AN4068 datasheet(21/63 Pages) STMICROELECTRONICS

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AN4068

ST7580 reference design description

Doc ID 022923 Rev 2

21/63

7.1.1

Transmission active filter

The transmission active filter is based on the ST7580 internal power amplifier (PA), whose

input and output pins are externally available to allow a filtering network tailored around the

amplifier.

For the ST7580 reference design, a 3-pole low-pass filter has been realized by cascading a

simple R-C low-pass stage and a Sallen-Key 2-pole cell.

The R16-C22 low-pass stage has a corner frequency at nearly 110 kHz for a first filtering of

the TX_OUT harmonics. The 1 nF value of C22 has been found to be the optimal value to

obtain a good filtering action without yielding unwanted capacitive load distortion on the

TX_OUT line.

The transfer function of the 2nd order Sallen-Key cell is:

Equation 1

where:

Equation 2

Equation 3

Equation 4

Figure 7 represents the measured transfer function of the transmission active filter. It shows

good rejection at signal harmonic frequencies.

)

(

⋅

)

(

=4.3=12.7 db

=150 kHz

⋅

=1.03

)

(

−

⋅

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