Low Voltage SPM Detector
5
MX631
Application Information ......
Input Gain Calculation
The input amplifier, with its external circuitry, is
provided on-chip to set the sensitivity of the MX631 to
conform to the user's national level specification with
regard to ‘Must’ and ‘Must-Not’ decode signal levels.
With reference to Figure 4, the following steps will
assist in the determination of the required gain/
attenuation.
Step 1
Draw two horizontal lines from the Y-axis (Signal
Levels (dB)). The upper line represents your
required ‘Must’ decode level. The lower line
represents your required ‘Must-Not’ decode level.
Step 2
Mark the intersection of the upper horizontal line
and the upper sloping line; drop a vertical line from
this point to the X-axis (Amplifier Gain (dB)).
The point where the vertical line meets the X-axis
indicates the MINIMUM Input Amp gain required for
reliable decoding of valid signals.
Step 3
Mark the intersection of the lower horizontal line
and the lower sloping line; drop a vertical line from
this point to the X-axis.
The point where the vertical line meets the X-axis
indicates the MAXIMUM allowable Input Amp gain.
Input signals at or below the ‘Must-Not’ decode
level will not be detected as long as the amplifier gain
is no higher than this level.
Input Gain Components
The following paragraphs refer to the gain
components shown in Figures 2 and 3.
The user should calculate and select external
components (R
1
, R
2
/C
3
, R
3
/C
4
, R
4
) to provide an amplifier
gain within the limits obtained in Steps 2 and 3.
Component tolerances should not move the gain-
figure outside these limits.
It is recommended that the designed gain is near the
center of the calculated range. The graph in Figure 4 is
for calculations for the input gain components for an
MX631 using a V
DD
of 3.3 (±0.1) volts.
Use this area to keep a permanent record
of your calculated gains and components
Implementation Notes
Aliasing
Due to the switched-capacitor filters employed in the
MX631, be careful to avoid the effects of alias distortion
with the external components you choose.
Possible Alias Frequencies:
12kHz Mode= 52kHz
16kHz Mode= 69kHz
If these alias frequencies are liable to cause problems
and/or interference, it is recommended that anti-alias
capacitors are used across input resistors R
1
and R
4
.
Values of anti-alias capacitors should be chosen to
provide a highpass cutoff frequency, in conjunction with
R
1
(R
4
) of approximately 20kHz to 25kHz (12kHz system)
or 25kHz to 30kHz (16kHz system).
i.e. C =
1
2
x
π
x
f
0
x
R
1
When anti-alias capacitors are used, make allowance for
reduced gain at the SPM frequency (12kHz or 16kHz).
© 1997 MX•COM, INC.
www.mxcom.com Tele: 800 638 5577 910 744 5050 Fax: 910 744 5054
Doc.# 20480087.003
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
All trademarks and service marks are held by their respective companies
Signal Input Protection
Telephone systems may have high d.c. and a.c.
voltages present on the line. If the MX631 is part of host
equipment that has its own signal input protection
circuitry, there will be no need for further protection as
long as the voltage on any pin is limited to within V
DD
+
0.3V and V
SS
-0.3V.
If the host system does not have input protection, or
there are signals present outside the device's specified
limits, the MX631 will require protection diodes at its
signal inputs (+ and -). The breakdown voltage of
capacitors and the peak inverse voltage of the diodes
must be sufficient to withstand the sum of the d.c.
voltages plus all expected signal peaks.
CMLMICRO [ CML MICROCIRCUITS ]
