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19-1970; Rev 2; 1/02
10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI
General Description
The MAX3970 is a compact, low-power transimpedance amplifier (TIA) optimized for use in 10Gbps optical receivers. The TIA provides transimpedance at 600V/A with 50 differential CML outputs. The MAX3970 has a typical input-referred noise of 1.1A, and when coupled with a high-speed photodiode, achieves -18dBm sensitivity and +2mA input overload. A received-signal strength indicator (RSSI) simplifies optical assembly. The circuit operates from a single 3.3V supply over a junction temperature range from 0C to +110C.
Features
o 150mW Power Dissipation at 3.3V Supply o 1.1ARMS Noise (-18dBm Sensitivity) o 9GHz Bandwidth o 2mAP-P Input Overload o Received-Signal Strength Indication o 8psP-P Typical Jitter Generation at 1.3mAP-P Input Current o 600V/A Transimpedance
MAX3970
Applications
10.3Gbps Ethernet Optical Receivers OC-192 VSR Optical Receivers Fibre-Channel Optical Receivers
PART MAX3970U/D
Ordering Information
TEMP RANGE 0C to +85C PIN-PACKAGE Dice
Note: Dice are designed to operate over a 0C to +110C junction temperature (TJ) range, but are tested and guaranteed at TA = +25C.
Typical Application Circuit
3.3V SUPPLY FILTERING VCC1 VCC2
MAX3970
FILTER 200pF IN OUT+ OUT1.0V RF 0.01F
3.3V
LIMITING AMPLIFIER 0.01F
RSSI
________________________________________________________________ Maxim Integrated Products
1
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.
10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI MAX3970
ABSOLUTE MAXIMUM RATINGS
Terminal Voltage Voltage VCC1 and VCC2 ...................................-0.3V to +5.0V Voltage at FILTER.................................-0.3V to (VCC1 + 0.3V) Voltage at OUT+, OUT-, RSSI ........................0V to (VCC + 0.5V) Input Current IN, TEST ............................................................-5mA to +5mA Operating Junction Temperature Range ...........-40C to +125C Storage Temperature Range .............................-60C to +150C Die Attach Process Temperature.....................................+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.
ELECTRICAL CHARACTERISTICS
(VCC = +3.0V to +3.6V, output loads = 50 to VCC, TJ = 0C to +110C. Typical values are at VCC = +3.3V, CIN = 0.25pF, LIN = 1.7nH, TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Supply Current Maximum DC Input Current Input Linear Range Input-Referred RMS Noise Input-Referred Noise Density Output Resistance (per side) Small-Signal Transimpedance Small-Signal Bandwidth Low-Frequency Cutoff Deterministic Jitter Input Bias Voltage RSSI Gain RSSI Bandwidth Photodiode Filter Resistance Maximum Differential Output Voltage RFILTER VOD-MAX Input = 1mAP-P DJ VIN IIN = 100A to 1mA IIN = 10A to 100A 900 1200 10 330 350 IIN < 1.3mA IIN = 2.0mA ROUT Z21 BW Differential output 10AP-P < Input < 100AP-P in SYMBOL ICC IIN-MAX 0.95 < linearity < 1.05 f = 7.5GHz (Note 2) f = 10GHz (Note 2) f = 10GHz (Note 2) 43 450 7.4 1.6 100 130 1.1 1.1 11 50 600 9 70 8 16 0.9 1200 1800 70 410 470 500 700 22 0.96 1500 3000 58 875 13.2 150 1.45 1.45 CONDITIONS MIN TYP 46 MAX 62 UNITS mA mA AP-P A pA/Hz GHz kHz psP-P V V/A kHz mVP-P
Note 1: AC characteristics are guaranteed by design and characterization. Note 2: Input-referred noise is calculated as RMS output noise / (gain at f = 10MHz). Noise density is (input-referred noise) / bandwidth. Noise measurements are made using 4-pole Bessel filters.
2
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10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI
Typical Operating Characteristics
(VCC = +3.3V, TA = +25C, input bondwire inductance = 1.0nH, unless otherwise noted. CIN is total source capacitance to die. All measurements made on MAX3970 EV Kit.)
