73k222al ETC-unknow, 73k222al Datasheet - Page 22

• Manufacturer Part Number: 73k222al
• Description: V.22, V.21, Bell 212a, Single-chip Modem
• Manufacturer: ETC-unknow

73K222AL
V.22, V.21, Bell 212A, 103
Single-Chip
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 amplitude than
REFERENCE
VOLTAGE
TXA
RXA
HOOK
RING
0.1 µF
0.1 µF
C6
R11
C3
*
+5V
20K 1%
R10
FIGURE 2: Single 5V Hybrid Version
R7
*
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
C8
11
+5V
4
+
-
*
U1B
10
7
1
9
R14
10K
22
IN914
D2
D3
D4
20K 1%
20K 1%
ZENERS
0.033 µF
5.1-6.2V
475 1%
R1
R2
R3
C2
+5V
Q1
2N2222A
MIDCOM
671-8005
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 products
include all basic modem functions. This makes
these devices adaptable for use in a variety of
applications,
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
used.
0.47 µF
R12
22K
250V
C10
4N35
U2
and
R13
22K
+5V
as
V250L20
MOV
VR1
T
R
easy
to
control
as