MAXIM MAX3970U

19-1970; Rev 2; 1/02
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
Features
♦ 150mW Power Dissipation at 3.3V Supply
♦ 1.1µARMS Noise (-18dBm Sensitivity)
♦ 9GHz Bandwidth
♦ 2mAP-P Input Overload
♦ Received-Signal Strength Indication
♦ 8psP-P Typical Jitter Generation at 1.3mAP-P Input
Current
♦ 600V/A Transimpedance
Applications
Ordering Information
10.3Gbps Ethernet Optical Receivers
OC-192 VSR Optical Receivers
Fibre-Channel Optical Receivers
PART
TEMP RANGE
MAX3970U/D
0°C to +85°C
PIN-PACKAGE
Dice
Note: Dice are designed to operate over a 0°C to +110°C junction temperature (TJ) range, but are tested and guaranteed at
TA = +25°C.
Typical Application Circuit
3.3V
SUPPLY
FILTERING
VCC1
VCC2
MAX3970
FILTER
3.3V
RF
200pF
0.01µF
OUT+
IN
OUT1.0V
LIMITING
AMPLIFIER
0.01µF
RSSI
________________________________________________________________ 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
MAX3970
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 50Ω differential CML outputs. The
MAX3970 has a typical input-referred noise of 1.1µA,
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 0°C
to +110°C.
MAX3970
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
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 ...........-40°C to +125°C
Storage Temperature Range .............................-60°C to +150°C
Die Attach Process Temperature.....................................+400°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.
ELECTRICAL CHARACTERISTICS
(VCC = +3.0V to +3.6V, output loads = 50Ω to VCC, TJ = 0°C to +110°C. Typical values are at VCC = +3.3V, CIN = 0.25pF, LIN =
1.7nH, TA = +25°C, unless otherwise noted.) (Note 1)
PARAMETER
Supply Current
Maximum DC Input Current
SYMBOL
in
Z21
Small-Signal Bandwidth
BW
100
DJ
Input Bias Voltage
VIN
RSSI Gain
RFILTER
Maximum Differential Output
Voltage
VOD-MAX
62
130
1.45
1.45
f = 10GHz (Note 2)
11
µA
pA/√Hz
43
50
58
Ω
450
600
875
Ω
9
13.2
GHz
70
150
kHz
7.4
IIN < 1.3mA
8
IIN = 2.0mA
16
22
0.9
0.96
IIN = 100µA to 1mA
900
1200
1500
IIN = 10µA to 100µA
1200
1800
3000
Input = 1mAP-P
mA
µAP-P
1.1
Differential output
10µAP-P < Input < 100µAP-P
UNITS
mA
1.1
RSSI Bandwidth
Photodiode Filter Resistance
46
f = 10GHz (Note 2)
Low-Frequency Cutoff
Deterministic Jitter
MAX
f = 7.5GHz (Note 2)
ROUT
Small-Signal Transimpedance
TYP
1.6
0.95 < linearity < 1.05
Input-Referred Noise Density
Output Resistance (per side)
MIN
ICC
IIN-MAX
Input Linear Range
Input-Referred RMS Noise
CONDITIONS
psP-P
V
V/A
10
70
330
410
500
kHz
Ω
350
470
700
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
_______________________________________________________________________________________
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
INPUT-REFERRED RMS NOISE CURRENT
vs. AVERAGE INPUT CURRENT
TJ = 100°C
TJ = 50°C
0.3
0.4
0.5
0.6
MAX3970 toc02
5
10
100
1000
OUTPUT AMPLITUDE vs. TEMPERATURE
0
100
10
1000
INPUT = 1mAP-P, 00–11 PATTERN AT 10.0Gbps
600
500
400
300
200
-50
-25
0
25
50
1000
10000
DC TRANSFER FUNCTION
OUTPUT VOLTAGE (mV)
10
100
AMPLITUDE (µAP-P)
MAX3970 toc05
DETERMINISTIC JITTER
vs. AVERAGE INPUT CURRENT
700
10
10000
DC INPUT CURRENT (µA)
20
15
10
1
0.7
30
20
1.0
CIN (pF)
SIGNAL INPUT = 50µAP-P
75
100
300
DC CANCELLATION
250 CIRCUIT DISABLED,
200 VFILTER = GND
150
100
50
0
-50
-100
-150
-200
-250
-300
-2500 -1500
-500
500
1500
2500
INPUT CURRENT (µA)
AMBIENT TEMPERATURE (°C)
INPUT CURRENT (µA)
EYE DIAGRAM (50µAP-P INPUT)
EYE DIAGRAM (2.0mAP-P INPUT)
SIMULATED FREQUENCY RESPONSE
vs. INPUT INDUCTANCE
100mV/div
MAX3970 toc09
70
65
LIN = 2.0nH
60
MAGNITUDE S21 (dB)
5mV/div
223 - 1PRBS 2mA INPUT
MAX3970 toc08
223 - 1PRBS 50µA INPUT
MAX3970 toc07
1
1.5
0
0.2
DIFFERENTIAL AMPLITUDE (mVP-P)
40
25
0.5
TJ = 0°C
0.9
2.0
INPUT = k28.5 PATTERN
MAX3970 toc06
1.1
30
JITTER (psP-P)
1.2
0.8
0.1
JITTER (psP-P)
2.5
RMS NOISE CURRENT (µA)
1.3
1.0
3.0
MAX3970 toc01
NOISE IS MEASURED IN A
BANDWIDTH OF 7.5GHz.
MAX3970 toc04
INPUT-REFFERED NOISE (µARMS)
1.4
DETERMINISTIC JITTER
vs. INPUT AMPLITUDE
MAX3970 toc03
INPUT-REFERRED NOISE vs. CAPACITANCE
55
LIN = 1.5nH
LIN = 1.0nH
LIN = 0.5nH
50
45
40
35
30
25
20ps/div
20ps/div
1k
10k 100k 1M 10M 100M 1G
10G 100G
FREQUENCY (Hz)
_______________________________________________________________________________________
3
MAX3970
Typical Operating Characteristics
(VCC = +3.3V, TA = +25°C, input bondwire inductance = 1.0nH, unless otherwise noted. CIN is total source capacitance to die. All
measurements made on MAX3970 EV Kit.)
Typical Operating Characteristics (continued)
(VCC = +3.3V, TA = +25°C, input bondwire inductance = 1.0nH, unless otherwise noted. CIN is total source capacitance to die. All
measurements made on MAX3970 EV Kit.)
LIN = 2.0nH
LIN = 1.5nH
52
