MAXIM MAX3725E

19-3181; Rev 0; 2/04
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
The MAX3724/MAX3725 transimpedance amplifiers provide a compact, low-power solution for communication
up to 3.2Gbps. They feature 325nA input-referred noise
at 2.1GHz bandwidth (BW) with 0.6pF input capacitance. The parts also have >2mAP-P AC input overload.
Both parts operate from a single +3.3V supply and consume 93mW. The MAX3724/MAX3725 are in a compact
30-mil x 50-mil die and require no external compensation capacitor. A space-saving filter connection is provided for positive bias to the photodiode through an
on-chip 580Ω resistor to V CC. These features allow
easy assembly into a low-cost TO-46 or TO-56 header
with a photodiode.
The MAX3724 and MAX3748A receiver chip set provides an RSSI output using a Maxim-proprietary* interface technique. The MAX3724 preamplifier, MAX3748A
postamplifier, and the DS1858/DS1859 SFP controller
meet all the SFF-8472 digital diagnostic requirements.
Features
♦ Wider Bandwidth than MAX3744/MAX3745
♦ RSSI Implementation in 4-Pin TO-46 Header
(MAX3724 and MAX3748A)
♦ 8psP-P Deterministic Jitter for <100µAP-P Input
Current
♦ 325nARMS Input-Referred Noise at 2.1GHz
Bandwidth
♦ 28mA Supply Current at +3.3V
♦ 2.5GHz Small-Signal Bandwidth
♦ 2.0mAP-P AC Overload
♦ Die Size: 30 mils x 50 mils (Identical to the
MAX3744/MAX3745)
Applications
Ordering Information
Up to 3.2Gbps SFF/SFP Optical Receivers
PART
Optimized for Small-Form-Factor Pluggable (SFP)
Optical Receivers
TEMP RANGE
PIN-PACKAGE
MAX3724E/D
-40°C to +85°C
Dice**
MAX3725E/D
-40°C to +85°C
Dice**
**Dice are designed to operate with junction temperatures of
-40°C to +110°C but are tested and guaranteed only at TA =
+25°C.
*Patent pending.
Typical Application Circuit
SFP OPTICAL RECEIVER
VCC
400pF
400pF
VCC
VCC
FILTER OUT+
IN
MAX3724
OUTGND
0.1µF
MAX3748A
IN-
OUTRSSI
4-PIN TO CAN
0.1µF
OUT+
IN+
DISABLE
HOST
BOARD
VCC = 3.3V
4.7kΩ
TO
10kΩ
LOS
3.3kΩ
MOD-DEF1
DS1858/
DS1859
MOD-DEF2
________________________________________________________________ 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
MAX3724/MAX3725
General Description
MAX3724/MAX3725
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
ABSOLUTE MAXIMUM RATINGS
Power-Supply Voltage (VCC) .................................-0.5V to +6.0V
Continuous CML Output Current
(OUT+, OUT-) ............................................. -25mA to +25mA
Continuous Input Current (IN)...............................-4mA to +4mA
Continuous Input Current (FILTER).......................-8mA to +8mA
Operating Junction Temperature Range (TJ) ....-55°C to +150°C
Storage Ambient Temperature Range (TSTG) ...-55°C to +150°C
Die Attach 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 = +2.97V to +3.63V and TA = -40°C to +85°C. Typical values are at VCC = +3.3V, source capacitance (CIN) = 0.85pF, and TA =
+25°C, unless otherwise noted.) (Notes 1, 2)
PARAMETER
Supply Current
SYMBOL
ICC
CONDITIONS
MIN
Including CML output current (IIN = 0)
TYP
MAX
UNITS
28
41
mA
Input Bias Voltage
1.0
Input Overload
Input-Referred Noise (Note 4)
(Note 3)
IN
2
CIN = 0.85pF, BW = 933MHz
200
CIN = 0.85pF, BW = 2.1GHz (Note 3)
360
CIN = 0.85pF, BW = 18GHz (Note 3)
