Detailed Description
The MAX2678 IC contains two LNA stages tuned for use
at 1575MHz.
Amplier 1
Amplifier 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 selection of external components to optimize for
noise or gain.
Amplier 2 with Gain Step
The output of Amplifier 2 (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. For maximum
gain, the GAIN_SELECT pin should be left open. 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 tem-
perature and supply voltage.
Alternate Supply (ALT_V
CC
)
The IC power can be supplied from the navigation system
through RFOUT2/V
CC
(pin 5). An integrated filter is con-
nected to the output of AMP 2 to separate the supply volt-
age from the GPS signal. Alternatively, the supply voltage
can be applied to the EXTCAP/ALT_V
CC
pin (external
capacitor pin 4).
Layout Considerations
For best performance, carefully lay out the PCB using
high-frequency techniques. Use controlled-impedance
transmission lines to interface with the MAX2678 high-
speed inputs and outputs and isolate the input sig-
nals from the output signals as much as possible. 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 con-
nected 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 PCB trace. Good grounding is
critical for this device. The backside ground plane should
be as close as possible.
Visit www.maximintegrated.com/evkitsoftware to
download the latest MAX2678 EV kit BOM, schematic
and PCB layout diagrams, including Gerber data.
MAX2678 GPS/GNSS Front-End Amplier
www.maximintegrated.com
Maxim Integrated
│
7
1
RFIN2RFOUT1
4
5
6
7
2, 3, 8, 9
10
SAW
V
CC
REG
V
CC
REG
AMP 2
ALT_V
CC
RFOUT2/V
CC
GAIN_SELECT
EXPOSED PAD
AMP 1
MAX2678
RFIN1
EXTCAP/ALT_V
CC
Typical Operating Circuit