INPUT-REFERRED NOISE vs. CAPACITANCE
MAX3970 toc01
MAX3970
INPUT-REFERRED RMS NOISE CURRENT vs. AVERAGE INPUT CURRENT
MAX3970 toc02
DETERMINISTIC JITTER vs. INPUT AMPLITUDE
INPUT = k28.5 PATTERN
MAX3970 toc03
1.4 INPUT-REFFERED NOISE (ARMS) 1.3 1.2 1.1 1.0
NOISE IS MEASURED IN A BANDWIDTH OF 7.5GHz.
3.0 2.5 RMS NOISE CURRENT (A) 2.0 1.5 1.0 0.5 0
30
25 JITTER (psP-P)
20
15
TJ = 100C TJ = 50C TJ = 0C
0.9 0.8 0.1
10
5 1 10 100 1000 10000 10 100 1000 10000 DC INPUT CURRENT (A) AMPLITUDE (AP-P)
0.2
0.3
0.4 CIN (pF)
0.5
0.6
0.7
DETERMINISTIC JITTER vs. AVERAGE INPUT CURRENT
MAX3970 toc04
OUTPUT AMPLITUDE vs. TEMPERATURE
MAX3970 toc05
DC TRANSFER FUNCTION
300 DC CANCELLATION 250 CIRCUIT DISABLED, 200 VFILTER = GND 150 100 50 0 -50 -100 -150 -200 -250 -300 -2500 -1500 -500
MAX3970 toc06
40
SIGNAL INPUT = 50AP-P
700 DIFFERENTIAL AMPLITUDE (mVP-P) 600 500 400 300
INPUT = 1mAP-P, 00-11 PATTERN AT 10.0Gbps
20
10
0 1 10 100 1000 INPUT CURRENT (A)
200 -50
-25
0
25
50
75
100
OUTPUT VOLTAGE (mV)
30 JITTER (psP-P)
500
1500
2500
AMBIENT TEMPERATURE (C)
INPUT CURRENT (A)
EYE DIAGRAM (50AP-P INPUT)
MAX3970 toc07
EYE DIAGRAM (2.0mAP-P INPUT)
223 - 1PRBS 2mA INPUT
MAX3970 toc08
SIMULATED FREQUENCY RESPONSE vs. INPUT INDUCTANCE
65 60 MAGNITUDE S21 (dB) 55 50 45 40 35 30 25 LIN = 1.5nH LIN = 1.0nH LIN = 0.5nH LIN = 2.0nH
MAX3970 toc09
70
223 - 1PRBS 50A INPUT
5mV/div
100mV/div
20ps/div
20ps/div
1k
10k 100k 1M 10M 100M 1G FREQUENCY (Hz)
10G 100G
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10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI MAX3970
Typical Operating Characteristics (continued)
(VCC = +3.3V, TA = +25C, input bondwire inductance = 1.0nH, unless otherwise noted. CIN is total source capacitance to die. All measurements made on MAX3970 EV Kit.)
