Rainbow Electronics MAX2685 User Manual

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.

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 900MHz ISM-Band Radios

Wireless Data 868MHz European ISM Band

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

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier

and Downconverter Mixer

________________________________________________________________ Maxim Integrated Products 1

Functional Diagram

19-1493; Rev 1; 12/00

PART

MAX2685EEE -40°C to +85°C

TEMP. RANGE PIN-PACKAGE

16 QSOP

EVALUATION KIT

AVAILABLE

Pin Configuration

Ordering Information

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.

MIXIN

LNAOUT

LNA

LNA BYPASS

MAX2685

POWER

MIXER

BUFFER

TOP VIEW

GND

LNAIN

GND

GAIN

SHDN

GND

MAX2685

QSOP

LNAOUT

GND

MIXIN

GND

IFOUT+

IFOUT-

LNAIN

GAIN

SHDN

GND

MANAGEMENT

IFOUT+

IFOUT-

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier and Downconverter Mixer

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

DC ELECTRICAL CHARACTERISTICS

(VCC= +2.7V to +5.5V, V

SHDN

= +2V, V

GAIN

= +2V, LNAIN = LNAOUT = MIXIN = LO = unconnected, IFOUT+ = IFOUT- = VCC, T

= -40°C to +85°C. Typical values are at TA= +25°C and VCC= +3V, unless otherwise noted.)

AC ELECTRICAL CHARACTERISTICS

(MAX2685 EV kit, VCC= V

SHDN

= +3V, f

LNAIN

= f

MIXIN

= 880MHz, fLO= 960MHz, P

LNAIN

= -30dBm, PLO= -8dBm, P

MIXIN

= -25dBm,

differential IFOUT operation, Z

= 50Ω, TA= +25°C, unless otherwise noted.)

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.

VCCto 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 (V

+ 0.3V)

Continuous Power Dissipation (T

= +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

SHDN = GND (Note 1)

GAIN = V

(Note 1)

GAIN, SHDN GAIN, SHDN

SHDN

= V

GAIN

= 0 to 5.5V (Note 1)

CONDITIONS

µA0.1 1.0Shutdown Supply Current

8.5 14.1

V2.7 5.5Supply Voltage Range

V2.0Logic Input Voltage High

V0.5Logic Input Voltage Low

µA±0.01 ±1Logic Input Current

UNITSMIN TYP MAXPARAMETER

GAIN = GND (Note 1)

3.8 6.4

Operating Supply Current

GAIN = GND (Note 1)

-14 -12 -10.3

GAIN = VCC, TA= T

MIN

to T

MAX

(Note 3)

LNA Gain

GAIN = V

(Note 1)

GAIN = V

GAIN = GND

GAIN = V

CONDITIONS

dB0.9 1.6LNA Gain Variation over Temperature

13 15 16.2

1.4 dB

12.2

LNA Noise Figure

-18.4LNA Input 1dB Compression

UNITSMIN TYP MAXPARAMETER

MHz800 1000RF Frequency Range (Note 2)

GAIN = V

GAIN = GND

dBm

-18.6 dB

-11.3

LNAOUT Port Return Loss

MHz800 1000RF Frequency Range (Note 2)

GAIN = GND (Note 1)

2.5 4.6 6.0

Mixer Conversion Gain

GAIN = V

(Note 1) 4.7 6.1 7.0

GAIN = V

(Note 4)

GAIN = GND (Note 5)

-4.1 dBm

+16.2

LNA Input IP3

LOW-NOISE AMPLIFIER (LNA)

DOWNCONVERTER MIXER

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier

and Downconverter Mixer

_______________________________________________________________________________________ 3

AC ELECTRICAL CHARACTERISTICS (continued)

(MAX2685 EV kit, VCC= V

SHDN

= +3V, f

LNAIN

= f

MIXIN

= 880MHz, fLO= 960MHz, P

LNAIN

= -30dBm, PLO= -8dBm, P

MIXIN

= -25dBm,

differential IFOUT operation, Z

= 50Ω, TA= +25°C, unless otherwise noted.)