CIN = 0.6pF, BW = 933MHz
200
CIN = 0.6pF, BW = 2.1GHz (Note 3)
325
Small-Signal Bandwidth (Note 3)
Differential output, IIN = 40µAAVE
BW
Low-Frequency Cutoff
Deterministic Jitter
(Notes 3, 5)
nARMS
440
635
2.8
3.5
-3dB, CIN = 0.6pF
2
2.5
-3dB, CIN = 0.85pF
1.8
2.3
-3dB, input current = 20µAAVE (Note 3)
DJ
485
700
CIN = 0.6pF, BW = 18GHz (Note 3)
Differential Transimpedance
V
mAP-P
4.5
GHz
30
100µAP-P < input ≤ 2.1Gbps, K28.5 pattern
2mAP-P
2.7Gbps, 231 - 1 pattern
11
10µAP-P < input ≤
100µAP-P
8
2.7Gbps, 231 - 1 pattern
kHz
33
15
2.1Gbps, K28.5 pattern
kΩ
17
psP-P
12
Filter Resistance
510
580
690
Ω
Differential Output Resistance
(OUT+, OUT-)
85
100
115
Ω
220
280
400
mVP-P
Maximum Differential Output
Voltage
2
VOD
Input > 50µAAVE, output termination 50Ω to
VCC (output in limited state)
_______________________________________________________________________________________
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
(VCC = +2.97V to +3.63V and TA = -40°C to +85°C. Typical values are at VCC = +3.3V, source capacitance (CIN) = 0.85pF, and TA =
+25°C, unless otherwise noted.) (Notes 1, 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
Single-Ended Output CommonMode Minimum Level (MAX3724)
Relative to VCC, IIN = 1mAAVE
540
490
Output Data Transition Time
Input > 200µAP-P 20% to 80% rise/fall time
(Note 3)
70
Frequency ≤ 1GHz
17
1GHz < frequency ≤ 2GHz
10
Differential Output Return Loss
Power-Supply Noise Rejection
PSNR
IIN = 0
(Note 6)
RSSI Gain (MAX3724)
ARSSI
(Note 7)
RSSI Gain Stability (MAX3724)
f < 1MHz
46
1MHz ≤ f < 10MHz
34
10log(ARSSI/ARSSI-NOM) where ARSSI-NOM =
ARSSI at 3.3V, +25°C (Note 3)
MAX
UNITS
mV
100
ps
dB
dB
21
A/A
0.24
dB
Note 1: Dice are designed to operate with junction temperatures of -40°C to +110°C but are tested and guaranteed only at TA = 25°C.
Note 2: Source capacitance represents the total capacitance at the IN pad during characterization of the noise and bandwidth parameters.
Note 3: Guaranteed by design and characterization.
Note 4: Input-referred noise is:
 RMS output noise 
 Gain at f = 100MHz 


Note 5: Deterministic jitter is the sum of pulse-width distortion (PWD) and pattern-dependent jitter (PDJ).
Note 6: Power-supply noise rejection PSNR = -20log(∆VOUT / ∆VCC), where ∆VOUT is the differential output voltage and ∆VCC is the
noise on VCC.
Note 7:
IOUT _ CM (IIN = 400µA) − IOUT _ CM (IIN = 0µA)
ARSSI =
400µA
IOUT + + IOUT −
where IOUT _ CM =
2
RSSI range is from IIN = 6µA to 500µA
_______________________________________________________________________________________
3
MAX3724/MAX3725
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VCC = +3.3V, CIN = 0.85pF, TA = +25°C, unless otherwise noted.)
CIN = 0.85pF
500
CIN = 0.5pF
400
300
BW = 2.1GHz
200
700
0
600
500
CIN = 1.5pF
CIN = 0.85pF
400
20
40
60
80
5
CIN = 0.5pF
-5
-40
-20
0
20
40
60
80
100
-10
100M
1G
10G
TEMPERATURE (°C)
FREQUENCY (Hz)
DETERMINISTIC JITTER
vs. INPUT VOLTAGE
SMALL-SIGNAL TRANSIMPEDANCE
vs. TEMPERATURE
EYE DIAGRAM
INPUT = 20µAP-P, DATA RATE = 2.1Gbps
MAX3724/25 toc04
50
45
40
35
30
25
20
2.7Gbps, 27-1PBRS
15
MAX3724/25 toc06
75
K28.5 PATTERN
70
10mV/div
65
10
2.1Gbps, K28.5
5
0
0.01
0.1
1
10
60
-40
-20
0
20
40
60
80
TEMPERATURE (°C)
EYE DIAGRAM
