73K224L-28IH/F TERIDIAN [Teridian Semiconductor Corporation], 73K224L-28IH/F Datasheet - Page 26

• Manufacturer Part Number: 73K224L-28IH/F
• Description: Single-Chip Modem
• Manufacturer: TERIDIAN [Teridian Semiconductor Corporation]

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 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)
Page: 26 of 31
REFERENCE
VOLTAGE
RXA
TXA
HOOK
RING
0.1 µF
0.1 µF
R11
C6
C3
*
+5V
20K 1%
R10
R7
FIGURE 2: Single 5V Hybrid Version
*
0.0047 µF
0.1 µF
©
C4
C7
5
6
2
3
2005, 2008 TERIDIAN Semiconductor Corporation
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
11
C8
+5V
4
+
-
*
U1B
10
7
1
9
R14
10K
IN914
D2
D3
D4
20K 1%
20K 1%
ZENERS
475 1%
0.033 µF
R1
3.3V
R2
R3
C2
+5V
Q1
2N2222A
MIDCOM
671-8005
K1
T1
IN4004
D1
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
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
constant and the receive signal is unaffected.
DESIGN CONSIDERATIONS
TERIDIAN
products include all basic modem functions. This
makes these devices adaptable for use in a variety
of applications, and as easy to control as
conventional digital bus peripherals. Unlike digital
logic circuitry, modem designs must properly
contend with precise frequency tolerances and
very
acceptable performance. Using good analog
circuit design practices will generally result in a
sound
recommendations, which should be taken into
consideration when starting new designs.
0.47 µF
R12
22K
250V
C10
4N35
U2
impedance
Note: Op-amp U1
must be rated for
single 5V
operation. R10 &
R11 values depend
low
R13
22K
V.22bis, V.22, V.21, Bell 212A, 103
design.
+5V
level
Semiconductor's
and
V250L20
MOV
VR1
T
R
DATA SHEET
matching
analog
resistor
Following
Single-Chip Modem
signals,
remains
resistor
1-chip
are
73K224L
to
additional
and
relatively
modem
Rev 7.1
ensure
the