73k322l ETC-unknow, 73k322l Datasheet - Page 25

• Manufacturer Part Number: 73k322l
• Description: Ccitt V.23, V.22, V.21 Single-chip Modem
• Manufacturer: ETC-unknow

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
REFERENCE
VOLTAGE
RXA
TXA
HOOK
RING
0.1 µF
0.1 µF
C6
R11
C3
*
+5V
20K 1%
R10
R7
*
0.0047 µF
FIGURE 2: Single 5V Hybrid Version
0.1 µF
C4
C7
5
6
2
3
8
*
+
+
-
-
37.4K 1%
20K 1%
20K 1%
750 pF
U1C
22.1K
390 pF
R5 3.3K
*
+
R6
C5
R9
R8
C1
R4
10 µF
U1A
C8
11
+5V
4
+
-
*
U1B
10
7
1
9
R14
10K
IN914
D2
D3
0.033 µF
D4
20K 1%
20K 1%
ZENERS
475 1%
C2
R1
3.3V
R2
R3
+5V
25
Q1
2N2222A
MIDCOM
671-8005
K1
undistorted signals to be sent with a single 5 volt
supply. Because DTMF tones utilize a higher
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.
T1
IN4004
D1
0.47 µF
R12
22K
250V
C10
4N35
* Note: Op-amp U1
U2
must be rated for
single 5V operation.
R10 & R11 values
depend on Op-amp
used.
CCITT V.23, V.22, V.21
R13
22K
+5V
Single-Chip Modem
V250L20
MOV
VR1
R
T