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Doc.# 20480087.003

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Low Voltage SPM Detector

5

MX631

Application Information ......

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

0.3V and V

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.

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

Values of anti-alias capacitors should be chosen to

provide a highpass cutoff frequency, in conjunction with

R

or 25kHz to 30kHz (16kHz system).

i.e. C =

1

0

x

R

1

When anti-alias capacitors are used, make allowance for

reduced gain at the SPM frequency (12kHz or 16kHz).

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

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

Use this area to keep a permanent record

of your calculated gains and components