All linear re-drivers will have an associated linearity range. Put simply, the linear range is the range of output signal swing that the re-driver can support. If the re-driver output swing exceeds its linear range, the output will no longer correspond to the input. This will negatively impact the signal quality and can cause issues with compliance testing.
To understand this concept, lets look at the linearity curve plot in the TDP142 datasheet. We can see that this plots the output voltage swing in relation to the input voltage swing:
We can see that the output voltage swing levels out around 1.4V as the input voltage swing increases. From this we can gather that the linearity range ends at around 1.4V. Beyond 1.4V, the signal enters a compression region where the input voltage swing no longer affects the output voltage swing. In practice this can lead to bad signal quality and failed compliance tests.
Its best to consider the linearity range when deciding which re-driver EQ value to use in your system. For example, if designing a system where the TDP142 input voltage swing is 600mV, you should start by using lower EQ values (0 – 5). This will ensure that the re-driver stays within its linear range, and give headroom for pre-emphasis from the video source. If the input voltage swing is lower, you can get away with using higher EQ values before the signal enters the compression region on the TDP142.
Best,
Shane
