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19-4765; Rev 0; 7/98
622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers
General Description
The MAX3760 is a transimpedance preamplifier for 622Mbps ATM applications. It operates from a single +5V supply and typically consumes only 100mW power. The preamplifier converts a small photodiode current to a differential voltage. A DC cancellation circuit provides a true differential output swing over a wide range of input current levels, thus reducing pulsewidth distortion. 6.5k transimpedance gain and 560MHz bandwidth, combined with low 73nA input-referred noise, provide -31.5dBm typical sensitivity in 1300nm receivers. The circuit accepts a 1mAp-p input current, resulting in a typical optical overload of -3dBm. The device operates over an extended temperature range of -40C to +85C. The MAX3760 is internally compensated and requires few external components. In die form it includes a space-saving filter connection, which provides positive bias for the photodiode through a 1k resistor to VCC. These features, combined with the die aspect ratio and dimensioning, allow the MAX3760 to assemble easily into a TO-style header with a photodiode. The MAX3760 is designed to be used with either the MAX3761 or the MAX3762 limiting-amplifier ICs. When combined with a photodiode, the chipset forms a complete 5V, 622Mbps receiver. The MAX3760 is available in die form and in an 8-pin SO package.
KIT ATION EVALU BLE AVAILA
____________________________Features
o 73nA RMS Input-Referred Noise o 560MHz Bandwidth o 1mA Peak Input Current o 6.5k Gain o Operation from -40C to +85C o 100mW Typical Power Consumption o Single +5V Supply
MAX3760
Ordering Information
PART MAX3760ESA MAX3760E/D TEMP. RANGE -40C to +85C -40C to +85C PIN-PACKAGE 8 SO Dice*
*Dice are designed to operate over a -40C to +100C junction temperature (T j ) range, but are tested and guaranteed at TA = +25C.
________________________Applications
622Mbps ATM LAN Optical Receivers 622Mbps WAN Optical Receivers
Typical Application Circuit
+5V 0.01F VCC (FILTER) 1k
Pin Configuration
TOP VIEW
100pF INREF
MAX3760
0.01F OUT+ 100 (OPTIONAL) OUT-
MAX3761 MAX3762
LIMITING AMPLIFIER
VCC 1 IN 2
8
COMP OUT+ OUTGND
GND COMP* IN
MAX3760
7 6 5
INREF 3 GND 4
0.01F
SO
( ) INDICATE PINS AVAILABLE ONLY ON THE DIE. *NOT CONNECTED
________________________________________________________________ Maxim Integrated Products
1
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622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers MAX3760
ABSOLUTE MAXIMUM RATINGS
VCC ........................................................................-0.5V to +7.0V Continuous Current IN, FILTER ..........................................................-5mA to +5mA OUT+, OUT-...................................................-25mA to +25mA Voltage at INREF ...................................................-0.5V to +0.5V Voltage at COMP........................................-0.5V to (VCC + 0.5V) Continuous Power Dissipation (TA = +85C) SO (derate 5.88mW/C above +85C) ..........................383mW Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C Operating Junction Temperature Range (die) .....-55C to +150C Processing Temperature (die) .........................................+400C
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 = +4.5V to +5.5V, COMP = GND, 100 load between OUT+ and OUT-, TA = -40C to +85C, unless otherwise noted. Typical values are at VCC = +5.0V, TA = +25C.) (Note 1) PARAMETER Input Bias Voltage Supply Current Small-Signal Transimpedance Output Common-Mode Level Differential Output Offset Output Impedance (per side) Maximum Differential Output Voltage FILTER Resistance Power-Supply Rejection Ratio VOS ZOUT VOUT(MAX) RFILTER PSRR f < 1MHz, referred to output IIN = 500A, total peak-to-peak, differential signal 800 20 IIN = 500A, COMP = open -25 40 50 550 1000 45 SYMBOL VIN ICC z21 Input = open Input = open Differential output, input <10A 5.2 CONDITIONS MIN TYP 0.8 20 6.5 VCC - 2.0 25 60 950 1200 MAX 0.95 30 7.8 UNITS V mA k V mV mV dB
Note 1: Dice are tested at TA = +25C.
AC ELECTRICAL CHARACTERISTICS
(VCC = +4.5V to +5.5V, COMP = open, CIN = 0.75pF, outputs terminated differentially into 100, 8-pin SO package in MAX3760 EV kit, TA = +25C, unless otherwise noted. Typical values are at VCC = +5V.) (Notes 2, 3) PARAMETER Small-Signal Bandwidth Low-Frequency Cutoff Pulse-Width Distortion RMS Noise Referred to Input Data-Dependent Jitter PWD in DDJ SYMBOL BW 20A average input current (Note 4) CIN = 0.75pF (Notes 3, 5) CONDITIONS MIN 455 TYP 565 50 75 73 200 200 93.5 MAX UNITS MHz kHz ps nA ps
Note 2: AC characteristics are guaranteed by design and characterization. Note 3: CIN is the source capacitance presented to the die. Includes package parasitic, photo diode, and parasitic interconnect capacitance. Note 4: Input is a 622Mbps 1-0 pattern, signal amplitude = 0 to 1mA, extinction ratio (re) = 10. Note 5: Measured with a 4-pole, 470MHz Bessel filter.
