TLK110 Datasheet, PDF (30/104 Pages) Texas Instruments – Industrial Temp, Single Port 10/100Mbs Ethernet Physical Layer

TLK110
SLLS901A – DECEMBER 2011 – REVISED FEBRUARY 2012
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5.1.5 Digital to Analog Converter
The multipurpose programmable transmit Digital to Analog Converter (DAC) receives digital coded
symbols and generates filtered analog symbols to be transmitted on the line. In 100B-TX the DAC applies
a low-pass shaping filter to minimize EMI. The DAC is designed to improve the return loss requirements
and enable the use of low-cost transformers.
Digital pulse-shape filtering is also applied in order to conform to the pulse masks defined by standard and
to reduce EMI and high frequency signal harmonics.
5.2 100Base-TX Receive Path
In 100B-TX, the ADC sampled data is passed to an adaptive equalizer. The adaptive equalizer drives the
received symbols to the MLT3 decoder. The decoded NRZ symbols are transferred to the descrambler
block for descrambling and deserialization.
5.2.1 Analog Front End
The Receiver Analog Front End (AFE) resides in front of the 100B-TX receiver. It consists of an Analog to
Digital Converter (ADC), receive filters and a Programmable Gain Amplifier (PGA).
The ADC samples the input signal at the 125MHz clock recovered by the timing loop and feeds the data
into the adaptive equalizer. The ADC is designed to optimize the SNR performance at the receiver input
while maintaining high power-supply rejection ratio and low power consumption. There is only one ADC in
the TLK110, which receives the analog input data from the relevant cable pair, according to MDI-MDIX
resolution.
The PGA, digitally controlled by the adaptive equalizer, fully uses the dynamic range of the ADC by
adjusting the incoming-signal amplitude. Generally, the PGA attenuates short-cable strong signals and
amplifies long-cable weak signals.
5.2.2 Adaptive Equalizer
The adaptive equalizer removes Inter-Symbol Interference (ISI) from the received signal introduced by the
channel and analog Tx/Rx filters. The TLK110 includes both Feed Forward Equalization (FFE) and
Decision Feedback Equalization (DFE). The combination of both adaptive modules with the adaptive gain
control results in a powerful equalizer that can eliminate ISI and compensate for cable attenuation for
longer-reach cables. In addition, the Equalizer includes a Shift Gear Step mechanism to provide fast
convergence on the one hand and small residual-adaptive noise in steady state on the other hand.
5.2.3 Baseline Wander Correction
The DC offset of the transmitted signal is shifted down or up based on the polarity of the transmitted data
because the MLT-3 data is coupled onto the CAT 5 cable through a transformer that is high-pass in
nature. This phenomenon is called Baseline wander. To prevent corruption of the received data because
of this phenomenon, the receiver corrects the baseline wander and can receive the ANSI TP-PMD-defined
"killer packet" with no bit errors.
5.2.4 NRZI and MLT-3 Decoding
The TLK110 decodes the MLT-3 information from the Digital Adaptive Equalizer block to binary NRZI
data. The NRZI-to-NRZ decoder is used to present NRZ-formatted data to the descrambler.
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Architecture
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