INPUT = 20µAP-P, DATA RATE = 3.2Gbps
EYE DIAGRAM
INPUT = 2mAP-P, DATA RATE = 2.1Gbps
MAX3724/25 toc07
10mV/div
EYE DIAGRAM
INPUT = 2mAP-P, DATA RATE = 3.2Gbps
MAX3724/25 toc08
MAX3724/25 toc09
K28.5 PATTERN
50mV/div
50ps/div
60ps/div
100
INPUT CURRENT (mAP-P)
K28.5 PATTERN
4
15
10
0
300
100
20
TEMPERATURE (°C)
TRANSIMPEDANCE (dB Ω)
-20
30
25
200
-40
35
GAIN (dB)
CIN = 1.5pF
40
MAX3724/25 toc02
700
800
MAX3724/25 toc05
INPUT-REFERRED NOISE (nARMS)
UNFILTER
INPUT-REFERRED NOISE (nARMS)
MAX3724/25 toc01
800
600
OPTICAL FREQUENCY RESPONSE
(70µm InGaAS PHOTODIODE, INPUT -22dBm)
INPUT-REFERRED NOISE
vs. TEMPERATURE
MAX3724/25 toc03
INPUT-REFERRED NOISE
vs. TEMPERATURE
DETERMINISTIC JITTER (psP-P)
MAX3724/MAX3725
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
K28.5 PATTERN
50mV/div
60ps/div
50ps/div
_______________________________________________________________________________________
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
SUPPLY CURRENT
vs. TEMPERATURE
S22 (dB)
-15
MAX3725
-20
60
-25
50
40
30
20
-20
0
RSSI
MAX3724, MAX3748A
400
350
300
250
40
60
80
100
-100
-50
0
50
100
INPUT CURRENT (mAP-P)
BANDWIDTH vs. TEMPERATURE
EYE DIAGRAM
TEMPERATURE = +100°C INPUT = 20µAP-P
DATA RATE = 2.7Gbps
MAX3724/25 toc15
3
TA = +85°C
-50
MAX3724/25 toc14
450
3dB BANDWIDTH
MAX3748 RSSI OUTPUT (µA)
500
20
4
MAX3724/25 toc13
TA = -40°C
0
TEMPERATURE (°C)
FREQUENCY (MHz)
550
50
-200
-40
500 1000 1500 2000 2500 3000 3500 4000
100
-150
0
0
150
-100
10
-30
200
MAX3724/25 toc12
MAX3724
-10
MAX3724/25 toc11
-5
70
SUPPLY CURRENT (mA)
MAX3724/25 toc10
0
DC TRANSFER FUNCTION
(VFILT = 0V)
OUTPUT VOLTAGE (mVP-P)
DIFFERENTIAL S22 vs. FREQUENCY
223 - 1 PRBS
MAX3724 CIN = 0.6pF
2
6mV/div
MAX3744 CIN = 0.6pF
1
200
150
0
0
500
1000
1500
2000
-40
-20
0
20
40
60
80
100
AVERAGE INPUT CURRENT (µA)
TEMPERATURE (°C)
OPTICAL EYE DIAGRAM
INPUT = 0dBm, DATA RATE = 3.2Gbps
OPTICAL EYE DIAGRAM
INPUT = 0dBm, DATA RATE = 4.25Gbps
MAX3724/25 toc16
MAX3724/25 toc18
K28.5 PATTERN
30mV/div
60ps/div
OPTICAL EYE DIAGRAM
INPUT = -18dBm, DATA RATE = 4.25Gbps
MAX3724/25 toc17
K28.5 PATTERN
100mV/div
60ps/div
K28.5 PATTERN
3mV/div
60ps/div
50ps/div
_______________________________________________________________________________________
5
MAX3724/MAX3725
Typical Operating Characteristics (continued)
(VCC = +3.3V, CIN = 0.85pF, TA = +25°C, unless otherwise noted.)
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
MAX3724/MAX3725
Pin Description
MAX3724/
MAX3725
BOND PAD
NAME
1, 3
VCC
2, 7
N.C.
4
IN
5
FILTER
6, 10
GND
Supply Ground
8
OUT-
Inverting Data Output. Current flowing into IN causes the voltage at OUT- to decrease. For the
MAX3724, the common mode between OUT+ and OUT- is proportional to the average input
current.
9
OUT+
Noninverting Data Output. Current flowing into IN causes the voltage at OUT+ to increase. For
the MAX3724, the common mode between OUT+ and OUT- is proportional to the average input
current.
FUNCTION
Supply Voltage
No Connection
TIA Input. Signal current from photodiode flows into this pin.
Provides bias voltage for the photodiode through a 580Ω resistor to VCC. When grounded, this
pin disables the DC cancellation amplifier to allow a DC path from IN to OUT+ and OUT- for
testing.
