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PDF ATR4251 Data sheet ( Hoja de datos )
| Número de pieza | ATR4251 | |
| Descripción | High-dynamic-range Antenna Amplifier IC | |
| Fabricantes | ATMEL Corporation | |
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Features
• High Dynamic Range for AM and FM
• Integrated AGC for AM and FM
• High Intercept Point 3rd Order for FM
• FM Amplifier Adjustable to Various Cable Impedances
• High Intercept Point 2nd and 3rd Order for AM
• Low Noise Output Voltage
• Low Power Consumption
• Low Output Impedance AM
1. Description
The ATR4251 is an integrated low-noise AM/FM antenna amplifier with integrated
AGC in BiCMOS2S technology. The device is designed in particular for car applica-
tions, and is suitable for windshield and roof antennas.
Figure 1-1.
Block Diagram QFN24 Package
FM FM
FM AGC
VREF1 IN GAIN GND2 OUT IN
NC*
1
24 23 22 21 20 19
FM
amplifier
18
Paddle = GND
NC*
GND
AGC1
2
3
BAND
GAP
AGC
VS
17
AGCCONST
16
AGC2
4
VREF4
15
VREF2
AMIN
5
6
AM
78
AGC
(AM)
AMOUT1
14
GND1
13
9 10 11 12
NC* CREG AGC AGC T NC*
AMIN AM CONST
* Pin must not be connected to any other pin or supply chain except GND.
www.DataSheet4U.com
Low-noise,
High-dynamic-
range AM/FM
Antenna
Amplifier IC
ATR4251
4913J–AUDR–10/09
1 page  
ATR4251
www.DataSheet4U.com
3. Functional Description
The ATR4251 is an integrated AM/FM antenna impedance matching circuit. It compensates
cable losses between the antenna (for example windshield, roof, or bumper antennas) and the
car radio which is usually placed far away from the antenna.
AM refers to the long wave (LW), medium wave (MW) and short wave (SW) frequency bands
(150 kHz to 30 MHz) that are usually used for AM transmission, and FM means any of the fre-
quency bands used world-wide for FM radio broadcast (70 MHz to 110 MHz).
Two separate amplifiers are used for AM and FM due to the different operating frequencies and
requirements in the AM and FM band. This allows the use of separate antennas (for example,
windshield antennas) for AM and FM. Of course, both amplifiers can also be connected to one
antenna (for example, the roof antenna).
Both amplifiers have automatic gain control (AGC) circuits in order to avoid overdriving the
amplifiers under large-signal conditions. The two separate AGC circuits prevent strong AM sig-
nals from blocking FM stations, and vice versa.
3.1 AM Amplifier
Due to the long wavelength in AM bands, the antennas used for AM reception in automotive
applications must be short compared to the wavelength. Therefore these antennas do not pro-
vide 50Ω output impedance, but have an output impedance of some pF. If these (passive)
antennas are connected to the car radio by a long cable, the capacitive load of this cable (some
100 pF) dramatically reduces the signal level at the tuner input.
In order to overcome this problem, ATR4251 provides an AM buffer amplifier with low input
capacitance (less than 2.5 pF) and low output impedance (5Ω). The low input capacitance of the
amplifier reduces the capacitive load at the antenna, and the low impedance output driver is able
to drive the capacitive load of the cable. The voltage gain of the amplifier is close to 1 (0 dB), but
the insertion gain that is achieved when the buffer amplifier is inserted between antenna output
and cable may be much higher (35 dB). The actual value depends, of course, on antenna and
cable impedance.
The input of the amplifier is connected by an external 4.7 MΩ resistor to the bias voltage (pin 7,
SSO20) in order to achieve high input impedance and low noise voltage.
AM tuners in car radios usually use PIN diode attenuators at their input. These PIN diode atten-
uators attenuate the signal by reducing the input impedance of the tuner. Therefore, a series
resistor is used at the AM amplifier output in the standard application. This series resistor guar-
antees a well-defined source impedance for the radio tuner and protects the output of the AM
amplifier from short circuit by the PIN diode attenuator in the car radio.
4913J–AUDR–10/09
5
5 Page 
8. Test Circuit FM/AM
Figure 8-1. Common Emitter Configuration
VS 4.7Ω +
10 µF 100 nF
GND
4.7Ω +
10 µF 100 nF
FMOUT
22 pF
150 nH
AGCIN
470 nF 500 pF
22 pF 4.7 µF 2.2 µF
++
47Ω1)
ATR4251
www.DataSheet4U.com
5 kΩ
AMOUT1
1 nF
2.2 nF
270Ω
FMIN
2.2 nF
1 µH
22Ω 56Ω
2.2 nF
2.2 nF
(1) Output impedance 50Ω adjustment
4.7 MΩ
68Ω
++
1 µF
33 pF
10 µF
220 nF
15 nF
Cant
AMINP1
50Ω
AMAGCIN
50Ω
4913J–AUDR–10/09
11
11 Page | ||
| Páginas | Total 21 Páginas | |
| PDF Descargar | [ Datasheet ATR4251.PDF ] | |
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