MAXIM MAX2685

19-1493; Rev 1; 12/00
KIT
ATION
EVALU
E
L
B
A
AVAIL
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
The MAX2685 includes an LNA, LNA bypass switch,
downconverter mixer, and local-oscillator (LO) buffer.
The LNA has a low noise figure of 1.4dB, a high gain of
15dB, and an input third-order intercept point (IP3) of
-4dBm. The mixer has a noise figure of 13dB, a gain of
6dB, and an input IP3 of +7dBm. In addition, an LNA
bypass switch allows two levels of gain, reducing
power consumption when high gain is not needed.
The downconverter mixer has a single-ended RF input
port and differential IF output ports. Differential operation of the IF ports offers improved even-order harmonic
rejection and increased immunity to noise. An LO buffer
allows the LO port to be driven with only -8dBm of LO
power. The MAX2685 is offered in a space-saving 16pin QSOP package.
Applications
Cellular/Cordless Phones
Wireless Data
Features
♦ 800MHz to 1000MHz RF Frequency Range
♦ +2.7V to +5.5V Single-Supply Operation
♦ Integrated LNA + Mixer + LO Buffer
♦ Logic-Controlled LNA Bypass Switch Reduces
Supply Current
♦ LNA Performance (High/Low Gain)
Gain: +15dB/-12dB
NF: 1.4dB/12dB
Input IP3: -4dBm/+16dBm
♦ Mixer Performance (High/Low Gain)
Gain: 6dB/4.6dB
NF: 13dB/12dB
Input IP3: +7dBm/-1.5dBm
♦ Supply Current
8.5mA (High Gain)
3.8mA (Low Gain)
♦ <0.1µA Supply Current in Shutdown Mode
♦ 0.8µs Receiver Enable Time
Ordering Information
PART
MAX2685EEE
TEMP. RANGE
-40°C to +85°C
PIN-PACKAGE
16 QSOP
900MHz ISM-Band Radios
868MHz European ISM Band
Functional Diagram
Pin Configuration
LNAOUT MIXIN
TOP VIEW
LNA
GND
LNAIN
GND
GAIN
VCC
SHDN
LO
GND
1
16
2
15
3
14
4
13
5
12
6
MAX2685
11
7
10
8
9
QSOP
MIXER
LNAOUT
VCC
IFOUT+
LNAIN
IFOUT-
GND
MIXIN
LNA BYPASS
GAIN
GND
GND
IFOUT+
IFOUT-
MAX2685
VCC
SHDN
GND
POWER
MANAGEMENT
LO
BUFFER
LO
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX2685
General Description
The MAX2685’s low-noise amplifier (LNA) and downconverter mixer comprise the major blocks of an RF
front-end receiver. Optimized for 900MHz operation,
the device’s low noise figure, high gain, and high linearity make it ideal for cellular, cordless, and wireless data
applications. A low supply current of 8.5mA (high-gain
mode) and 3.8mA (low-gain mode) plus a low operating
supply voltage range of +2.7V to +5.5V make it suitable
for use in 3-cell NiCd or 1-cell lithium-ion (Li+) battery
applications. A low-power shutdown mode further
extends battery life by reducing supply current below
0.1µA.
MAX2685
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
ABSOLUTE MAXIMUM RATINGS
VCC to GND ..............................................................-0.3V to +6V
LNAIN Input Power (50Ω source)...................................+10dBm
LO Input Power (50Ω source) ........................................+10dBm
MIXIN Input Power (50Ω source) ...................................+10dBm
IFOUT+, IFOUT- to GND ..........................................-0.3V to +6V
LNAOUT to GND ......................................................-0.3V to +6V
GAIN, SHDN to GND..................................-0.3V to (VCC + 0.3V)
Continuous Power Dissipation (TA = +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)............667mW
Junction Temperature ......................................................+150°C
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
DC ELECTRICAL CHARACTERISTICS
(VCC = +2.7V to +5.5V, V SHDN = +2V, VGAIN = +2V, LNAIN = LNAOUT = MIXIN = LO = unconnected, IFOUT+ = IFOUT- = VCC, TA
= -40°C to +85°C. Typical values are at TA = +25°C and VCC = +3V, unless otherwise noted.)
