MSAN-108 Zarlink Semiconductor, Inc., MSAN-108 Datasheet - Page 9

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

Application Note
attenuation at 685Hz, the required input time
constant may be derived from;
Now,
impedance (440K in the passband) and C=
unity gain in the passband we choose R
R
critical and could be set at, say... 68K. Bias resistor
R
differential amplifier but has no affect on the inverting
/R=6.9nF (use a standard value of 10 nF). For
b
a
R
convenient value greater
than 30K .
M( )
where M( )
Therefore
adds a zero to the non-inverting path through the
a
are biasing resistors. The choice of R
can be chosen to be a
choosing
R
Voltage Gain;
Input Impedance;
3dB Cutoff Frequency;
dB
b
selection; R
=20 log
dB
-0.1=20 log
is the input time constant
is the radian frequency
is the amplifier gain in decibels
10
or = 1.52mS
R=220K
b
R
R
=
f
+20 log
R
AV =
R
a
10
a
+ R
R
{
f
f
+
_
Z( )
gives
(2
10
V
V
V
o
i
(2
i
f c =
Figure 9 - Differential Input Configuration
{
=-
= 2R 1 + (1/ RC)
(
a
2 RC
C
C
R
R
)
f
2
f
=R. R
1
high
+ 1
}
S + 1/RC
}
1/2
a
1/2
is not
a
input
S
and
2
R
R
R
1
2
a
path. This zero can be exactly cancelled by the
added pole due to R
With appropriate input transient
circuit will provide an excellent bridging interface
across a properly terminated telephone line for end-
to-end or key system applications.
protection may be achieved by splitting the input
resistors and inserting zener diodes to achieve
voltage clamping (Fig. 10). This allows the transient
energy to be dissipated in the resistors and diodes
and limits the maximum voltage that may appear at
the op-amp inputs.
It is important to consider the amount of shunt
capacitance introduced by the protection devices. In
this case the parasitic capacitances of the zener
diodes are in series which reduces their effect.
Relatively large shunt capacitances will attenuate the
high group frequencies causing the input signal to
”twist“ which degrades receiver performance.
R
f
R
b
(3)
(2)
(1)
(4)
R
IN-
IN+
V
GS
b
Ref
b
=
if R
+
-
R
R
b
a
a
R
is chosen as;
+R
MT8870
f
f
MSAN-108
.
protection, this
V
o
Transient
A-53