SIMULATED FREQUENCY RESPONSE vs. INPUT INDUCTANCE
MAX3970 toc10
POWER-SUPPLY REJECTION RATIO vs. FREQUENCY
5 SUPPLY REJECTION (dB) 10 15 20 25 30 35 40 -0.40 100k 1M 10M FREQUENCY (Hz) 100M 1G PSRR = -20 LOG VOUT VCC
MAX3970 toc11
OUTPUT COMMON-MODE VOLTAGE (REFERENCED TO VCC) vs. TEMPERATURE
IIN = 0, iIN = 0
MAX3970 toc12
58 56 MAGNITUDE S21 (dB) 54 52 LIN = 1.0nH 50 LIN = 0.5nH 48 46 1G 10G FREQUENCY (Hz) LIN = 2.0nH LIN = 1.5nH
0
-0.20 COMMON-MODE VOLTAGE (V)
-0.25 VCC = +3.0V -0.30 VCC = +3.3V VCC = +3.6V -0.35
100G
0
20
40
60
80
AMBIENT TEMPERATURE (C)
S22 vs. FREQUENCY
MAX3970 toc13
OUTPUT VSWR (DIFFERENTIAL)
2.8 2.6 2.4 2.2 VSWR 2.0 1.8 1.6 1.4 1.2 1.0 0.8 100M 0.5 VRSSI (V) 1.5
MAX3970 toc14
RSSI OUTPUT VOLTAGE vs. AVERAGE INPUT CURRENT
MAX3970 toc15
10 0 MAGNITUDE S22 (dB) -10 -20 -30 -40 -50 -60 100M
3.0
2.5
2.0
1.0
0 1G FREQUENCY (Hz) 10G 0 500 1000 CURRENT (mA) 1500 2000
1G FREQUENCY (Hz)
10G
SIMULATED SMALL-SIGNAL BANDWIDTH vs. CAPACITANCE
MAX3970 toc16
SMALL-SIGNAL TRANSIMPEDANCE vs. TEMPERATURE
MAX3970 toc17
15 14 13 -3dB BANDWIDTH (GHz) 12 11 10 9 8 7 6 5 4 0.1 0.2 0.3 0.4 CIN (pF) 0.5 0.6 TJ = 100C TJ = 0C TJ = 50C
700 600 TRANSIMPEDANCE (V/A) 500 400 300 200 100 0
0.7
-50
-25
0
25
50
75
100
AMBIENT TEMPERATURE (C)
4
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10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI
Pad Description
PAD BP1, BP2, BP18 BP3 BP4 BP5 BP6, BP7 BP8, BP9 BP10, BP13 BP11 BP12 BP14, BP15 BP16 BP17 NAME VCC1 FILTER TEST IN GND1 GND2 GND3 OUTOUT+ VCC2 RSSI FUNCTION Power Supply. Provides supply voltage to input circuitry and bias to the photodiode via an internal 410 resistor. Provides bias voltage for the photodiode through a 410 resistor to VCC1. When grounded, this pin disables the DC cancellation circuit to allow a DC path from IN to OUT+ and OUT- for testing. Test Pad. This pad is connected to IN via a 1k resistor. Amplifier Input. Accepts photodiode input current. Ground Ground Ground Negative CML Output. Current flowing into IN causes OUT- to decrease. Positive CML Output. Current flowing into IN causes OUT+ to increase. Power Supply. Provides supply voltage to the output buffers. Received-Signal Strength Indicator. This pin provides a voltage proportional to the DC input current. Monitor this output during assembly to optimally align the photodiode to the optics.
MAX3970
Detailed Description
The MAX3970 transimpedance amplifier is optimized for 10Gbps fiber optic receivers. Figure 1 is a functional diagram of the MAX3970, which comprises a transimpedance amplifier, a voltage amplifier, an output buffer, a received-signal strength indicator, and a DCcancellation circuit.
DC Cancellation Circuit
The DC cancellation circuit centers the input signal within the transimpedance amplifier's linear range (Figure 3). Low-frequency feedback is employed to remove the input signal's DC component. The DC cancellation circuit is internally compensated and therefore does not require external capacitors. This circuit minimizes pulse-width distortion for data sequences that exhibit a 50% mark density. A mark density significantly different from 50% will cause the MAX3970 to generate pulse-width distortion.
Transimpedance Amplifier
Photodiode signal current flows into the summing node of a high-gain amplifier. Shunt feedback through RF converts this current into a voltage with a gain of approximately 400. Schottky diodes clamp the output voltage for large input currents, as shown in Figure 2.
Received-Signal Strength Indicator
The received-signal strength indicator (RSSI) provides a voltage proportional to the DC input current. The RSSI circuitry is designed to drive a 10k load and is used during the assembly process to optimally align the photodiode. The lowpass filter in the DC cancellation circuit determines the response time of the RSSI circuit.
Voltage Amplifier
The voltage amplifier converts single-ended signals to differential signals and introduces approximately 4dB of gain.
