ST7285C datasheet(67/117 Pages) STMICROELECTRONICS

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ST7285C

RDS FILTER (Cont’d)

4.7.2 Functional Description

The RDS filter is of classical Switched Capacitor

type, comprising: an anti-aliasing filter, the SC-fil-

ter proper and a smoothing filter connected in cas-

cade. The block diagram is given inFigure 33.

The Filter peripheral is composed of the following

functional blocks:

– Prefilter. The anti-aliasing filter consists of a 2

order Sallen-Key filter. The phase response in

the passband is linear. The cut-off frequency is

located at about 360KHz. The prefilter includes

an operational amplifier with a gain of 80 dB.

– SC Filter. The SC filter is a 8

th order bandpass

filter. It comprises 4 cascaded biquads. The bi-

quads all have the same scheme and differ only

in their capacitor values. The switches are con-

trolled by a clock generator which produces non-

overlapping clock phases.

– Buffer. The output of the SC-Filter cannot be

connected to resistive loads, since this would se-

verely reduce its gain. A buffer is therefore con-

nected between the SC filter and the smoothing

filter.

– Smoothing filter. The smoothing filter connect-

ed to the Sc-filter (through the Buffer) is a simple

RC low pass filter. The output is connected to

RDSFIL pin (external connection) and to the

Comparator.

– Comparator. The comparator is connected to

the smoothing filter and is able to detect zero-

crossing in less than 125ns. The digital output of

the comparator is connected via a port to the

RDS demodulator.

– D/A Converter. A maximum offset of 1mV is al-

lowed on the comparator’s inputs. The offset

compensation is achieved as follows:

in a software selectable test mode, the input of

the filter is switched to RDSREF (=2.5V). The D/

A converter register (RDSFi1) is set to zero and

then incremented by software until the compara-

tor changes its sign.

– Test Mode Selector. This function is controlled

via 4 bits in the filter control register (RDS Fi2). It

selects the various test modes. (see next point).

– ST7 interface registers. These are described

below.

RDS Fi1

Address 005Ah:

—

Read/Write Register

b7 = reserved.

b6 = COMP

Comparator output (read only).

b5-0 = AD5-AD0

Offset

correction

value

output

D/A

(1LSB=2mV).

RDS Fi2

Address 005Bh

—

Read/Write Register

b7-5 = reserved.

b4 = PDB

Power down bit (1 = Run; 0 = Power-down)

b3-0 = TM3-TM0

Mode select. Only Modes shown in the table below

are valid; other modes are reserved.

When the internal filter is switched off, the RDS-

COMP pin can be used as an input and to feed the

demodulator from an external filter.

COMP

AD5

AD4

AD3

AD2

AD1

AD0

4321

PDB

TM3

TM2

TM1

TM0

TM3

TM2

TM1

TM0

Mode

0000

filter off (reset state)

0011

normal operating mode

1101

offset compensation

mode

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