Note 1: Performance at temperatures greater than or equal to +25°C are guaranteed by production test; performance at tempera-

tures 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, f

LNAIN

= f

MIXIN

= 880MHz, fLO= 960MHz, P

LNAIN

= -30dBm, PLO= -8dBm, P

MIXIN

= -25dBm,

differential IFOUT operation, Z

= 50Ω, TA= +25°C, unless otherwise noted.)

2.5 3.53.0 4.0 4.5 5.0 5.5

SUPPLY CURRENT vs. SUPPLY VOLTAGE

MAX2685 toc01

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (mA)

GAIN = HIGH

GAIN = LOW

TA = +85°C

TA = +25°C

TA = -40°C

TA = +85°C

TA = +25°C

TA = -40°C

2.5 3.53.0 4.0 4.5 5.0 5.5

SHUTDOWN SUPPLY CURRENT

vs. SUPPLY VOLTAGE

MAX2685 toc02

SUPPLY VOLTAGE (V)

SHUTDOWN SUPPLY CURRENT (nA)

SHDN = GND

TA = -40°C

TA = +25°C

TA = +85°C

-15

-5

-10

2.5 3.5 4.03.0 4.5 5.0 5.5

LNA GAIN vs. SUPPLY VOLTAGE

MAX2685 toc03

SUPPLY VOLTAGE (V)

GAIN (dB)

GAIN = HIGH

GAIN = LOW

TA = +85°C

TA = +25°C

TA = -40°C

TA = +85°C

TA = +25°C

TA = -40°C

GAIN = GND

12.1

Mixer Noise Figure (SSB)

GAIN = VCC, TA= T

MIN

to T

MAX

(Note 3)

GAIN = V

dB12

Mixer Conversion Gain Variation over Temperature

(Note 7)

GAIN = V

GAIN = GND

CONDITIONS

µs0.8Receiver Enable Time

dBm

-1.5

Mixer Input IP3 (Note 6)

11LO Port Return Loss

UNITSMIN TYP MAXPARAMETER

SHDN = VCCor GND

53LO-to-LNAIN Isolation

28LNAOUT-to-MIXIN Isolation

31LO-to-MIXIN Isolation

OVERALL SYSTEM

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier and Downconverter Mixer

4 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(MAX2685 EV kit, VCC= V

SHDN

= +3V, f

LNAIN

= f

MIXIN

= 880MHz, fLO= 960MHz, P

LNAIN

= -30dBm, PLO= -8dBm, P

MIXIN

= -25dBm,

differential IFOUT operation, Z

= 50Ω, TA= +25°C, unless otherwise noted.)

-10

-5

2.5 3.5 4.03.0 4.5 5.0 5.5

LNA INPUT IP3 vs. SUPPLY VOLTAGE

MAX2685 toc04

SUPPLY VOLTAGE (V)

INPUT IP3 (dBm)

GAIN = HIGH

GAIN = LOW

TA = +85°C

TA = +25°C

TA = -40°C

TA = +85°C

TA = +25°C

TA = -40°C

-20

-10

-15

-5

800 850 875825 900 925 950 975 1000

LNA GAIN vs. FREQUENCY

MAX2685 toc05

FREQUENCY (MHz)

GAIN (dB)

GAIN = HIGH

GAIN = LOW

1.0

1.2

1.1

1.4

1.3

1.6

1.5

1.7

1.9

1.8

2.0

870 890 900 910880 920 930 940 950 960

LNA NOISE FIGURE vs. FREQUENCY

MAX2685 toc06

FREQUENCY (MHz)

NOISE FIGURE (dB)

-40

-30

-35

-20

-25

-15

-10

-5

800 850 875825 900 925 950 975 1000

LNA REVERSE ISOLATION vs. FREQUENCY

MAX2685 toc07

FREQUENCY (MHz)

REVERSE ISOLATION (dB)

GAIN = HIGH

GAIN = LOW

LNA S11 vs. FREQUENCY

(800MHz to 1000MHz UNMATCHED)

