8.3.9 Differential Driver With FIR Filter

The DS250DF210 output driver has a three-tap finite impulse response (FIR) filter which allows for precursor and postcursor equalization to compensate for a wide variety of output channel media. The filter consists of a weighted sum of three consecutive retimed bits as shown in the following diagram. C[0] can take on values in the range [-31, +31]. C[-1] and C[+1] can take on values in the range [-15, 15].

Figure 7. FIR Filter Functional Model

When using 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 precursor or postcursor 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 precursor or postcursor tap must be equal to the main-cursor tap to realize attenuation effect

The FIR filter is used to predistort the transmitted waveform in order to compensate for frequency-dependant loss in the output channel. The most common way of predistorting the signal is to accentuate the transitions and de-emphasize the non-transitions. The bit before a transition is accentuated via the precursor tap, and the bit after the transition is accentuated through the postcursor 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.

• VOD_pk-pk= v₇ - v₈
• VOD_{low-frequency} = v₂ - v₅
• Rpre_dB = 20 * log₁₀ (v₃ ⁄v₂ )
• Rpst_dB = 20 * log₁₀ (v₁ ⁄v₂ )

Figure 8. Conceptual FIR Waveform With Postcursor Only

Figure 9. Conceptual FIR Waveform With Precursor Only

Figure 10. Conceptual FIR Waveform With Both Precursor and Postcursor