AFE1224 Datasheet, PDF (9/11 Pages) Burr-Brown (TI) – 2Mbps, Single Pair ANALOG FRONT END

HDSL DSP
REF_P
V_CM
REF_N
txLINE+
txLINE–
rxbaudCLK
rx48xCLK
Data Out
txbaudCLK
tx48xCLK
Data In
AFE1224
rxHYB+
rxHYB–
rxLINE–
GNDA
GNDA
GNDA
DV_DD DV_DD
rxLINE+
AV_DD AV_DD AV_DD
0.1µF
0.1µF
Input Anti-Alias Filter
fc ≅ Bit Rate
375Ω
100pF
375Ω
375Ω
100pF
375Ω
0.1µF
13Ω
4.7nF
13Ω
–
+
Compromise
Hybrid
–
+
4.7nF
5V to 3.3V Digital
0.1µF
0.1µF
0.1µF 0.1µF 0.1µF
5V Analog
1 - 10µF
1:2 Transformer
Tip
Ring
FIGURE 6. Basic Connection Diagram.
For the transmit channel, the pulse shape and the power
spectral density scale directly with the clock rate. The power
spectral density shown in Curve 1 and the pulse template
shown in Curve 2 are measured at the output of the trans-
former. The transformer and the RC circuit on the output
provide some smoothing for the output transmission. At
lower bit rates, the amount of smoothing will be less and
more output filtering may be needed.
rxHYB AND rxLINE INPUT ANTI-ALIASING FILTERS
An external input anti-aliasing filter is needed on the hybrid
and line inputs as shown in the basic connection diagram
(Figure 6). The –3dB frequency of the input anti-aliasing
filter for the rxLINE and rxHYB differential inputs should
be approximately 2MHz for single pair T1 and E1 symbol
rates. Suggested values for the filter are 375Ω for each of the
two input resistors and 100pF for the capacitor. Together the
two 375Ω resistors and the 100pF capacitor result in a 3dB
frequency of just over 2MHz. The 375Ω input resistors will
result in minimal voltage divider loss with the input imped-
ance of the AFE1224. This circuit applies at rates of 1Mbps
to 2Mbps. For slower rates, the anti-aliasing filters will give
best performance with 3dB frequency approximately equal
to the bit rate. For instance, a 3dB frequency of 320kHz may
be used for a single line bit rate of 320k bits per second.
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AFE1224