If you look in just about any design guide on differential pairs, you’ll find a single circuit that is used to terminate differential pairs in an LVDS channel. This circuit involves placing a resistor in parallel to the two ends of the differential pair at the receiver end of the channel. It’s not that this guideline is wrong, but it can easily be mis-applied in situations where it might not be applicable.
The correct way to terminate LVDS channels depends on the coupling method being used to inject signals into an interconnect. In addition to applying termination at the required impedance value, there is also the matter of applying biasing in these termination schemes. Let’s take a look and see how to apply LVDS termination and biasing as these same ideas apply to other differential interfaces.
LVDS Termination Depends on Coupling Method
To understand LVDS termination, we have to look at the two possible coupling methods for signals: AC and DC coupling. In DC coupling, the driver is connected directly to a transmission line, or possibly connected with a series resistor. In AC coupling, the driver is connected to its transmission line with a series capacitor. The basic idea in applying AC coupling is to eliminate any DC offset that might exist on the driver’s output.
There are two termination schemes that are used in DC-coupled LVDS. These are direct parallel termination across the receiver input terminals, and split termination involving a center-tapped capacitor connected to ground. In these configurations, it is assumed that any common-mode noise does not create an appreciable deviation from the target DC offset of 1.2 V.
Parallel termination and split termination in LVDS channels.
The parallel resistor method is used in cases where there is not excessive skew between each side of the channel, and as such the receiver can easily filter common-mode noise. The split termination method uses a capacitor to provide a low-impedance path to ground for common-mode noise and the AC component of the signal in the presence of skew. In both cases, the result is formation of the required signal level across the resistors.
In the PCB layout, termination must be applied at the receiver such that the termination circuit, the receiver input pins, and the buffer appear to be a lumped circuit. This is appropriate up to approximately GHz bandwidths (Gbps data rates). Faster channels will use on-die termination (see below for a discussion).
In AC-coupled LVDS, we have capacitors along the transmission line that remove all DC offset along the transmission line. This is followed by termination circuits that restore the common-mode DC offset voltage to the desired value at the receiver end of the board. This would be used when large common-mode noises are present along the interconnect, or when large ground offsets are expected (equivalent to a large DC common-mode noise).
The first version uses a resistor divider on Vcc to set the DC offset on the signal level. Typical resistor values are in the kOhm range, and typical capacitor values are on the order of 100 nF.
This termination circuit for AC-coupled LVDS sets the required DC offset at the receiver through a resistor divider on each trace.
The following version of LVDS termination provides the same function, but it sets the offset through a connection to Vcc with a voltage divider. An alternative to this could also involve a direct connection to the desired DC offset voltage value without the voltage divider, such as with a precision voltage reference or a dedicated power supply.
This termination circuit for AC-coupled LVDS is the classic kind of differential termination circuit. It also provides high-frequency common-mode noise shunting directly to ground.
This termination circuit provides split termination as well as setting the DC bias to the level required at the receiver. This is the most useful coupling and termination scheme if you are working with components that have different bais offsets, or if you suspect a large ground offset between the two ends of the link. This is a common problem when routing SerDes channels across a cable between two pieces of equipment on different circuits.
Do You Need to Build These Termination Circuits?
If you’re routing a single point-to-point LVDS channel, then the short answer is “probably not”, although it is important to identify how to route a channel from an LVDS based on the termination applied in the receiver. These days, many chips will use on-die termination that falls within one of the above categories. In other words, the termination circuit is built into the semiconductor die.
For a component like an FPGA, the termination circuit available on a given I/O might be selectable while programming the device. If the device you want to use has this available, then there will be some information on selectable termination options in your datasheets. This can apply to any SerDes channel or differential interface you want to implement on the device, not just LVDS.
In other cases, such as with older LVDS driver/receiver pairs, you might need to apply termination manually based on your driver layout and routing. Make sure to check your product datasheets when making your schematics and apply the appropriate termination scheme.
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