8.4.9 Setting the Differential Output Driver to Non-Standard Amplitudes
In some applications, it may be desirable to have larger or smaller differential amplitudes than those produced by the standard LVPECL
and LVDS settings, as selected by CBPro. In these cases, the following information describes how to implement these amplitudes by
writing to the OUTx_CM and OUTx_AMPL setting names. Contact Skyworks for assistance if you want your custom configured device
to be programmed for any of the settings described here.
The differential output driver has a variable output amplitude capability and two basic formats, normal and low-power format. The
difference between these two formats is that the normal format has an output impedance of ~100 Ω differential, and the low-power
format has an output impedance of > 500 Ω differential. Note that the rise/fall time is slower when using the Low Power Differential
Format. See the Si5391/Si5391P data sheet for rise/fall time specifications.
If the standard LVDS or LVPECL compatible output amplitudes will not work for a particular application, the variable amplitude capability
can be used to achieve higher or lower amplitudes. For example, a “CML” format is sometimes desired for an application. However,
CML is not a defined standard, and hence the amplitude of a CML signal for one receiver may be different than that of another receiver.
When the output amplitude needs to be different than standard LVDS or LVPECL, the Common Mode Voltage settings must be set as
shown in
Table 8.11 Output Differential Common Mode Voltage Settings on page 35
. No settings other than these are supported as
the signal integrity could be compromised. In addition, the output driver should be ac-coupled to the load so that the common-mode
voltage of the driver is not affected by the load.
Table 8.11. Output Differential Common Mode Voltage Settings
VDDOx (Volts)
Differential
Format
OUTx_FORMAT
Common
Mode Voltage (Volts)
OUTx_CM
3.3
Normal
0x1
2.0
0xB
3.3
Low Power
0x2
1.6
0x7
2.5
Normal
0x1
1.3
0xC
2.5
Low Power
0x2
1.1
0xA
1.8
Normal
0x1
0.8
0xD
1.8
Low Power
0x2
0.8
0xD
The differential amplitude can be set as shown in the following table.
Table 8.12. Typical Differential Amplitudes
OUTx_AMPL
Normal Differential Format
(Vpp SE mV – Typical)
Low-Power Differential Format
(Vpp SE mV – Typical)
0
130
200
1
230
400
2
350
620
3
450
820
4
575
1010
5
700
1200
6
810
7
920
Note:
1. These amplitudes are based upon a 100 Ω differential termination.
2. In low-power mode and VDDOx = 1.8 V, OUTx_AMPL may not be set to 6 or 7.
Si5391 Reference Manual • Outputs
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
35
Rev. 0.5 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • January 11, 2022
35