MAX12000 Просмотр технического описания (PDF) - Maxim Integrated
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MAX12000 Datasheet PDF : 9 Pages
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MAX12000
1575MHz GPS Front-End Amplifier
Detailed Description
The MAX12000 IC contains two LNA stages tuned for use
at 1575MHz.
AMP 1
AMP 1 has an internal load that limits the bandwidth and
provides a 50Ω output impedance through a DC-blocking
capacitor. The internal biasing for AMP 1 suppresses gain
variation with changes in temperature and supply voltage.
At the input, an integrated DC-blocking capacitor and
matching network are intentionally omitted to allow selec-
tion of external components to optimize for noise or gain.
AMP 2 with Gain Step
The output of AMP 2 has the dual role of providing both
the RF output drive and receiving the DC power supply
through a single cable. Both the input and output ports
of AMP 2 are internally matched to 50Ω impedance at
1575MHz. A 3.4dB gain switch can be used to adjust
the gain for different applications. The gain-select pin is
connected to an inverter with an internal pullup resistor.
Hence, the gain-select pin is set by default to high-gain
mode. Shorting the gain-select pin to ground sets the gain
stage to a 3.4dB lower gain. As with AMP 1, AMP 2 has an
internal load that limits the bandwidth, and the amplifier’s
internal biasing suppresses gain variation with changes in
temperature and supply voltage.
Supply
The IC power can be supplied from the navigation system
through the RF cable (pin 5). An integrated filter circuit is
connected to the output of LNA 2 to separate the supply
voltage from the GPS signal. Alternatively, the supply volt-
age can be applied to the external capacitor pin (pin 4).
Layout Considerations
For best performance, carefully lay out the printed cir-
cuit board using high-frequency techniques. Use con-
trolled-impedance transmission lines to interface with the
MAX12000 high-speed inputs and outputs and isolate the
input signals from the output signals as much as possi-
ble. For improved noise figure, keep the connection to
the input of LNA 1 as short as possible. A power-supply
decoupling capacitor should be placed very close to pin
4 and connected directly to a ground plane. If low-gain
selection for LNA 2 is required, connect pin 7 directly to
the ground plane with a very short printed circuit board
trace. Good grounding is critical for this device. The back-
side ground plane should be as close as possible. See
Figure 1 for printed circuit board layout.
Typical Operating Circuit
PINNING:
1 = LNA 2 INPUT
2 = GND
3 = GND
4 = EXTCAP/ALT_VCC
5 = LNA 2 OUTPUT/VCC
6 = LNA 1 INPUT
7 = GAIN SETTING
VCC
REG
8 = GND
9 = GND
10 = LNA 1 OUTPUT
SAW
10
1
VCC
REG
MATCHING
NETWORK/
DC BLOCK
6
LNA 1
8/9 EXPOSED PADDLE
MAX12000
LNA 2
2/3
4
5 OUTPUT
7
GAIN SETTING
OPEN = 31.4dB GAIN
SHORT TO GROUND = 28dB GAIN
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