VCC
MAX3724
RF
RF
TRANSIMPEDANCE
AMPLIFIER
TRANSIMPEDANCE
AMPLIFIER
OUT+
OUT-
IN
DC CANCELLATION
CIRCUIT
VCC
DC CANCELLATION
CIRCUIT
RSSI
MAX3725
FILTER
FILTER
50Ω
OUT+
OUT-
IN
50Ω
50Ω
VCC
50Ω
Figure 1. Functional Diagram
Detailed Description
The MAX3724/MAX3725 are transimpedance amplifiers
designed for up to 3.2Gbps SFF/SFP transceiver modules. A functional diagram of the MAX3724/MAX3725 is
shown in Figure 1. The MAX3724/MAX3725 comprise a
transimpedance amplifier stage, a voltage amplifier
stage, an output buffer, and a direct-current (DC) feedback cancellation circuit. The MAX3724 also includes a
signal strength indicator (RSSI). To provide this signal in
6
a standard 4-pin TO header, the RSSI level is added to
the common mode of the differential data output pins.
Transimpedance Amplifier Stage
The signal current at the input flows into the summing
node of a high-gain amplifier. Shunt feedback through
the resistor RF converts this current to a voltage. In parallel with the feedback resistor are two back-to-back
Schottky diodes that clamp the output signal for large
input currents, as shown in Figure 2.
_______________________________________________________________________________________
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
AMPLITUDE
MAX3724/MAX3725
AMPLITUDE
INPUT FROM PHOTODIODE
TIME
TIME
OUTPUT (SMALL SIGNALS)
INPUT AFTER DC CANCELLATION
OUTPUT (LARGE SIGNALS)
Figure 2. MAX3724/MAX3725 Limited Output
Voltage Amplifier Stage
The voltage amplifier stage provides gain and converts
the single-ended input to differential outputs.
Figure 3. DC Cancellation Effect on Input
VCC
100Ω
DC Cancellation Circuit
The DC cancellation circuit uses low-frequency feedback to remove the DC component of the input signal
(Figure 3). This feature centers the input signal within
the transimpedance amplifier’s linear range, thereby
reducing pulse-width distortion caused by large input
signals. The DC cancellation circuit is internally compensated and therefore does not require external
capacitors.
OUT+
OUT-
Output Buffer
The output buffer provides a reverse-terminated voltage
output. The buffer is designed to drive a 100Ω differential
load between OUT+ and OUT-. The MAX3724 must be
DC-coupled to the MAX3748A. See Figures 4 and 5.
For optimum supply-noise rejection, the MAX3725
should be terminated with a matched load. If a singleended output is required, the unused output should be
terminated to a 50Ω resistor to VCC. The MAX3725
does not drive a DC-coupled, 50Ω grounded load;
however, it does drive a compatible 50Ω CML input.
Figure 4. Equivalent Output MAX3724
VCC
50Ω
50Ω
Signal-Strength Indicator
OUT+
The MAX3724 produces a signal proportional to the
average photodiode current. This is added to the common mode of the data outputs OUT+ and OUT-. This
signal is intended for use with the MAX3748A to provide a ground-referenced RSSI voltage.
OUT-
Applications Information
Signal-Strength Indicator
The SFF-8472 digital diagnostic specification requires
monitoring of input receive power. The MAX3748A and
MAX3724 receiver chipset allows for the monitoring of
the average receive power by measuring the average
DC current of the photodiode.
Figure 5. Equivalent Output MAX3725
_______________________________________________________________________________________
7
MAX3724/MAX3725
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
The MAX3724 preamp measures the average photodiode current and provides the information to the output
common mode. The MAX3748A RSSI detect block
senses the common-mode DC level of input signals IN+
and IN- and provides a ground-level-referenced output
signal of the photodiode current. The advantage of this
implementation is that it allows the TIA to be packaged
in a low-cost conventional 4-pin TO-46 header.
The MAX3748A RSSI output is connected to an analog
input channel of the DS1858/DS1859 SFP controller to
convert the analog information into a 16-bit word. The
DS1858/DS1859 provide the received power information to the host board of the optical receiver through a
2-wire interface. The DS1859 allows for internal calibration of the receive power monitor.
The MAX3724 and the MAX3748A have been optimized
to achieve RSSI stability of better than 2.5dB within the
6µA to 500µA range of average input photodiode current. To achieve the best accuracy, Maxim recommends receive power calibration to be 6µA at the low
end, and 500µA at the high end of the required range.
Optical Power Relations
Many of the MAX3724/MAX3725 specifications relate to
the input signal amplitude. When working with optical
receivers, the input is sometimes expressed in terms of
average optical power and extinction ratio. Figure 6
and Table 1 show relations that are helpful for converting optical power to input signal when designing with
the MAX3724/MAX3725. (Refer to Application Note
HFAN–3.0.0: Accurately Estimating Optical Receiver
Sensitivity.)