2
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622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers
__________________________________________Typical Operating Characteristics
(MAX3760 EV kit, VCC = +5.0V, COMP = open, TA = +25C, unless otherwise noted.)
INPUT-REFERRED NOISE vs. JUNCTION TEMPERATURE
MAX3760-01
MAX3760
SMALL-SIGNAL GAIN vs. FREQUENCY
MAX3760-02
PULSE-WIDTH DISTORTION vs. INPUT SIGNAL AMPLITUDE
re = 10 622Mbps, ONE-ZERO PATTERN
MAX3760-03
110 100 INPUT-REFERRED NOISE (nA) 90
CIN IS SOURCE CAPACITANCE PRESENTED TO DIE. INCLUDES PACKAGE PARASITIC, PIN DIODE, AND PARASITIC INTERCONNECT CAPACITANCE CIN = 1.25pF CIN = 0.75pF CIN = 0.25pF
80 78 76 74 GAIN (dB)
200
150 PWD (ps) 10k 100k 1M 10M 100M 1G 10G
80 70 60 50
72 70 68 66 64
100
50
470MHz BANDWIDTH 40 -40 -5 30 65 100 JUNCTION TEMPERATURE (C)
62 60 FREQUENCY (Hz) 0 0 200 400 600 800 1000 INPUT SIGNAL AMPLITUDE (Ap-p)
INPUT-REFERRED RMS NOISE CURRENT vs. DC INPUT CURRENT
MAX3760toc04
TRANSIMPEDANCE vs. AMBIENT TEMPERATURE
MAX3760-05
TYPICAL BANDWIDTH vs. JUNCTION TEMPERATURE
IN TO-56 HEADER 520 3dB BANDWIDTH (MHz) CIN = 0.75pF 500 480 460 440 CIN IS SOURCE CAPACITANCE PRESENTED TO DIE. INCLUDES PACKAGE PARASITIC, PIN DIODE, AND PARASITIC INTERCONNECT CAPACITANCE -40 -5 30 65 100 CIN = 1.25pF CIN = 0.25pF
MAX760toc06
450 400 INPUT-REFERRED NOISE (nA) 350 300 250 200 150 100 50 0 CIN = 0.75pF 470MHz BANDWIDTH
7000
540
TRANSIMPEDANCE ()
6500
6000
5500
5000 1.00 2.15 4.64 10.0 21.5 46.4 100 215 464 1000 DC INPUT CURRENT (A) -40 -20 0 20 40 60 80 100 AMBIENT TEMPERATURE (C)
420 JUNCTION TEMPERATURE (C)
TYPICAL EYEWIDTH vs. INPUT SIGNAL
MAX3760 toc07
DATA-DEPENDENT JITTER vs. INPUT SIGNAL AMPLITUDE
MAX3760-08
OUTPUT COMMON-MODE VOLTAGE vs. AMBIENT TEMPERATURE
-1.2 -1.4 -1.6 REFERENCED TO VCC
MAX3760-09
1.6 1.5 EYEWIDTH (ns) 1.4 1.3 1.2 TA = -40C 1.1 1.0 TA = +85C TA = +25C
800 223 - 1 PRBS 622Mbps re = 10
-1.0
600 JITTER (ps pp)
VOLTAGE (V) 0 200 400 600 800 1000
-1.8 -2.0 -2.2 -2.4 -2.6 -2.8
400
200
0 10 20 50 100 150 200 250 300 400 500 600 INPUT SIGNAL (A) INPUT SIGNAL AMPLITUDE (Ap-p)
-3.0 -40 -20 0 20 40 60 80 100 AMBIENT TEMPERATURE (C)
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622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers MAX3760
Typical Operating Characteristics (continued)
(MAX3760 EV kit, VCC = +5.0V, COMP = open, TA = +25C, unless otherwise noted.)
