5

Input Gain Calculation

Input Gain Calculation

Input Gain Calculation

Input Gain Calculation

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

Step 1

Step 1

Step 1

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.

Step 2

Step 2

Step 2

Step 2

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

Step 3

Step 3

Step 3

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.

Application Inf

Application Inf

Application Inf

Application Inf

Application Information ......

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Input Gain Components

Input Gain Components

Input Gain Components

Input Gain Components

Input Gain Components

With reference 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

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

Use this area to keep a permanent record

of your calculated gains and components

Implementation Notes

Implementation Notes

Implementation Notes

Implementation Notes

Implementation Notes

Aliasing

Aliasing

Aliasing

Aliasing

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 R

Values of anti-alias capacitors should be chosen so as to

provide a highpass cutoff frequency, in conjunction with R

1

(R

25kHz to 30kHz (16kHz system).

i.e. C =

1

0

x

R

1

When anti-alias capacitors are used, allowance must be

made for reduced gain at the SPM frequency (12kHz or

16kHz).

Microcircuit Protection

Microcircuit Protection

Microcircuit Protection

Microcircuit Protection

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

V

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

Clock Out

Clock Out

Clock Out

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.