MAX12000 Datasheet, PDF (6/9 Pages) Maxim Integrated Products – 1575MHz GPS Front-End Amplifier

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 sup-
presses gain variation with changes in temperature and
supply voltage. At the input, an integrated DC-blocking
capacitor and matching network are intentionally omit-
ted to allow selection of external components to opti-
mize 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 sys-
tem 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 voltage 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 iso-
late the input signals from the output signals as much
as possible. For improved noise figure, keep the con-
nection 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, con-
nect pin 7 directly to the ground plane with a very short
printed circuit board trace. Good grounding is critical
for this device. The backside ground plane should be
as close as possible. See Figure 1 for printed circuit
board layout.
Typical Operating Circuit
SAW
PINNING:
1 = LNA 2 INPUT
2 = GND
3 = GND
4 = EXTCAP/ALT_V_CC
10
1
4
5 = LNA 2 OUTPUT/V_CC
6 = LNA 1 INPUT
7 = GAIN SETTING
V_CC
REG
V_CC
REG
8 = GND
9 = GND
10 = LNA 1 OUTPUT
MATCHING
NETWORK/
DC BLOCK
6
LNA 1
8/9 EXPOSED PADDLE
MAX12000
LNA 2
2/3
5 _OUTPUT
7 ^{GAIN SETTING}
OPEN = 31.4dB GAIN
SHORT TO GROUND = 28dB GAIN
6 _______________________________________________________________________________________