PARAMETER
CONDITIONS
Supply Voltage Range
Operating Supply Current
MIN
TYP
2.7
MAX
UNITS
5.5
V
GAIN = VCC (Note 1)
8.5
14.1
GAIN = GND (Note 1)
3.8
6.4
0.1
1.0
Shutdown Supply Current
SHDN = GND (Note 1)
Logic Input Voltage High
GAIN, SHDN
Logic Input Voltage Low
GAIN, SHDN
Logic Input Current
V SHDN = VGAIN = 0 to 5.5V (Note 1)
2.0
mA
µA
V
±0.01
0.5
V
±1
µA
AC ELECTRICAL CHARACTERISTICS
(MAX2685 EV kit, VCC = V SHDN = +3V, fLNAIN = fMIXIN = 880MHz, fLO = 960MHz, PLNAIN = -30dBm, PLO = -8dBm, PMIXIN = -25dBm,
differential IFOUT operation, Zo = 50Ω, TA = +25°C, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
1000
MHz
LOW-NOISE AMPLIFIER (LNA)
RF Frequency Range (Note 2)
LNA Gain
LNA Gain Variation over Temperature
LNA Noise Figure
LNA Input IP3
LNA Input 1dB Compression
LNAOUT Port Return Loss
800
GAIN = VCC (Note 1)
13
15
16.2
GAIN = GND (Note 1)
-14
-12
-10.3
0.9
1.6
GAIN = VCC, TA = TMIN to TMAX (Note 3)
GAIN = VCC
1.4
GAIN = GND
12.2
GAIN = VCC (Note 4)
-4.1
GAIN = GND (Note 5)
+16.2
GAIN = VCC
-18.4
GAIN = VCC
-18.6
GAIN = GND
-11.3
dB
dB
dB
dBm
dBm
dB
DOWNCONVERTER MIXER
RF Frequency Range (Note 2)
Mixer Conversion Gain
2
800
1000
GAIN = VCC (Note 1)
4.7
6.1
7.0
GAIN = GND (Note 1)
2.5
4.6
6.0
_______________________________________________________________________________________
MHz
dB
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
(MAX2685 EV kit, VCC = V SHDN = +3V, fLNAIN = fMIXIN = 880MHz, fLO = 960MHz, PLNAIN = -30dBm, PLO = -8dBm, PMIXIN = -25dBm,
differential IFOUT operation, Zo = 50Ω, TA = +25°C, unless otherwise noted.)
PARAMETER
CONDITIONS
Mixer Conversion Gain Variation over
Temperature
MIN
GAIN = VCC, TA = TMIN to TMAX (Note 3)
Mixer Noise Figure (SSB)
Mixer Input IP3 (Note 6)
TYP
MAX
UNITS
1
2
dB
GAIN = VCC
13
GAIN = GND
12.1
GAIN = VCC
7
GAIN = GND
-1.5
LO Port Return Loss
dB
dBm
11
dB
53
dB
LO-to-MIXIN Isolation
31
dB
LNAOUT-to-MIXIN Isolation
28
dB
0.8
µs
SHDN = VCC or GND
LO-to-LNAIN Isolation
OVERALL SYSTEM
Receiver Enable Time
(Note 7)
Note 1: Performance at temperatures greater than or equal to +25°C are guaranteed by production test; performance at temperatures less than +25°C are guaranteed by design and characterization.
Note 2: This is the recommended operating frequency range.
Note 3: Maximum and minimum limits are guaranteed by design and device characterization and are not production tested.
Note 4: Two tones at 880MHz and 880.1MHz, -30dBm per tone.
Note 5: Two tones at 880MHz and 880.1MHz, -10dBm per tone.
Note 6: Two tones at 880MHz and 880.1MHz, -25dBm per tone.
Note 7: Time from SHDN = high, until the cascaded receive gain is within 1dB of its final value. Measured with 47pF blocking
capacitors on LNAIN and LNAOUT. Matching network removed from IFOUT output.
Typical Operating Characteristics
(MAX2685 EV kit, VCC = V SHDN = +3V, fLNAIN = fMIXIN = 880MHz, fLO = 960MHz, PLNAIN = -30dBm, PLO = -8dBm, PMIXIN = -25dBm,
differential IFOUT operation, Zo = 50Ω, TA = +25°C, unless otherwise noted.)
SHUTDOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
TA = +25°C
7
6
GAIN = HIGH
TA = -40°C
TA = +85°C
5
4
3
GAIN = LOW
TA = +25°C
2
TA = -40°C
8
TA = +85°C
5
4
3
2.5
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
5.0
5.5
0
-5
2
-10
TA = -40°C
0
0
5
TA = +25°C
1
1
TA = +85°C
TA = +25°C
10
7
6
TA = -40°C
GAIN = HIGH
15
GAIN (dB)
SUPPLY CURRENT (mA)
8
SHDN = GND
9
LNA GAIN vs. SUPPLY VOLTAGE
20
MAX2685 toc02
TA = +85°C
10
SHUTDOWN SUPPLY CURRENT (nA)
9
MAX2685 toc01
10
MAX2685 toc03
SUPPLY CURRENT vs. SUPPLY VOLTAGE
2.5
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
5.0
TA = +25°C T = -40°C
A
GAIN = LOW TA = +85°C
-15
5.5
2.5
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
_______________________________________________________________________________________
3
MAX2685
AC ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics (continued)
(MAX2685 EV kit, VCC = V SHDN = +3V, fLNAIN = fMIXIN = 880MHz, fLO = 960MHz, PLNAIN = -30dBm, PLO = -8dBm, PMIXIN = -25dBm,
differential IFOUT operation, Zo = 50Ω, TA = +25°C, unless otherwise noted.)
0
-5
-10
-5
TA = -40°C
3.0
3.5
4.0
4.5
5.0
5.5
LNA REVERSE ISOLATION vs. FREQUENCY
GAIN = LOW
-15
-20
-25
-30
-35
GAIN = HIGH
0
MAX2685 toc08
-7
-8
GAIN = HIGH
-9
-10
-11
-12
-2
GAIN = LOW
-4
-6
-8
GAIN = LOW
-10
-12
-14
-13
-16
-14
-18
GAIN = HIGH
-20
800
840
880
920
960
1000
800
840
880
FREQUENCY (MHz)
FREQUENCY (MHz)
LNA S11 vs. FREQUENCY
(800MHz to 1000MHz UNMATCHED)
MAX2685 toc10
TA = -40°C
GAIN = HIGH
5.0
HI
LO
LOW
GAIN =
TA = -40°C
TA = +85°C
4.5
1000
TA = +85°C
6
4
2 TA = +85°C
TA = +25°C GAIN = HIGH
TA = -40°C
0
TA = +25°C
-2
GAIN = LOW
-4
TA = +25°C
4.0
-6
TA = +85°C
3.5
800
8
INPUT IP3 (dBm)
GAIN (dB)
GAIN = LOW
1000 800
10
6.0
5.5
960
MIXER INPUT IP3 vs. SUPPLY VOLTAGE
MIXER GAIN vs. SUPPLY VOLTAGE
7.0
6.5
920
FREQUENCY (MHz)
TA = +25°C
4
MAX2685 toc06
LNA OUTPUT RETURN LOSS vs. FREQUENCY
-15
-40
800 825 850 875 900 925 950 975 1000
1000
GAIN = HIGH
1.0
870 880 890 900 910 920 930 940 950 960
LNA INPUT RETURN LOSS vs. FREQUENCY
-6
INPUT RETURN LOSS (dB)
-10
1.3
FREQUENCY (MHz)
-5
MAX2685 toc07
-5
1.4
FREQUENCY (MHz)
SUPPLY VOLTAGE (V)
0
1.5
1.1
-20
800 825 850 875 900 925 950 975 1000
OUTPUT RETURN LOSS (dB)
2.5
1.6
1.2
GAIN = LOW
-15
-10
1.7
MAX2685 toc09
TA = +85°C
MAX2685 toc11
GAIN = HIGH
5
MAX2685 toc12
5
TA = +25°C
1.8
NOISE FIGURE (dB)
GAIN (dB)
10
0
1.9
10
TA = +85°C
TA = -40°C
GAIN = HIGH
15
2.0
MAX2685 toc05
15
INPUT IP3 (dBm)
20
MAX2685 toc04
TA = +25°C
GAIN = LOW
LNA NOISE FIGURE vs. FREQUENCY
LNA GAIN vs. FREQUENCY
LNA INPUT IP3 vs. SUPPLY VOLTAGE
20
REVERSE ISOLATION (dB)
MAX2685
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
TA = -40°C
-8
3.0
-10
2.5
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
5.0
5.5
2.5
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
_______________________________________________________________________________________
5.0
5.5
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
MIXER GAIN vs. IF FREQUENCY
MIXER GAIN vs. LO POWER
GAIN = HIGH
5
0
-2
GAIN (dB)
GAIN (dB)
GAIN = HIGH
6
2
GAIN = LOW
-4
-6
GAIN = LOW
4
3
2
-8
BANDWIDTH DETERMINED BY
DIFFERENTIAL-TO-SINGLE-ENDED
CONVERTER CIRCUIT
-10
-12
60
70
80
90
1
0
100
-15
-10
IF FREQUENCY (MHz)
13.0
GAIN = LOW
12.5
12.0
GAIN = HIGH
11.5
-6
-7
RETURN LOSS (dB)
13.5
MAX2685 toc16
14.0
0
LO PORT RETURN LOSS vs. FREQUENCY
-5
MAX2685 toc15
14.5
-5
LO POWER (dBm)
MIXER NOISE FIGURE vs. LO POWER
15.0
NOISE FIGURE (dB)
MAX2685 toc14
6
4
7
MAX2685 toc13
8
-8
-9
-10
-11
-12
11.0
-13
10.5
-14
-15
10.0
-15
-10
-5
LO POWER (dBm)
0
750
800
850
900
950
1000 1050 1100
FREQUENCY (MHz)
_______________________________________________________________________________________
5
MAX2685
Typical Operating Characteristics (continued)
(MAX2685 EV kit, VCC = V SHDN = +3V, fLNAIN = fMIXIN = 880MHz, fLO = 960MHz, PLNAIN = -30dBm, PLO = -8dBm, PMIXIN = -25dBm,
differential IFOUT operation, Zo = 50Ω, TA = +25°C, unless otherwise noted.)
