CMX867A(1999) 查看數據表（PDF） - CML Microsystems Plc

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CMX867A
(Rev.:1999)

Low-Power SPM Detector

CML Microsystems Plc 

AN/Telecom/631A/1 June 1999

CMX631A Low-Power SPM Detector

External Component Value Calculation - A Worked Example

German 16KHz FTZ Specification:

Must Not Decode Level =

Must Decode Level

=

-27dB

-21dB

Using the graph supplied in the FX631 datasheet, section 4.3 and following the steps 1, 2 and 3, the required

‘gain-range’ of -4.75dB to -6.80dB is obtained.

Note: This graph is produced using a VDD of 3.0V.

For 5.0V, VDD operation the graph provided in Figure 5 of the datasheet should be used. This will provide a

‘gain-range’ of -0.50dB to -2.50dB.

Using the ‘gain-range’ for 5.0V, VDD, the mid-point gives a gain requirement of -1.5dB.

Assuming a Differential input:

R4 =

R1

C4 =

C3

R= R

3

2

The amplifier gain (G) is set by: G =

R1

R2 2

+

X2

C3

= -1.5dB or 0.8414

For a system frequency of 16kHz and an input capacitor value of 330pF.

1

X=

C3

2π × 16000 × 330 pF

= 30.143kW

Choosing R1 as 100kW (datasheet recommends R1 ³33kW )

R1

G=

R2 2

+

X2

C3

then by transposition and calculation;

R2 = 115kW

Choosing R2 = 120kW ±1% and R1 = 100kW ±1% and C3 = 330pF ± 10%;

and recalculating gives a variation in gain of -1.64dB to -2.10dB. This figure is still within the

-0.5dB to -2.5dB ‘gain-range’.

This same method and gain calculation can be used with common mode input.

NB: This application note also applies to the FX631

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