Output Buffer
The output buffer is optimized to drive a 100 differential load between OUT+ and OUT-. Although short-circuit protection is provided, this stage will not drive a 50 load to ground. For proper operation, the load must be AC-coupled. For large signals, the output buffer produces a limited, 500mVP-P differential output voltage. Terminate the MAX3970 outputs differentially for optimum supply-noise rejection. If a single-ended output is required, terminate the used and unused outputs similarly.
Design Procedure
Power Supply
The MAX3970 requires wide-band power-supply decoupling. Power-supply bypassing should provide low impedance between VCC and ground for frequencies between 50kHz and 10GHz. Use LC filtering at the main supply terminal and decoupling capacitors as close to the die as possible.
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10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI MAX3970
RF 420 TRANSIMPEDANCE AMPLIFIER
MAX3970
VOLTAGE AMPLIFIER OUTPUT BUFFER
50 OUT+
IN OUT50 DC CANCELLATION CIRCUIT 1k TEST RSSI VCC1 BUF 410 FILTER DISABLE BUF LOWPASS FILTER
Figure 1. Functional Diagram
AMPLITUDE AMPLITUDE INPUT FROM PHOTODIODE OUTPUT (LARGE SIGNALS) OUTPUT (SMALL SIGNALS) TIME
TIME
INPUT AFTER DC CANCELATION
Figure 2. MAX3970 Limited Output
Figure 3. Effects of DC Cancellation on Input Signal
Photodiode Filter
Supply-voltage noise at the cathode of the photodiode produces a current I = CPD V/t, which reduces the receiver sensitivity (C PD is the photodiode capacitance). The MAX3970 contains an internal lowpass filter to reduce photodiode noise current and improve receiver sensitivity. An external capacitor connected between the FILTER pad and ground can further reduce this noise (see the Typical Application Circuit). Current generated by supply-noise voltage is divided between the filter capacitance and photodiode capacitance. Assuming the filter capacitance is much larger than the photodiode capacitance, the input noise current due to supply noise is:
6
INOISE = (VNOISE)(CPD) / (RFILTER)(CFILTER) where C FILTER is the external capacitance plus the internal 22pF capacitor. If the amount of tolerable noise is known, the filter capacitance can be easily selected: CFILTER = (VNOISE)(CPD) / (RFILTER)(INOISE) For example, with maximum noise voltage = 100mVP-P, CPD = 0.25pF, RFILTER = 410, and INOISE selected to be 300nA (1/4 of the MAX3970's input noise): CFILTER = (100mV)(0.25pF) / (410)(300nA) 200pF Thus, the required external filter capacitance is 200pF -22pF = 178pF.
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10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI
Wire Bonding
For high current density and reliable operation, the MAX3970 uses gold metalization. Connections to the die should be made with gold wire only. Aluminum bonding is not recommended. Die thickness is typically 8mils (0.203mm). Bondwire inductance between the photodiode and the IN pad can be optimized to obtain best performance. Higher inductance improves bandwidth while lower bondwire inductance reduces time domain ringing. See the Frequency Response vs. Input Inductance plot in the Typical Operating Characteristics. Bondwires on all other pads should be kept as short as possible (<30mil) to optimize performance. The MAX3970 has two power-supply connections (VCC1 and VCC2) and three ground connections (GND1, GND2, and GND3). Maxim recommends connecting all power supply and ground pads. At a minimum, connect at least one pad from each section. The backside of the MAX3970 die is fully insulated and can be connected to VCC, ground, or left floating.
Output Coupling Capacitors
The output coupling capacitors should be low impedance over a frequency range from 50kHz to 10GHz. For more information on selecting coupling capacitors, visit Maxim's website and follow the links to HFAN1.1, Choosing AC-Coupling Capacitors.
MAX3970
Applications Information
Interface Schematics
Figures 4 through 7 show interface pads for the MAX3970. Back termination is provided by integrated 50 pullup resistors.
Optical Power Relations
Many MAX3970 specifications relate to the input signal amplitude. When working with fiber optic receivers, the input is sometimes expressed in terms of average optical power and extinction ratio. Figure 8 shows the relations that are helpful for converting optical power to optical modulation amplitude when designing with the MAX3970. Optical power relations are shown in Table 1 for an average mark density of 50% and an average duty cyle of 50%.
