73K224L
V.22bis, V.22, V.21, Bell 212A, 103
Single-Chip Modem
DATA SHEET
DIRECT ACCESS ARRANGEMENT (DAA)
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.
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.
DESIGN CONSIDERATIONS
TERIDIAN Semiconductor's 1-chip modem
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 low level analog signals, to ensure
acceptable performance. Using good analog
circuit design practices will generally result in a
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)
sound
design.
Following
are
additional
recommendations, which should be taken into
consideration when starting new designs.
C1
390 pF
R4
37.4K 1%
R1
20K 1%
C3
0.1 µF
U1C
9
*
8
-
Note: Op-amp U1
must be rated for
single 5V
RXA
10
C4
0.0047 µF
+
R2
20K 1%
R5 3.3K
operation. R10 &
R11 values depend
+5V
4
R3
475 1%
5
+
7
6
T1
MIDCOM
671-8005
-
U1B
*
11
T
+5V
C10
0.47 µF
250V
R6
22.1K
C2
0.033 µF
R13
22K
R7
20K 1%
C6
0.1 µF
C5
750 pF
U2
4N35
TXA
R9
20K 1%
VR1
MOV
V250L20
D1
IN4004
D2
3.3V
ZENERS
R8
20K 1%
R12
22K
D3
2
U1A
*
-
1
3
+
+5V
+5V
K1
VOLTAGE
REFERENCE
D4
IN914
R10*
R
R14
10K
Q1
2N2222A
+
C7
0.1 µF
C8
R11*
10 µF
HOOK
RING
FIGURE 2: Single 5V Hybrid Version
Page: 26 of 31
© 2005, 2008 TERIDIAN Semiconductor Corporation
Rev 7.1
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
