DS280DF810ABVT datasheet(21/51 Pages) TI1

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Differential

Voltage

Time [UI]

0 V

Transmitted

Bits: 0

1 UI

Delay

1 UI

Delay

C[-1]

Pre-cursor

C[0]

Main-curosr

C[+1]

Post-cursor

Retimed

Data

FIR filter

output

DS280DF810

www.ti.com

SNLS538A – SEPTEMBER 2016 – REVISED OCTOBER 2019

Product Folder Links: DS280DF810

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Figure 7. FIR filter functional model

When utilizing the FIR filter, it is important to abide by the following general rules:

•

|C[-1]| + |C[0]| + |C[+1]|

≤ 31; the FIR tap coefficients absolute sum must be less or equal to 31).

•

sgn(C[-1]) = sgn(C[+1])

≠ sgn(C[0]), for high-pass filter effect; the sign for the pre-cursor or post-cursor tap

must be different from main-cursor tap to realize boost effect.

•

sgn(C[-1]) = sgn(C[+1]) = sgn(C[0]), for low-pass filter effect; the sign for the pre-cursor or post-cursor tap

must be equal to the main-cursor tap to realize attenuation effect.

The FIR filter is used to pre-distort the transmitted waveform in order to compensate for frequency-dependant

loss in the output channel. The most common way of pre-distorting the signal is to accentuate the transitions and

de-emphasize the non-transitions. The bit before a transition is accentuated via the pre-cursor tap, and the bit

after the transition is accentuated via the post-cursor tap. The figures below give a conceptual illustration of how

the FIR filter affects the output waveform. The following characteristics can be derived from the example

waveforms.

•

VODpk-pk=v7 - v8

•

VODlow-frequency = v2 - v5

•

RpredB = 20 * log10 (v3 ⁄v2 )

•

RpstdB = 20 * log10 (v1 ⁄v2 )

Figure 8. Conceptual FIR Waveform With Post-Cursor Only

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