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
MAX2685
Pin Description
PIN
NAME
FUNCTION
1, 3, 8,
11, 12, 14
GND
2
LNAIN
RF Input to LNA and LNA Bypass Switch. Requires an external matching network and a series
DC-blocking capacitor.
4
GAIN
Gain Control Logic-Level Input. Drive high to enable the LNA, open the LNA bypass switch, and
increase the receiver’s gain. Drive low to disable the LNA, close the LNA bypass switch, and reduce
the receiver’s gain.
5, 15
VCC
Supply Voltage. Bypass VCC to GND at each pin with a 47pF capacitor as close to the pin as possible.
6
SHDN
7
LO
Local-Oscillator Input to Downconverter Mixer. Requires a series DC-blocking capacitor and an impedance-setting resistor (typically 75Ω to ground).
9
IFOUT-
Inverting Side to Downconverter Mixer’s Differential Open-Collector IF Output. Requires a pull-up inductor to VCC for proper biasing, as well as a matching network to ensure optimum output power.
10
IFOUT+
Noninverting Side of Downconverter Mixer’s Differential Open-Collector IF Output. Requires a pull-up
inductor to VCC for proper biasing, as well as a matching network to ensure optimum output power.
13
MIXIN
16
LNAOUT
Ground. Connect to ground plane with a low-inductance connection.
Shutdown Control Logic-Level Input. Drive high or connect to VCC for normal operation. Drive low to
place the device in low-power shutdown mode.
RF Input to Downconverter Mixer. Requires an external matching network and series DC-blocking
capacitor.
LNA Output. Internally matched to 50Ω. LNAOUT has an internal blocking capacitor.
300pF
1
LNA
INPUT
880MHz
0.1µF
12nH
2
3
GND
LNAOUT
VCC
LNAIN
15
VCC
47pF
GND
GND
GAIN
MIXIN
14
MIXER
INPUT
880MHz
300pF
GAINCONTROL
INPUT
4
5
VCC
VCC
47pF
6
SHUTDOWN
INPUT
SHDN
13
3.3pF
MAX2685
GND
GND
6pF
12
11
12pF
330pF
LO
INPUT
960MHz
LNA
OUTPUT
16
7
IFOUT+
10
LO
R*
75Ω
8
GND
IFOUT-
820nH
680nH
IF
OUTPUT
80MHz
VCC
9
1000pF
6pF
*OPTIONAL FOR BROADBAND MATCH.
Figure 1. Typical Operating Circuit
6
_______________________________________________________________________________________
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
The MAX2685 consists of five major components: a
low-noise amplifier (LNA), an LNA bypass switch, a
downconverter mixer, a local-oscillator (LO) buffer, and
a power-management block.
When a large input signal is present, enable the LNA
bypass function to increase linearity and reduce supply
current. Set GAIN low to enable the LNA bypass function.
Low-Noise Amplifier (LNA)
The LNA is a wideband, single-ended cascode amplifier that operates over a wide range of frequencies. The
input of the LNA (LNAIN) requires an appropriate
matching network and a DC-blocking capacitor. The
typical operating circuit shown in Figure 1 is optimized
for frequencies around 880MHz, requiring only a 0.1µF
capacitor in series with a 12nH inductor. See Table 1
for the LNA “S” parameters for matching to other frequencies.
The output of the LNA (LNAOUT) is internally biased to
VCC. It is internally matched to 50Ω and incorporates
an internal DC-blocking capacitor.
Receive Mixer
The downconverter mixer is a wideband, single-balanced design with a low noise figure and high linearity.
