CPC5604
Receive Signal Path (Refer to Block Diagram)
Transmit Signal Path (Refer to Block Diagram)
Signals to and from the telephone line to the LiteLinkTM
appear on Tip and Ring connections. The receive signal
is extracted from the transmit signal via the 2-4 wire
hybrid block. The receive signal is then converted to
infrared light by the receive photodiode amplifier and LED
front end. The intensity of the infrared light is modulated
by the receive signal and this light is transferred across
the electrical isolation barrier via reflective dome to a pho-
todiode where the light is converted to a photocurrent.
This photocurrent is a highly linear representation of the
receive signal and is amplified and converted to a volt-
age. This single ended voltage is converted to a differen-
tial voltage signal where it is presented as RX+ and RX-
and connects to the receive inputs of the host data pump.
Signals that are to be sent from the host to the tele-
phone line are placed differentially on TX+ and TX-. The
maximum value of the transmit signal should not
exceed 0dBm or 2.18Vpp. The differential transmit sig-
nal is converted to a single ended signal by the
LiteLinkTM. This signal is coupled to the transmit photo-
diode amplifier in a similar manner to the receive path.
At the output of this amplifier the voltage signal is cou-
pled to a voltage to current converter via a transcon-
ductance stage where the transmit signal modulates the
telephone line loop current. As in the receive stage, the
gain of the transmit photodiode amplifier is set to unity
automatically thereby limiting insertion loss to 0 1dB.
Figures 2C and 2D illustrate connection to the host dif-
ferentially and single ended respectively.
Variations in gain due to quantum efficiency of the optics
are virtually eliminated by an on chip AGC circuit which
automatically sets the input to output gain of the pho-
toamplifier to unity. This means that the receive signal on
the telephone line is faithfully reproduced at the RX out-
puts in terms of amplitude to within 2dB of the received
signal. Distortion at the RX outputs is -80dB maximum at
a receive level of -3dB over the band of 30Hz-4kHz.
Figure 2B Connection To Host Single Ended
(Receive)
CPC5604A LITELINKTM
HOST DATA PUMP/CODEC
0.1uF
RX+
Single supply operation requires that the RX outputs be
biased at 2.5V DC, therefore, it is necessary to use 0.1uf
blocking capacitors for coupling the receive signal to the
host. Figures 2.4.A and 2.4.B. illustrate connection to the
host differentially and single ended respectively.
RX+
RX-
RX-
Figure 2A Connection To Host Differential
(Receive)
CPC5604A LITELINKTM
HOST DATA PUMP/CODEC
SINGLE ENDED CONNECTION TO CPC5604A
0.1uF
RX+
Figure 2C Connection To Host Differential
(Transmit)
RX+
CPC5604A LITELINKTM
HOST DATA PUMP/CODEC
0.1uF
RX-
RX-
0.1uf
TX-
TXA1
TXA2
-
DIFFERENTIAL CONNECTION TO CPC5604A
+
TX+
0.1uf
SINGLE ENDED CONNECTION TO CPC5604A
XXX
16
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