MAX2685 toc10

GAIN = HIGH

GAIN = LOW

800

1000

800

1000

-15

-12

-13

-14

-11

-10

-9

-8

-7

-6

-5

800 880840 920 960 1000

LNA INPUT RETURN LOSS vs. FREQUENCY

MAX2685 toc08

FREQUENCY (MHz)

INPUT RETURN LOSS (dB)

GAIN = HIGH

GAIN = LOW

-20

-14

-16

-18

-12

-10

-8

-6

-4

-2

800 880840 920 960 1000

LNA OUTPUT RETURN LOSS vs. FREQUENCY

MAX2685 toc09

FREQUENCY (MHz)

OUTPUT RETURN LOSS (dB)

GAIN = HIGH

GAIN = LOW

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

2.5 3.53.0 4.0 4.5 5.0 5.5

MIXER GAIN vs. SUPPLY VOLTAGE

MAX2685 toc11

SUPPLY VOLTAGE (V)

GAIN (dB)

TA = -40°C

TA = +85°C

TA = +25°C

TA = -40°C

GAIN = HIGH

GAIN = LOW

-10

-4

-6

-8

-2

2.5 3.53.0 4.0 4.5 5.0 5.5

MIXER INPUT IP3 vs. SUPPLY VOLTAGE

MAX2685 toc12

SUPPLY VOLTAGE (V)

INPUT IP3 (dBm)

GAIN = HIGH

GAIN = LOW

TA = +85°C

TA = +25°C

TA = -40°C

TA = +85°C

TA = +25°C

TA = -40°C

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier

and Downconverter Mixer

_______________________________________________________________________________________ 5

Typical Operating Characteristics (continued)

(MAX2685 EV kit, VCC= V

SHDN

= +3V, f

LNAIN

= f

MIXIN

= 880MHz, fLO= 960MHz, P

LNAIN

= -30dBm, PLO= -8dBm, P

MIXIN

= -25dBm,

differential IFOUT operation, Z

= 50Ω, TA= +25°C, unless otherwise noted.)

-12

-6

-8

-10

-4

-2

60 70 80 90 100

MIXER GAIN vs. IF FREQUENCY

MAX2685 toc13

IF FREQUENCY (MHz)

GAIN (dB)

BANDWIDTH DETERMINED BY DIFFERENTIAL-TO-SINGLE-ENDED CONVERTER CIRCUIT

GAIN = HIGH

GAIN = LOW

-15 -10 -5 0

MIXER GAIN vs. LO POWER

MAX2685 toc14

LO POWER (dBm)

GAIN (dB)

GAIN = HIGH

GAIN = LOW

10.0

11.5

11.0

10.5

12.0

12.5

13.0

13.5

14.0

14.5

15.0

-15 -10 -5 0

MIXER NOISE FIGURE vs. LO POWER

MAX2685 toc15

LO POWER (dBm)

NOISE FIGURE (dB)

GAIN = HIGH

GAIN = LOW

-15

-12

-13

-14

-10

-11

-6

-7

-8

-9

-5

750 800 850 900 950 1000 1050 1100

LO PORT RETURN LOSS vs. FREQUENCY

MAX2685 toc16

FREQUENCY (MHz)

RETURN LOSS (dB)

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier and Downconverter Mixer

6 _______________________________________________________________________________________

Pin Description

NAME FUNCTION

1, 3, 8,

11, 12, 14

GND Ground. Connect to ground plane with a low-inductance connection.

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 V

Supply Voltage. Bypass VCCto GND at each pin with a 47pF capacitor as close to the pin as possible.

SHDN

Shutdown Control Logic-Level Input. Drive high or connect to VCCfor normal operation. Drive low to place the device in low-power shutdown mode.

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 VCCfor 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 VCCfor proper biasing, as well as a matching network to ensure optimum output power.

13 MIXIN

RF Input to Downconverter Mixer. Requires an external matching network and series DC-blocking capacitor.