LIN = 1.0nH
50
LIN = 0.5nH
15
20
25
30
-0.20
IIN = 0, iIN = 0
MAX3970 toc12
MAX3970 toc11
∆VCC
10
48
-0.25
VCC = +3.0V
VCC = +3.3V
-0.30
VCC = +3.6V
-0.35
35
40
1G
10G
-0.40
100k
100G
1M
10M
100M
FREQUENCY (Hz)
FREQUENCY (Hz)
S22 vs. FREQUENCY
OUTPUT VSWR
(DIFFERENTIAL)
20
40
60
80
RSSI OUTPUT VOLTAGE
vs. AVERAGE INPUT CURRENT
2.5
MAX3970 toc14
0
0
AMBIENT TEMPERATURE (°C)
3.0
MAX3970 toc13
10
1G
2.8
2.6
MAX3970 toc15
46
2.0
2.4
-10
VSWR
-30
VRSSI (V)
2.2
-20
2.0
1.8
-40
1.5
1.0
1.6
1.4
0.5
1.2
-50
1.0
-60
100M
1G
0
0.8
100M
10G
1G
SIMULATED SMALL-SIGNAL BANDWIDTH
vs. CAPACITANCE
700
12
TJ = 50°C
10
9
TJ = 0°C
TJ = 100°C
600
TRANSIMPEDANCE (V/A)
-3dB BANDWIDTH (GHz)
13
7
6
1000
CURRENT (mA)
MAX3970 toc17
14
8
500
SMALL-SIGNAL TRANSIMPEDANCE
vs. TEMPERATURE
MAX3970 toc16
15
11
0
10G
FREQUENCY (Hz)
FREQUENCY (Hz)
500
400
300
200
100
5
4
0
0.1
0.2
0.3
0.4
CIN (pF)
4
∆VOUT
COMMON-MODE VOLTAGE (V)
PSRR = -20 LOG
5
SUPPLY REJECTION (dB)
56
MAGNITUDE S21 (dB)
0
MAX3970 toc10
58
54
OUTPUT COMMON-MODE VOLTAGE
(REFERENCED TO VCC) vs. TEMPERATURE
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
SIMULATED FREQUENCY RESPONSE
vs. INPUT INDUCTANCE
MAGNITUDE S22 (dB)
MAX3970
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
0.5
0.6
0.7
-50
-25
0
25
50
75
100
AMBIENT TEMPERATURE (°C)
_______________________________________________________________________________________
1500
2000
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
PAD
NAME
FUNCTION
BP1, BP2,
BP18
VCC1
BP3
FILTER
BP4
TEST
BP5
IN
BP6, BP7
GND1
BP8, BP9
GND2
Ground
BP10, BP13
GND3
Ground
BP11
OUT-
Negative CML Output. Current flowing into IN causes OUT- to decrease.
BP12
OUT+
Positive CML Output. Current flowing into IN causes OUT+ to increase.
BP14, BP15
BP16
VCC2
Power Supply. Provides supply voltage to the output buffers.
BP17
RSSI
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.
Power Supply. Provides supply voltage to input circuitry and bias to the photodiode via an
internal 410Ω resistor.
Provides bias voltage for the photodiode through a 410Ω 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 1kΩ resistor.
Amplifier Input. Accepts photodiode input current.
Ground
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.
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 400Ω. Schottky diodes clamp the output
voltage for large input currents, as shown in Figure 2.
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.
Received-Signal Strength Indicator
The voltage amplifier converts single-ended signals to
differential signals and introduces approximately 4dB
of gain.
The received-signal strength indicator (RSSI) provides a
voltage proportional to the DC input current. The RSSI
circuitry is designed to drive a 10kΩ 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.
Output Buffer
Design Procedure
Voltage Amplifier
The output buffer is optimized to drive a 100Ω differential
load between OUT+ and OUT-. Although short-circuit
protection is provided, this stage will not drive a 50Ω
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.
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.
_______________________________________________________________________________________
5
MAX3970
Pad Description
MAX3970
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
RF
MAX3970
420Ω
TRANSIMPEDANCE
AMPLIFIER
VOLTAGE
AMPLIFIER
OUTPUT
BUFFER
50Ω
OUT+
IN
OUT50Ω
DC CANCELLATION CIRCUIT
1kΩ
TEST
RSSI
BUF
LOWPASS
FILTER
VCC1
BUF
410Ω
FILTER
DISABLE
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 = 410Ω, and INOISE selected to
be 300nA (1/4 of the MAX3970's input noise):
CFILTER = (100mV)(0.25pF) / (410Ω)(300nA) ≈ 200pF
Thus, the required external filter capacitance is
200pF -22pF = 178pF.
_______________________________________________________________________________________
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
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.
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.
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.
Applications Information
Interface Schematics
Figures 4 through 7 show interface pads for the
MAX3970. Back termination is provided by integrated
50Ω 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%.
VCC
TEST
50Ω
50Ω
OUT+
1kΩ
OUT-
12.5Ω
IN
12.5Ω
GND
GND
Figure 4. OUT Pads
Figure 5. IN and TEST Pads
_______________________________________________________________________________________
7
MAX3970
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.
MAX3970
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
VCC
VCC1
410Ω
FILTER
22pF
RSSI
GND
GND
Figure 7. RSSI Pad
Figure 6. FILTER Pad
Input Optical Overload
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:
OPTICAL POWER
P1
 2mA

