MSAN-108 Zarlink Semiconductor, Inc., MSAN-108 Datasheet - Page 8

• Manufacturer Part Number: MSAN-108
• Description: Applications of The MT8870 Integrated DTMF Receiver
• Manufacturer: Zarlink Semiconductor, Inc.

MSAN-108
The design of a
generally be broken down into three functional
blocks (Fig. 7).
interface to the transmission medium. This may be
as simple as a few passive components to
adequately configure the MT8870’s input stage or as
complex as some form of demodulation, multiplexing
or analog switching system. The second functional
block is the DTMF receiver itself. This is where the
receiving system’s parameters can be optimized for
the specific signal conditions delivered from the
"
widely varying function, is the output control logic.
This may be as simple as a 4 to 16 line decoder,
controlling a specific function for each DTMF code,
or as complex as a full blown computer handling
system protocols and adaptively varying the tone
receiver’s parameters to adjust for changing signal
conditions.
conceptually designed applications are described
subsequently but first let’s consider the design of a
typical input stage.
A-52
front end
SOURCE OF
DTMF SIGNALS
Voltage Gain;
Input Impedance;
3dB Cutoff Frequency;
"
interface. The third, and perhaps most
Several
The first consideration is the
TRANSMISSION
DTMF receiving system can
INTERFACE
AV =
MEDIUM
Figure 7 - Modular Approach to DTMF Receiver Systems
currently
Z( ) = R 1 + (1/ RC)
Vo
Vi
V
i
f
c
Figure 8 - Single Ended Input Configuration
=-
=
2 RC
Rf
applied
R
C
1
S + 1/RC
RECEIVER
MT8870
DTMF
S
and
2
R
1
2
The input arrangement of the MT8870 provides a
differential input op amp as well as a bias source
(V
The output of this op amp is available to provide
feedback for gain adjustment.
A typical single ended input configuration having
unity gain is shown in Figure 8.
For balanced line applications good common mode
rejection is offered by the differential configuration
(Fig. 9). In both cases, the inputs are biased to 1/2
V
interface to a 600
mode noise signals, a balanced differential amplifier
input provides the solution.
With the input configured for unity gain the MT8870
will accept maximum signal levels of +1 dBm (into
600 ). The lowest DTMF frequency that must be
detected is approximately 685Hz. Allowing 0.1dB of
DD
REF
by V
) which is used to bias the inputs at mid-rail.
R
CONTROL
f
Ref
LOGIC
.
Consider an input stage which will
(3)
(2)
(1)
(4)
IN-
IN+
V
GS
balanced line. To reject common
Ref
+
-
MT8870
Application Note
FUNCTIONAL
INTERFACE
V
o