Le87213A
Data Sheet
functions. If the AFE does not have such amplifiers, then external low noise amplifiers are required to implement these functions.
Figure 4 shows a typical implementation of the receive and hybrid functions.
Figure 4. Receive and Hybrid Circuit
RECEIVER AND HYBRID CIRCUIT
R6
R7
R4
HYB-
REC+
REC-
HYB+
-
UPP
C12
+
C10
DATA AFE
AGND
R8
R9
+
R5
UPN
-
C13
C11
COMPONENTS SELECTION
The DC blocking capacitor C6 is required on the primary or line side of the transformer in order to avoid applying a DC short across
TIP (A) and RING (B) terminals. The value of the line side capacitor depends on transformer primary inductance chosen to give
desired –3 dB high pass point. According to T1.413, the impedance measured across TIP (A) and RING (B) at 4 kHz must be
greater than 1100 Ω, which requires a capacitance of less than 35 nF for the series combination of C6 and any external DC
blocking capacitor.
The minimum transformer primary inductance required is dictated by the low end of the upstream frequency band, which is
typically around 25-30 kHz. Since the transformer primary inductance shunts the line, it should be relatively high to avoid signal
attenuation and maintain an acceptable match across the desired bandwidth. A high impedance can be obtained by using a
transformer with a high primary side inductance while maintaining relatively low DC resistance for minimum insertion loss.
Besides using it for its isolation and high common-mode rejection it is also used as part of the high-pass splitter function.
If the -3 dB point is chosen to be around 20 kHz, the minimum primary (line side) inductance required would be
ZLOOP
LPRI = ----------------------------- =
2π(20 kHz)
100
-----------------------------
≈ 796 µH
.
2π(20 kHz)
For the Le87213A device, a transformer primary side inductance of >800 µH is recommended
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