MAXIMUM OUTPUT SIGNAL AMPLITUDE vs. AMBIENT TEMPERATURE
MAX OUTPUT SIGNAL AMPLITUDE (mVp-p)
MAX3760-10
800
700
600
500
400 -40 -20 0 20 40 60 80 100 AMBIENT TEMPERATURE (C)
EYE DIAGRAM (INPUT = 10A)
MAX3760-11
EYE DIAGRAM (INPUT = 1mA)
223 - 1 PRBS
MAX3760-12
50mV
500mV
223 - 1 PRBS
10mV/ div
100mV/ div
-50mV 200ps/div
-500mV 200ps/div
Pin Description
PIN 1 2 3 4, 5 6 7 8 NAME VCC IN INREF GND OUTOUT+ COMP Supply-Voltage Input Signal Input Input Reference Connection. Connect to photodetector AC ground. Ground Inverting Voltage Output. Current flowing into the input causes OUT- to decrease. Noninverting Voltage Output. Current flowing into the input causes OUT+ to increase. Compensation Capacitor Connection. Connection for optional external compensation capacitor for DCcancellation circuit. Add capacitance here to reduce the low-frequency cutoff of the DC cancellation circuit. Connect COMP directly to GND to disable the DC cancellation circuit. Filter Connection. Provides positive bias for photodiode through a 1k resistor to VCC (see the Designing Filters section). Available on the die only. FUNCTION
--
FILTER
4
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622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers MAX3760
VCC D1 RF
MAX3760
VCC VCC Q2
1k (FILTER)
R1 TRANSIMPEDANCE AMPLIFIER IN INREF Q3 R2 R3 R4 Q4 GND DC CANCELLATION AMPLIFIER OUT+ Q1 PARAPHASE AMP VCC OUT-
COMP
GND
( ) INDICATE PINS AVAILABLE ON DIE ONLY.
Figure 1. Functional Diagram
Detailed Description
The MAX3760 is a transimpedance amplifier designed for 622Mbps fiber optic applications. As shown in the Functional Diagram (Figure 1), it comprises a transimpedance amplifier, a paraphase amplifier with emitterfollower outputs, and a DC cancellation circuit.
be terminated with higher output impedances for increased gain and output voltage swing. The MAX3760 will not drive a 50 load to ground. For best noise rejection, terminate the MAX3760 with differential loads.
DC Cancellation Circuit
The DC cancellation circuit removes the input signal's DC component by employing low-frequency feedback. This feature centers the input signal within the transimpedance amplifier's dynamic range, thereby reducing pulse-width distortion on large input signals. The paraphase amplifier's output is sensed through resistors R3 and R4, then filtered, amplified, and fed back to the base of transistor Q4. The transistor draws the input signal's DC component away from the transimpedance amplifier's summing node. The MAX3760 DC cancellation loop is internally compensated and does not require external capacitors in most 622Mbps applications. Add external capacitance at the COMP pin to reduce the DC cancellation circuit's frequency response and improve data-dependent jitter. Connecting the COMP pin directly to GND disables the circuit. The DC cancellation circuit can sink up to 1mA at the input.
Transimpedance Amplifier
The signal current at the input flows into a high-gain amplifier's summing node. Shunt feedback through RF converts this current to a voltage with 6.5k gain. Diode D1 clamps the output voltage for large input currents. INREF is a direct connection to the input transistor's emitter, and must be connected directly to the photodetector AC ground return for best performance.
Paraphase Amplifier
The paraphase amplifier converts single-ended signals to differential signals and introduces a voltage gain of 2. This signal drives a pair of internally biased emitter followers, Q2 and Q3, which form the output stage. Resistors R1 and R2 provide back-termination at the output, delivering a 100 differential output impedance. The output emitter followers are designed to drive a 100 differential load between OUT+ and OUT-. The MAX3760 can also
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5
622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers MAX3760
The MAX3760 minimizes pulse-width distortion for data sequences exhibiting a 50% duty cycle. A duty cycle significantly different from 50% will cause the MAX3760 to generate pulse-width distortion. DC cancellation current is drawn from the input and creates noise. This is not a problem for low-level signals with little or no DC component. Preamplifier noise increases for signals with significant DC component (see Typical Operating Characteristics). The MAX3760 operates with the photodetector cathode connected to VCC, as shown in the Typical Application Circuit. Connecting the photodetector anode to GND and the cathode to IN defeats the DC cancellation circuit and causes pulse-width distortion. 1mA Overload = 10log 1000 dBm 2
Linear Range The MAX3760 has high gain, which limits the output when the input signal exceeds 20Ap-p. It operates with 90% linearity for inputs not exceeding the following:
20A r + 1 e 10log 1000 dBm r - 1 e
(
(
)
)
Input Reference
INREF is the reference point for IN. Connect it as close as possible to the photodetector diode's AC ground. The photodetector's AC ground is usually the ground of the photodetector's filter capacitor. The total length from INREF, through the filter capacitor and the diode, and back to the input should be no more than 2cm.
