73K302L TDK [TDK Electronics], 73K302L Datasheet - Page 24

• Manufacturer Part Number: 73K302L
• Description: Single-Chip Modem
• Manufacturer: TDK [TDK Electronics]

Bell 212A, 103, 202
Single-Chip Modem
APPLICATIONS INFORMATION
DIRECT ACCESS ARRANGEMENT (DAA)
The telephone line interfaces show two examples of
how the “hybrid” may be implemented. The split
supply design (Figure 1) is a typical two op-amp
hybrid. The receive op-amp serves two purposes. It
supplies gain to amplify the receive signal to the
proper level for the modem’s detectors and
demodulator, and it removes the transmitted signal
from the receive signal present at the transformer.
This is done by supplying a portion of the
transmitted signal to the non-inverting input of the
receive op-amp at the same amplitude as the signal
appearing at the transformer, making the transmit
signal common mode.
The single-supply hybrid is more complex than the
dual-supply version described above, but its use
eliminates the need for a second power supply. This
circuit (Figure 2) uses a bridged drive to allow
undistorted signals to be sent with a single 5V
supply. Because DTMF tones utilize a higher
REFERENCE
VOLTAGE
RXA
TXA
HOOK
RING
0.1 µF
0.1 µF
R11
C6
C3
*
FIGURE 2: Single 5V Hybrid Version
+5V
20K 1%
R10
R7
(continued)
*
0.0047 µF
0.1 µF
C4
C7
5
6
2
3
8
*
+
+
-
-
37.4K 1%
20K 1%
20K 1%
750 pF
390 pF
U1C
22.1K
+
R5 3.3K
*
R6
C5
R8
R9
C1
R4
10 µF
U1A
11
C8
+5V
4
+
-
*
U1B
10
7
1
9
R14
10K
IN914
D2
D3
D4
20K 1%
ZENERS
20K 1%
0.033 µF
475 1%
24
R1
3.3V
R2
R3
C2
+5V
Q1
2N2222A
MIDCOM
671-8005
amplitude than data, these signals will clip if a
single-ended drive approach is used. The bridged
driver uses an extra op-amp (U1A) to invert the
signal coming from the gain setting op-amp (U1B)
before sending it to the other leg of the transformer.
Each op-amp then supplies half the drive signal to
the transformer. The receive amplifier (U1C) picks
off its signal at the junction of the impedance
matching resistor and the transformer. Because the
bottom leg of the transformer is being driven in one
direction by U1A and the resistor is driven in the
opposite direction at the same time by U1B, the
junction of the transformer and resistor remains
relatively constant and the receive signal is
unaffected.
DESIGN CONSIDERATIONS
TDK Semiconductor's 1-chip modem
include all basic modem functions. This makes these
devices
applications, and as easy to control as conventional
digital bus peripherals.
K1
T1
IN4004
D1
Note: Op-amp U1
must be rated for
single 5V operation.
R10 & R11 values
depend on Op-amp
0.47 µF
R12
22K
250V
C10
adaptable
4N35
U2
R13
22K
+5V
for
V250L20
MOV
VR1
T
R
use
in
a
variety
products
of