FX631 Datasheet, PDF (5/9 Pages) CML Microcircuits – Low-Voltage SPM Detector

Application Information ......
Input Gain Calculation
The input amplifier, with its external circuitry, is
provided on-chip to set the sensitivity of the FX631 to
conform to the user's national level specification with
regard to ‘Must’ and ‘Must-Not’ decode signal levels.
With reference to the graphs in Figures 4 and 5, 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 will represent the required ‘Must’
decode level.
The lower line will represent the required ‘Must-
Not’ decode level.
Input Gain Components
With reference to the gain components shown in Figures
2 and 3.
The user should calculate and select external
components (R₁, R₂/C₃, R₃/C₄, R₄) 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
centre of the calculated range. The graphs in Figures 4 and 5
are for the calculation of input gain components for an FX631
using a V_DD of 3.3 (±0.1) or 5.0 (±0.5) volts respectively.
Use this area to keep a permanent record
of your calculated gains and components
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
will indicate 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
will indicate 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.
Select the gain components as described
opposite.
Implementation Notes
Aliasing
Due to the switched-capacitor filters employed in the
FX631, care should be taken, with the chosen external
components, to avoid the effects of alias distortion.
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
employed across input resistors R₁ and R₄.
Values of anti-alias capacitors should be chosen so as to
provide a highpass cutoff frequency, in conjunction with R₁
(R₄) of approximately 20kHz to 25kHz (12kHz system) or
25kHz to 30kHz (16kHz system).
i.e. C = 1
2 x π x f₀ x R₁
When anti-alias capacitors are used, allowance must be
made for reduced gain at the SPM frequency (12kHz or
16kHz).
Microcircuit Protection
Telephone systems may have high d.c. and a.c. voltages
present on the line. If the FX631 is part of a 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
FX631 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.
Clock Out
The Clock Out pin is intended to drive the FX631 Clock In
pin only. It is not recommended that it be used to clock other
devices within the host equipment.
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