TDA7400 Datasheet, PDF (19/28 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

Figure 18. Block Diagram of the Multipath Detector
TDA7400
LEVEL
MP_IN
BANDPASS
19KHz
VDD
CHARGE
1 bit
RECTIFIER
GAIN
2 BITS
GAIN
2 BITS
to SB
-
int. INFLUENCE
MPOUT
47nF
D98AU857
MPOUT pin is used as detector output connected
to a capacitor of about 47nF and additionally the
MPIN pin is selected to be the fieldstrength input.
Using the configuration an external adaptation to
the user’s requiremet is given in fig.25.
Selecting the ”internal influence” in the configura-
tion byte, the channel separation is automatically
reduced during a multipath condition according to
the voltage appearing at the MP_OUT pin. A
possible application is shown in fig. 26.
Programming
To obtain a good multipath performance an adap-
tation is necessary. Therefore tha gain of the
19kHz bandpass is programmable in four steps
as well as the rectifier gain. The attack and decay
times can be set by the external capacitor value.
QUALITY DETECTOR
The TDA7400D offers a quality detector output
which gives a voltage representing the FM recep-
tion conditions. To calculate this voltage the MPX
noise and the multipath detector output are
summed according to the following formula:
Quality = 1.6 (Vnoise -0.8V)+ a (REF5V- VMPOUT)
The noise signal is the PEAK signal without addi-
tional influences. The factor ”a” can be pro-
grammed from 0.7 to 1.15. the output is a low im-
pedance output able to drive external circuitry as
well as simply fed to an A/D converter for RDS
applications.
TEST MODE
During the test mode which can be activated by
setting bit D0 of the testing byte and bit D5 of the
subaddress byte to ”1” several internal signals
are available at the CASSR pin.
During this mode the input resistor of 100kOhm is
disconnected from the pin. The internal signals
available are shown in the software specification.
I²C BUS INTERFACE DESCRIPTION
Interface Protocol
The interface protocol comprises:
-a start condition (S)
-a chip address byte (the LSB bit determines read
/ write transmission)
-a subaddress byte
-a sequence of data (N-bytes + acknowledge)
-a stop condition (P)
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