19-1970; Rev 2; 1/02
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
General Description
The MAX3970 is a compact, low-power transimped-
ance amplifier (TIA) optimized for use in 10Gbps opti-
cal 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.
Features
o
150mW Power Dissipation at 3.3V Supply
o
1.1µA
RMS
Noise (-18dBm Sensitivity)
o
9GHz Bandwidth
o
2mA
P-P
Input Overload
o
Received-Signal Strength Indication
o
8ps
P-P
Typical Jitter Generation at 1.3mA
P-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
0°C to +85°C
PIN-PACKAGE
Dice
Note:
Dice are designed to operate over a 0°C to +110°C junc-
tion temperature (T
J
) range, but are tested and guaranteed at
T
A
= +25°C.
Typical Application Circuit
3.3V
SUPPLY
FILTERING
V
CC
1
V
CC
2
MAX3970
FILTER
200pF
IN
OUT+
OUT-
1.0V
R
F
0.01µF
3.3V
LIMITING
AMPLIFIER
0.01µF
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 V
CC
1 and V
CC
2 ...................................-0.3V to +5.0V
Voltage at FILTER.................................-0.3V to (V
CC
1 + 0.3V)
Voltage at OUT+, OUT-, RSSI ........................0V to (V
CC
+ 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
(V
CC
= +3.0V to +3.6V, output loads = 50Ω to V
CC
, T
J
= 0°C to +110°C. Typical values are at V
CC
= +3.3V, C
IN
= 0.25pF, L
IN
=
1.7nH, T
A
= +25°C, 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
R
FILTER
V
OD-MAX
Input = 1mA
P-P
DJ
V
IN
I
IN
= 100µA to 1mA
I
IN
= 10µA to 100µA
900
1200
10
330
350
I
IN
< 1.3mA
I
IN
= 2.0mA
R
OUT
Z
21
BW
Differential output
10µA
P-P
< Input < 100µA
P-P
i
n
SYMBOL
I
CC
I
IN-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
µA
P-P
µA
pA/√Hz
Ω
Ω
GHz
kHz
ps
P-P
V
V/A
kHz
Ω
mV
P-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
Typical Operating Characteristics
(V
CC
= +3.3V, T
A
= +25°C, input bondwire inductance = 1.0nH, unless otherwise noted. C
IN
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 (µA
RMS
)
1.3
1.2
1.1
1.0
NOISE IS MEASURED IN A
BANDWIDTH OF 7.5GHz.
3.0
2.5
RMS NOISE CURRENT (µA)
2.0
1.5
1.0
0.5
0
30
25
JITTER (ps
P-P
)
20
15
T
J
= 100°C
T
J
= 50°C
T
J
= 0°C
0.9
0.8
0.1
10
5
1
10
100
1000
10000
10
100
1000
10000
DC INPUT CURRENT (µA)
AMPLITUDE (µA
P-P
)
0.2
0.3
0.4
C
IN
(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 V
FILTER
= GND
150
100
50
0
-50
-100
-150
-200
-250
-300
-2500 -1500
-500
MAX3970 toc06
40
SIGNAL INPUT = 50µA
P-P
700
DIFFERENTIAL AMPLITUDE (mV
P-P
)
600
500
400
300
INPUT = 1mA
P-P
, 00–11 PATTERN AT 10.0Gbps
20
10
0
1
10
100
1000
INPUT CURRENT (µA)
200
-50
-25
0
25
50
75
100
OUTPUT VOLTAGE (mV)
30
JITTER (ps
P-P
)
500
1500
2500
AMBIENT TEMPERATURE (°C)
INPUT CURRENT (µA)
EYE DIAGRAM (50µA
P-P
INPUT)
MAX3970 toc07
EYE DIAGRAM (2.0mA
P-P
INPUT)
2
23
- 1PRBS 2mA INPUT
MAX3970 toc08
SIMULATED FREQUENCY RESPONSE
vs. INPUT INDUCTANCE
65
60
MAGNITUDE S21 (dB)
55
50
45
40
35
30
25
L
IN
= 1.5nH
L
IN
= 1.0nH
L
IN
= 0.5nH
L
IN
= 2.0nH
MAX3970 toc09
70
2
23
- 1PRBS 50µA INPUT
5mV/div
100mV/div
20ps/div
20ps/div
1k
10k 100k 1M 10M 100M 1G
FREQUENCY (Hz)
10G 100G
_______________________________________________________________________________________
3
10Gbps, 3.3V Low-Power Transimpedance
Amplifier with RSSI
MAX3970
Typical Operating Characteristics (continued)
(V
CC
= +3.3V, T
A
= +25°C, input bondwire inductance = 1.0nH, unless otherwise noted. C
IN
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
∆V
OUT
∆V
CC
MAX3970 toc11
OUTPUT COMMON-MODE VOLTAGE
(REFERENCED TO V
CC
) vs. TEMPERATURE
I
IN
= 0, i
IN
= 0
MAX3970 toc12
58
56
MAGNITUDE S21 (dB)
54
52
L
IN
= 1.0nH
50
L
IN
= 0.5nH
48
46
1G
10G
FREQUENCY (Hz)
L
IN
= 2.0nH
L
IN
= 1.5nH
0
-0.20
COMMON-MODE VOLTAGE (V)
-0.25
V
CC
= +3.0V
-0.30
V
CC
= +3.3V
V
CC
= +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
V
RSSI
(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
C
IN
(pF)
0.5
0.6
T
J
= 100°C
T
J
= 0°C
T
J
= 50°C
700
600
TRANSIMPEDANCE (V/A)
500
400
300
200
100
0
0.7
-50
-25
0
25
50
75
100
AMBIENT TEMPERATURE (°C)
4
_______________________________________________________________________________________
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
V
CC
1
FILTER
TEST
IN
GND1
GND2
GND3
OUT-
OUT+
V
CC
2
RSSI
FUNCTION
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 V
CC
1. 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
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 function-
al diagram of the MAX3970, which comprises a tran-
simpedance amplifier, a voltage amplifier, an output
buffer, a received-signal strength indicator, and a DC-
cancellation 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 R
F
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.
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 10kΩ load and is used
during the assembly process to optimally align the pho-
todiode. 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 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, 500mV
P-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 V
CC
and ground for frequen-
cies between 50kHz and 10GHz. Use LC filtering at the
main supply terminal and decoupling capacitors as
close to the die as possible.
_______________________________________________________________________________________
5
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