FAIRCHILD AN-5045

Fairchild Semiconductor
Application Note
August 2002
Revised March 2003
Using the VBB Reference on High Speed LVDS Repeaters
Abstract
Fairchild’s LVDS 2, 4, and 8-port repeaters are designed
for high speed interconnects providing low Electromagnetic
Interference (EMI) and low power dissipation at data rates
up to 1.2Gb/s. The devices accept and generate LVDS levels with a typical output voltage of 330 mV. The devices are
designed with a VBB output reference voltage for AC coupling on the inputs. This application brief will discuss when
to use AC coupling in an interconnect design and illustrate
how to use the VBB reference feature for optimizing the
link.
Output Reference Voltage (VBB)
The VBB reference is a design feature of Fairchild’s LVDS
repeaters which is simply a DC voltage reference used in
applications employing AC coupling on the inputs. It is an
internally generated DC voltage bias output that sources
up to 275 µA of current. Test conditions used to
characterize this feature were with VCC = 3.3V and an
IBB = 0 to −275 µA to guarantee minimum and maximum
limits of 1.125V and 1.375V respectively. The VBB pin does
not have to be used and can be left floating.
When would the
VBB Reference Voltage be used?
Application specific requirements will determine whether
the VBB reference is needed. Typically when AC coupling is
used to couple a transmitted differential signal to a
receiver, the coupling capacitor blocks DC and thus the offset is no longer present at the receiver input. By connecting
the VBB output reference voltage between a split termination, it effectively provides a common mode voltage point
for the differential signals. Refer to Figure 1 for the VBB
connection diagrams. The capacitor from VBB pin to ground
is optional and is used to stabilize the common mode voltage point. The value of the capacitor will be dependent on
the specific application environment.
FIGURE 1. AC Coupling ECL to LVDS using the Output Reference Voltage (VBB)
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AN500798
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AN-5045 Using the VBB Reference on High Speed LVDS Repeaters
AN-5045
AN-5045 Using the VBB Reference on High Speed LVDS Repeaters
Why AC-couple a Signal?
There are several reasons a designer would consider ACcoupling a signal, some of which include the following:
1. For interchangeability between different logic families
with different switching thresholds it may be necessary
to change the DC bias level to be compatible with the
switching thresholds of the receiving device.
2. To couple differential signals transmitted between
equipment with no DC connection required between
the chassis. This will allow easier reception of the signals by the receiver without concern of the commonmode voltage range.
AC coupling is typically used only with binary signals having an equal number of ones and zeros such as clocks with
a 50% duty cycle. A signal with DC balance is referred to
as a signal having an equal number of ones and zeros.
Without a DC bias, the receiver will not respond since the
internal circuitry has no DC operation point, effectively
never allowing the input transistors to bias up.
Summary
This application brief provides guidance for using the VBB
output reference feature on Fairchild’s LVDS repeaters.
VBB can ease interchangeability and interfacing between
different signaling standards.
Fairchild does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and
Fairchild reserves the right at any time without notice to change said circuitry and specifications.
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