Input Capacitance
Noise and bandwidth are adversely affected by capacitance on the MAX3970's input node as shown in Input Referred Noise vs. Capacitance and Small Signal Bandwidth vs. Capacitance in the Typical Operating Characteristics. Use any technique available to minimize input capacitance.
VCC TEST 50 50 OUT+ 1k
OUT-
IN
12.5
12.5
GND GND
Figure 4. OUT Pads
Figure 5. IN and TEST Pads _______________________________________________________________________________________ 7
10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI MAX3970
VCC
VCC1
410 FILTER
22pF
RSSI
GND
GND
Figure 6. FILTER Pad
Figure 7. RSSI Pad
Input Optical Overload
P1
The overload is the largest input that the MAX3970 accepts while meeting specifications. Optical overload can be estimated in terms of average power with the following equation: 2mA Overload = 10log x 1000 dBm 2x
OPTICAL POWER
PAVE
Optical Linear Range
P0 TIME
The MAX3970 has high gain, and operates in a linear range for inputs not exceeding: 60A(re + 1) Linear Range = 10log x 1000 dBm 2 x x (re - 1)
Figure 8. Optical Power Relations
Optical Sensitivity Calculation
The MAX3970 input-referred RMS noise current in generally determines the receiver sensitivity. To obtain a system bit error rate (BER) of 1 x 10-12, the signal-tonoise ratio must always exceed 14.1. The input sensitivity, expressed in average power, can be estimated as: 14.1 x in x (re + 1) Sensitivity = 10log x 1000 dBm 2 x x (re - 1) where is the photodiode responsivity in A/W.
8
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10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI MAX3970
Table 1. Optical Power Relations*
PARAMETER Average Power Extinction Ratio Optical Power of a "1" Optical Power of a "0" Optical Modulation Amplitude SYMBOL PAVG re P1 P0 PIN RELATION PAVG = (P0 + P1) / 2 re = P1 / P0
(129.8, 971.6)
P1 = 2PAVG
re re + 1
(0, 799.4) 3
MAX3970
P0 = 2PAVG / (re+1)
(512, 548.8) IN
r -1 PIN = P1 - P0 = 2PAVG e re + 1
*Assuming a 50% average mark density.
(0, 169.4) HF98Z (381.8, 0)
Table 2. MAX3970 Bondpad Information
PAD BP1 BP2 BP3 BP4 BP5 BP6 BP7 BP8 BP9 BP10 BP11 BP12 BP13 BP14 BP15 BP16 BP17 BP18 NAME VCC1 VCC1 FILTER TEST IN GND1 GND1 GND2 GND2 GND3 OUTOUT+ GND3 VCC2 VCC2 VCC2 RSSI VCC1 COORDINATES X 0 0 0 0 0 0 129.8 255.8 381.8 512 512 512 512 512 512 381.8 255.8 129.8 Y 799.4 673.4 547.4 421.4 295.4 169.4 0 0 0 170.8 296.8 422.8 548.8 674.8 800.8 971.6 971.6 971.6
y x
* ALL DIMENSIONS ARE IN microns. * PAD DIMENSIONS: METAL H = 102.4 microns W = 102.4 microns PASSIVATION OPENING 94.4 microns 94.4 microns
* COORDINATES SPECIFY LOWER LEFT CORNER OF THE PAD.
Figure 9. Bondpad Diagram
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9
10Gbps, 3.3V Low-Power Transimpedance Amplifier with RSSI MAX3970
Chip Topography
VCC1 RSSI VCC2
VCC1 VCC1 FILTER
VCC2 VCC2 GND3
TEST IN GND1
OUT+ 0.053" (1.345mm) OUT-
GND3
DIE IDENTIFICATION GND1 GND2
GND2
0.034" (0.864mm)
Chip Information
TRANSISTOR COUNT: 125 PROCESS: SILICON GERMANIUM BIPOLAR
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 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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