is not available for V
CC
< 2.5 V.
V
H-DC
V
H-PE
V
SW-PE
V
L-PE
V
L-DC
t
PE
V
TTO
V
OCM
V
SW-DC
07060-140
Figure 44. Simplified Output Voltage Levels Diagram
Table 20. Symbol Definitions
Symbol Formula Definition
I
DC
Programmable Output current that sets output level
I
PE
Programmable Output current for PE delayed tap
I
TTO
I
DC
+ I
PE
Total transmitter output current
V
DPP-DC
25 Ω × I
DC
× 2
Peak-to-peak differential voltage swing of
nonpreemphasized waveform
V
DPP-PE
25 Ω × I
TTO
× 2
Peak-to-peak differential voltage swing of preemphasized
waveform
V
SW-DC
V
DPP-DC
/2 = V
H-DC
– V
L-DC
DC single-ended voltage swing
V
SW-PE
V
DPP-PE
/2 = V
H-PE
– V
L-PE
Preemphasized single-ended voltage swing
∆V
OCM_DC-COUPLED
25 Ω × I
TTO
/2 Output common-mode shift, dc-coupled outputs
∆V
OCM_AC-COUPLED
50 Ω × I
TTO
/2 Output common-mode shift, ac-coupled outputs
V
OCM
V
TTO
− ∆V
OCM
= ( V
H-DC
+ V
L-DC
)/2 Output common-mode voltage
V
H-DC
V
TTO
− ∆V
OCM
+ V
DPP-DC
/2 DC single-ended output high voltage
V
L-DC
V
TTO
− ∆V
OCM
− V
DPP-DC
/2 DC single-ended output low voltage
V
H-PE
V
TTO
− ∆V
OCM
+ V
DPP-PE
/2 Maximum single-ended output voltage
V
L-PE
V
TTO
− ∆V
OCM
− V
DPP-PE
/2 Minimum single-ended output voltage