Overload = 10log 
× 1000 dBm
 2×ρ

PAVE
Optical Linear Range
The MAX3970 has high gain, and operates in a linear
range for inputs not exceeding:
P0
TIME
Figure 8. Optical Power Relations
 60µA(re + 1)

Linear Range = 10log 
× 1000 dBm
 2 × ρ × (re − 1)

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 × in × (re + 1)

Sensitivity = 10log 
× 1000 dBm
2
×
ρ
×
(
r
1
)
−


e
where ρ is the photodiode responsivity in A/W.
8
_______________________________________________________________________________________
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
PARAMETER
SYMBOL
MAX3970
Table 1. Optical Power Relations*
(129.8, 971.6)
RELATION
Average Power
PAVG
PAVG = (P0 + P1) / 2
Extinction Ratio
re
re = P1 / P0
Optical Power
of a “1”
P1
P1 = 2PAVG
Optical Power
of a “0”
P0
Optical Modulation
Amplitude
PIN
re
MAX3970
(0, 799.4)
re + 1
3
P0 = 2PAVG / (re+1)
(512, 548.8)
IN
r −1
PIN = P1 − P0 = 2PAVG e
re + 1
(0, 169.4)
*Assuming a 50% average mark density.
HF98Z
(381.8, 0)
Table 2. MAX3970 Bondpad Information
COORDINATES
PAD
NAME
X
Y
BP1
VCC1
0
799.4
BP2
VCC1
0
673.4
BP3
FILTER
0
547.4
BP4
TEST
0
421.4
BP5
IN
0
295.4
BP6
GND1
0
169.4
BP7
GND1
129.8
0
BP8
GND2
255.8
0
BP9
GND2
381.8
0
BP10
GND3
512
170.8
BP11
OUT-
512
296.8
BP12
OUT+
512
422.8
BP13
GND3
512
548.8
BP14
VCC2
512
674.8
BP15
VCC2
512
800.8
BP16
VCC2
381.8
971.6
BP17
RSSI
255.8
971.6
BP18
VCC1
129.8
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
_______________________________________________________________________________________
9
MAX3970
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
Chip Topography
VCC1
RSSI
VCC2
VCC2
VCC1
VCC2
VCC1
GND3
FILTER
OUT+ 0.053"
(1.345mm)
TEST
IN
OUT-
GND1
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.
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Printed USA
is a registered trademark of Maxim Integrated Products.