The RF signal at the MIXIN port is mixed with the signal
at the LO port, and is downconverted to an IF frequency at the differential IF port.
RF Input
The MIXIN input requires a simple external matching
network and a series DC-blocking capacitor. See
Figure 1 for a matching network example, optimized for
880MHz operation. Table 2 lists mixer “S” parameters
for matching to other frequencies.
Table 1. LNA Typical S-Parameters (VCC = +3V, TA = +25°C)
FREQUENCY
(MHz)
|S11|
MAG
S11
PHASE
(degrees)
|S21|
|S12|
MAG
S21
PHASE
(degrees)
|S22|
MAG
S12
PHASE
(degrees)
MAG
S22
PHASE
(degrees)
High-Gain Mode (GAIN = VCC)
800
0.761
-64.5
4.98
177.9
0.018
-163.7
0.376
-107.3
840
0.753
-68.6
5.06
167.2
0.022
-167.1
0.264
-107.0
880
0.747
-73.2
5.07
156.6
0.026
-171.3
0.172
-94.6
920
0.733
-78.0
4.91
146.6
0.030
-175.7
0.149
-62.9
960
0.719
-82.8
4.68
137.7
0.035
178.0
0.200
-42.4
1000
0.693
-87.5
4.40
130.3
0.039
171.0
0.263
-38.8
Low-Gain Mode (GAIN = GND)
800
0.625
-45.6
0.188
73.0
0.191
71.9
0.483
-91.3
840
0.621
-48.1
0.195
65.5
0.198
64.2
0.423
-91.3
880
0.619
-50.9
0.199
58.1
0.201
56.7
0.370
-89.9
920
0.611
-53.3
0.200
51.6
0.202
50.3
0.337
-86.1
960
0.608
-55.5
0.200
46.1
0.201
44.7
0.322
-80.9
1000
0.607
-57.5
0.200
41.2
0.200
40.0
0.317
-76.7
_______________________________________________________________________________________
7
MAX2685
LNA Bypass Switch
and Gain Control
Detailed Description
MAX2685
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
Table 2. Mixer Typical S-Parameters (VCC = +3V, TA = +25°C)
RF FREQUENCY
(MHz)
S11
MAG
|S11|
PHASE
(degrees)
IF FREQUENCY
(MHz)
MAG (IFOUT+
Port Only)
S22 PHASE
(IFOUT+ Port
Only)
(degrees)
|S22|
High-Gain Mode (GAIN = VCC)
800
0.355
152.7
10
0.996
-0.4
840
0.352
153.7
40
0.994
-1.8
880
0.351
154.5
80
0.993
-3.2
920
0.349
155.8
110
0.989
-4.2
960
0.352
156.2
170
0.988
-6.2
1000
0.353
156.9
240
0.983
-8.0
800
0.275
142.8
10
0.996
-0.5
840
0.268
144.1
40
0.995
-1.8
880
0.262
145.5
80
0.993
-3.2
920
0.255
147.7
110
0.989
-4.2
960
0.254
149.0
170
0.987
-6.2
1000
0.245
156.9
240
0.982
-7.9
Low-Gain Mode (GAIN = GND)
8
_______________________________________________________________________________________
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
IF Output Port
The mixer’s downconverted output appears on the differential IFOUT+ and IFOUT- pins. The differential output can be converted to a single-ended output, as
shown in the MAX2685 evaluation kit (EV kit). Refer to
the Detailed Description in the MAX2685 EV kit data
sheet.
Applications Information
Layout Considerations
A properly designed PC board is an essential part of
any RF/microwave circuit. Note the IC’s high-frequency
inputs and outputs, and be sure to decouple the DC
supply and control pins.
For power-supply traces and connections, a star topology works well. Each VCC node in the circuit has its
own path to the central VCC node and a decoupling
capacitor that provides a low impedance at the RF frequency of interest. The central VCC also has a large
decoupling capacitor. This provides good isolation
between the different sections of the MAX2685.
Shutdown
Drive SHDN low to disable all device functions and
place the MAX2685 in low-power shutdown mode.
Drive SHDN high or connect it to VCC to enable all
device functions.
Chip Information
TRANSISTOR COUNT: 295
_______________________________________________________________________________________
9
MAX2685
Local-Oscillator Input
The LO port is the high-impedance input of the localoscillator buffer. It requires a series DC-blocking
capacitor and a shunt resistor to ground to set the input
impedance. See the Typical Operating Characteristics
for a graph of LO Port Return Loss vs. Frequency.
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
QSOP.EPS
MAX2685
Package Information
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implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
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