Optical Sensitivity Calculation
The input-referred RMS noise current (I N ) of the
MAX3724/MAX3725 generally determines the receiver
sensitivity. To obtain a system bit-error rate (BER) of
1E-12, the signal-to-noise 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
(
r
−
)
2
ρ
1


e
where ρ is the photodiode responsivity in A/W and IN is
RMS current in amps.
Input Optical Overload
The overload is the largest input that the MAX3724/
MAX3725 can accept while meeting deterministic jitter
specifications. The optical overload can be estimated
in terms of average power with the following equation:
 2mARMS (re + 1)

Overload = 10log
1000 dBm
2ρ(re − 1)


Optical Linear Range
The MAX3724/MAX3725 have high gain, which limits
the output when the input signal exceeds 50µAP-P. The
MAX3724/MAX3725 operate in a linear range (10% linearity) for inputs not exceeding:
 50µARMS (re + 1)

Linear Range = 10log
1000 dBm
2ρ(re − 1)


Table 1. Optical Power Relations
PARAMETER
SYMBOL
RELATION
Average power
PAVG
Extinction ratio
re
PAVG = (P0 + P1) / 2
r e = P1 / P 0
Optical power of a 1
P1
P1 = 2PAVG(re) / (re + 1)
Optical power of a zero
P0
P0 = 2PAVG / (re + 1)
Signal amplitude
PIN
PIN = P1 - P0; PIN = 2PAVG(re - 1) / (re + 1)
Note: Assuming 50% average duty cycle and mark density.
8
_______________________________________________________________________________________
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
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 filter resistor of the MAX3724/MAX3725,
combined with an external capacitor, can be used to
reduce this noise (see the Typical Application Circuit).
Current generated by supply noise voltage is divided
between CFILTER and CPD. The input noise current due
to supply noise is (assuming the filter capacitor is much
larger than the photodiode capacitance):
INOISE = (VNOISE)(CPD) / (RFILTER)(CFILTER)
If the amount of tolerable noise is known, the filter
capacitor can be easily selected:
CFILTER = (VNOISE)(CPD) / (RFILTER)(INOISE)
For example, with maximum noise voltage = 100mVP-P,
CPD = 0.85pF, RFILTER = 600Ω, and INOISE selected to
be 350nA:
CFILTER = (100mV)(0.85pF) / (600Ω)(350nA) = 405pF
Wire Bonding
For high-current density and reliable operation, the
MAX3724/MAX3725 use gold metalization. Connections
to the die should be made with gold wire only, using ballbonding techniques. Die thickness is typically 14 mils
(0.4mm).
TOP VIEW OF TO-46 HEADER
CASE
GROUND
OPTICAL POWER
PI
400pF TO
1000pF
400pF TO
1000pF
VCC
PHOTODIODE
PAVG
OUT+
OUT-
PO
TIME
MAX3724
MAX3725
Figure 6. Optical Power Relations
Figure 7. Suggested Layout for TO-46 Header
_______________________________________________________________________________________
9
MAX3724/MAX3725
Layout Considerations
Noise performance and bandwidth are adversely affected by capacitance at the IN pad. Minimize capacitance
on this pad and select a low-capacitance photodiode.
Assembling the MAX3724/MAX3725 in die form using
chip and wire technology provides the best possible
performance. Figure 7 shows a suggested layout for a
TO header for the MAX3724/MAX3725. The placement
of the filter cap to minimize the ground loop of the photodiode is required to achieve the specified bandwidth.
The OUT+ and OUT- bond wire lengths should also be
minimized to meet the bandwidth specification. Special
care should be taken to ensure that ESD at IN does not
exceed 500V.
3.2Gbps SFP Transimpedance
Amplifiers with RSSI
MAX3724/MAX3725
Chip Topography
GND
VCC
1
N.C.
2
VCC
3
IN
4
FILTER
5
MAX3724
MAX3725
10
9
OUT+
0.03in
(0.76mm)
6
7
GND
N.C.
8
OUT-
0.05in
(1.26mm)
Pad Coordinates
Chip Information
PAD
COORDINATES (µm)
X
COORDINATES (µm)
Y
1
1.4
495.6
2
0
336
3
0
224
4
0
112
5
0
0
6
494.2
-1.4
7
865.2
-1.4
8
1005.2
-1.4
9
1005.2
495.6
10
490
495.6
TRANSISTOR COUNT: 301
PROCESS: SiGe Bipolar
SUBSTRATE: ISOLATED
DIE THICKNESS: 0.014in ±0.001in
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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is a registered trademark of Maxim Integrated Products.