16 LNAOUT LNA Output. Internally matched to 50Ω. LNAOUT has an internal blocking capacitor.

Figure 1. Typical Operating Circuit

LNA

INPUT

880MHz

SHUTDOWN

INPUT

960MHz

3.3pF

680nH

47pF

300pF

6pF

300pF

6pF

820nH

12pF

LNA OUTPUT

MIXER INPUT 880MHz

IF OUTPUT 80MHz

1000pF

GAIN-

CONTROL

INPUT

0.1µF

GND LNAOUT

12nH

47pF

330pF

75Ω

LNAIN

GND

GAIN

SHDN

GND

MAX2685

GND

MIXIN

GND

IFOUT+

IFOUT-

*OPTIONAL FOR BROADBAND MATCH.

Detailed Description

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.

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.

LNA Bypass Switch

and Gain Control

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.

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.

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier

and Downconverter Mixer

_______________________________________________________________________________________ 7

Table 1. LNA Typical S-Parameters (VCC= +3V, TA= +25°C)

S11

MAG

FREQUENCY

(MHz)

800 0.761

S11

PHASE

(degrees)

-64.5

S21

MAG

S21

PHASE

(degrees)

177.94.98

S12

MAG

S12

PHASE

(degrees)

-163.7

S22

MAG

S22

PHASE

(degrees)

-107.30.3760.018

-68.6 167.25.06 -167.1840 0.753 -107.00.2640.022

-73.2 156.65.07 -171.3880 0.747 -94.60.1720.026

-78.0 146.64.91 -175.7920 0.733 -62.90.1490.030

-82.8 137.74.68 178.0960 0.719 -42.40.2000.035

-87.5 130.34.40 171.01000 0.693 -38.80.2630.039

-45.6 73.00.188 71.9800 0.625 -91.30.4830.191

-48.1 65.50.195 64.2840 0.621 -91.30.4230.198

-50.9 58.10.199 56.7880 0.619 -89.90.3700.201

-53.3 51.60.200 50.3920 0.611 -86.10.3370.202

-55.5 46.10.200 44.7960 0.608 -80.90.3220.201

-57.5 41.20.200 40.01000 0.607 -76.70.3170.200

High-Gain Mode (GAIN = VCC)

Low-Gain Mode (GAIN = GND)

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier and Downconverter Mixer

8 _______________________________________________________________________________________

Table 2. Mixer Typical S-Parameters (VCC= +3V, TA= +25°C)

S11

MAG

S11

PHASE

(degrees)

152.7

S22

MAG (IFOUT+

Port Only)

S22 PHASE

(IFOUT+ Port

Only)

(degrees)

-0.4

IF FREQUENCY

(MHz)

RF FREQUENCY

(MHz)

0.996

-1.840

800 0.355

0.994

-3.280 0.993

153.7

-4.2110 0.989

840 0.352

-6.2170 0.988

154.5

-8.0240 0.983

880 0.351

155.8920 0.349

156.2960 0.352

156.91000 0.353

142.8 -0.510 0.996

-1.840

800 0.275

0.995

-3.280 0.993

144.1

-4.2110 0.989

840 0.268

-6.2170 0.987

145.5

-7.9240 0.982

880 0.262

147.7920 0.255

149.0960 0.254

156.91000 0.245

High-Gain Mode (GAIN = VCC)

Low-Gain Mode (GAIN = GND)

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier

and Downconverter Mixer

_______________________________________________________________________________________ 9

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.

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.

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 VCCto enable all device functions.

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 VCCnode in the circuit has its own path to the central VCCnode and a decoupling capacitor that provides a low impedance at the RF frequency of interest. The central VCCalso has a large decoupling capacitor. This provides good isolation between the different sections of the MAX2685.

Chip Information

TRANSISTOR COUNT: 295

MAX2685

Low-Cost, 900MHz, Low-Noise Amplifier and Downconverter Mixer

Package Information

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

10 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

QSOP.EPS

Rainbow Electronics MAX2685 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual