TLFD600 Datasheet, PDF (7/37 Pages) Texas Instruments – ADSL CODEC WITH INTEGRATED LINE DRIVER AND RECEIVER

TLFD600
ADSL CODEC WITH INTEGRATED LINE DRIVER AND RECEIVER
SLAS280B – MAY 2000 – REVISED NOVEMBER 2000
receive channel (continued)
For example: If R = 2063 Ω, the external capacitor (C) needs to be 240 pF. The gain setting for CPGA changes
to the following range: –18 dB to 0 dB in 6 dB/steps and 0 dB to 21 dB in 3 dB/steps. The 9 dB shifting is calculated
ǒ Ǔ ₊ ^{by using the following equation:}
20
log
2063
732
9 dB
The linearity of the external capacitors and inductors is very important to the whole RX channel performance.
Capacitors of NPO grade or better need to be used.
clock control-VCXO
A 12-bit, serial DAC is used to control the external 35.328-MHz VCXO (voltage control oscillator) that provides
the system clock to the CODEC. Two 8-bit registers, (each 2s complement) VCRM and VCRL, are used to
generate the 12-bit control code. This implies using two 8-bits to obtain a 12-bit code. The VCRM register
occupies the most significant 8 bits in the 12-bit code and the lower 4 bits of the VCRL register (VCRL[3:0]) are
used for the low 4 bits of the 12-bit code. The internal DAC register is updated only when VCRL is programmed.
Table 1 shows some representative analog outputs.
Table 1. VCXO DAC Digital-Analog Mapping
OPERATION
VCRM[7:0] × 24 + VCRL[3:0]
HEX RESULT
0x800
0x801
…
0xFFF
0x000
0x001
…
0x7FE
0x7FF
ANALOG OUTPUT
0V
∆V
…
2047∆V
2048∆V
2049∆V
…
4094∆V
4095∆V
COMMENTS
Min scale
Just above min
…
Just below mid
Mid scale
Just above mid
…
Just below max
Max scale
Where step-size, ∆ = (3/4095) V.
1. The analog output is computed as follows: ((VCRM[7:0] × 2⁴ + VCRL[3:0]) + 2048(decimal)) × ∆.
2. Step-size ∆ is computed as follows: 0 x 800 (–2048 decimal) is 0 V and 0x7FF (2047 decimal) is 3 V. Thus,
∆ = (3 – 0) / (2047 – (–2048)) V = (3/4095) V
clock generation
The clock generation block provides the necessary clocks for the different functional blocks on the board with
minimum skew and jitters. This is closely dependent on the performance of the external VCXO. The external
VCXO specification is:
D 3.3-V supply
D 35.328 MHz ±50 PPM
D Minimum duty cycle is 60/40 (50/50 is the best)
The on-chip clocks are shown in Table 2. CLK1OUT and CLK2OUT can be used as general clock sources, or
they can be disabled if they are not used.
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