Table 1. Optical Power Relations
PARAMETER Average Power Extinction Ratio Optical Power of a "1" Optical Power of a "0" Signal Amplitude SYMBOL PAVE re P1 P0 RELATION
PAVE = (P0 + P1) / 2 re = P1 / P0 P1 = 2PAVE re (re + 1)
Applications Information
Optical Power Definitions
Many of the MAX3760's specifications relate to the input signal amplitude. When working with fiber optic receivers, the optical input is usually expressed in terms of average optical power and extinction ratio. Use the relations given in Table 1 to convert optical power to input signal when designing with the MAX3760.
P0 = 2PAVE / (re + 1) PIN = P1 - P0 = 2PAVE
PIN
(re
-1
)
(re + 1)
Note: Assuming a 50% average input duty cycle.
Calculating Sensitivity, Overload, and Linear Range
Sensitivity Calculation The MAX3760's input-referred RMS noise current (in) generally dominates the receiver sensitivity. In a system where the bit error rate (BER) is 1E - 10, the signal-tonoise ratio must always exceed 12.7. The sensitivity, expressed in average power, can be estimated as:
12.7in (re + 1) Sensitivity = 10log 1000 dBm 2(re - 1) Where is the photodiode responsivity in A/W.
OPTICAL POWER P1
PAVE
P0
Input Overload The overload is the largest input that the MAX3760 accepts while meeting specifications. It is calculated as:
6
TIME
Figure 2. Optical Power Definitions
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622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers
Power Supply
The MAX3760 can operate from a power-supply voltage (VCC - GND) between 4.5V and 5.5V. GND can be any stable voltage, including -5.0V, as found in many systems using ECL interface levels. supply-noise voltage is divided between CFILTER and CPHOTO. The input noise current due to supply noise is as follows (assuming the filter capacitor is much larger than the photodiode capacitance): INOISE =
MAX3760
Layout Considerations
Use good high-frequency design and layout techniques. The use of a multilayer circuit board with separate ground and VCC planes is recommended. Take care to bypass VCC and to connect the GND pin to the ground plane with the shortest possible traces.
(VNOISE )(CPHOTO ) (RFILTER )(CFILTER )
If the amount of tolerable noise is known, the filter capacitor is easy to select, as follows: CFILTER >
Designing a Low-Capacitance Input
Noise performance and bandwidth will be adversely affected by stray capacitance at IN. Make every effort to minimize capacitance on this node. Select a lowcapacitance photodiode and use good high-frequency design and layout techniques. The MAX3760 is optimized for 0.75pF of capacitance on the input--approximately the capacitance of a low-cost photodetector packaged in a header. When using the MAX3760 in the SO package, note that the package capacitance is about 0.3pF. The PC board between the MAX3760 input and the photodetector will add parasitic capacitance. Keep the input line short, and remove power and ground planes beneath it. For the best possible performance, assemble the MAX3760 in die form using chip-and-wire technology, or package the die in a TO header. These techniques minimize parasitic capacitance, resulting in the lowest noise.
(VNOISE )(CPHOTO ) (RFILTER )(INOISE )
For example, with maximum noise voltage = 10mVRMS, CPHOTO = 0.75pF, RFILTER = 1k, and INOISE selected to be 30nA (half the MAX3760's input noise): CFILTER =
(10mV)(0.75pF) (1000)(30E - 9)
= 250pF
TOP VIEW OF TO-56 HEADER
VCC CFILTER
Designing Filters
The MAX3760's noise performance is strongly affected by the circuit's bandwidth, which changes over temperature and varies from lot to lot. Receiver sensitivity can be improved by adding filters to limit this bandwidth. Filter designs range from a one-pole filter using a single capacitor to more complex filters using inductors. The simple filter provides a moderate rolloff with minimal components, while the complex filter provides a sharper rolloff and better transient response. A simple 530MHz filter can be created by placing a 6pF capacitor between the OUT+ and OUT- pins. Supply-voltage noise at the photodiode cathode produces a current, I = C PHOTO V/t, which reduces receiver sensitivity (CPHOTO is the photodiode capacitance.) The MAX3760's FILTER resistor, combined with an external capacitor, can be used to reduce this noise (see Typical Application Circuit). Current generated by
PHOTODIODE
OUT-
OUT+
CASE IS GROUND
Figure 3. Suggested Layout for TO-56 Header
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7
622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers MAX3760
Wire Bonding
For high current density and reliable operation, the MAX3760 uses gold metalization. Make connections to the die with gold wire only, using ball-bonding techniques. Wedge bonding is not recommended. Die-pad size is 4 mils square, with 6 mil pitch. Die thickness is 15 mils.
___________________Chip Topography
INREF FILTER IN
GND
VCC
GND
COMP
0.042" (1.05mm)
OUT-
OUT+
0.030" (0.75mm)
TRANSISTOR COUNT: 131 SUBSTRATE CONNECTED TO GND
Package Information
SOICN.EPS
8
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