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19-2630; Rev 4; 10/08
6.25Gbps, 1.8V PC Board Equalizer
General Description
The MAX3785 6.25Gbps equalizer operates from a sin-
gle 1.8V supply and compensates for transmission-
medium losses encountered with FR-4 transmission
lines. Optimized for low-voltage, high-density, DC-cou-
pled interconnections between the line card and switch
card, the MAX3785 enables a system upgrade path
while maintaining a legacy rate of 2.5Gbps to
3.125Gbps. Roughly the size of two 0603 passive com-
ponents, the MAX3785 easily provides placement and
routing flexibility.
The MAX3785 is composed of an equalizer, limiting
amplifier, and output driver. For data rates of 3.2Gbps
and lower, the MAX3785 equalizes signals for spans up
to 40in of FR-4 board material. For data rates up to
6.25Gbps, the MAX3785 compensates for 30in of FR-4
board material. The MAX3785 is coding independent,
functioning equally well for 8b/10b or scrambled signals.
The MAX3785 features DC-coupled current-mode logic
(CML) data inputs and outputs. It is packaged in a tiny
1.5mm × 1.5mm chip-scale package (UCSP™) and a
6-pin TDFN package.
Applications
HSBI for 6.4Gbps
Double IEEE 802.3ae XAUI
Double STM-16/OC-48
UCSP is a trademark of Maxim Integrated Products, Inc.
Features
Single 1.8V Supply
Very Low Power, 60mW
Spans 30in with FR-4 at 6.25Gbps
Operates from 1.0Gbps to 6.4Gbps
Coding Independent, 8b/10b or Scrambled
DC-Coupled CML Inputs and Outputs
Small 1.5mm × 1.5mm Footprint
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX3785UBL
0°C to +85°C
6 UCSP (3  3)
MAX3785UWL+
0°C to +85°C
6 WLP
MAX3785UTT
0°C to +85°C
6 TDFN-EP*
MAX3785UTT+
0°C to +85°C
6 TDFN-EP*
MAX3785ITT
-20°C to +85°C
6 TDFN-EP*
MAX3785ITT+
-20°C to +85°C
6 TDFN-EP*
+Denotes a lead-free/RoHS-compliant package.
*EP = Exposed pad.
Pin Configurations
TOP VIEW (BUMPS ON BOTTOM OF DIE)
Δ
A1
IN-
A2 A3
GND OUT-
MAX3785UBL
C1 C2 C3
IN+ VCC OUT+
3 x 3 UCSP
IN- 1
6 IN+
GND 2 MAX3785UTT 5 VCC
OUT- 3
TDFN
4 OUT+
Typical Application Circuit
LINE CARD
BACKPLANE
SWITCH CARD
MAC
Rx Tx
Tx Rx
1.8V
VCC
OUT
MAX3785
IN
6.25Gbps
1.8V
VCC
IN
MAX3785
OUT
30in (0.75m)
SWITCH
ASIC WITH
SERDES
Rx
Tx
________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.

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6.25Gbps, 1.8V PC Board Equalizer
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, VCC to GND.................................-0.5V to +6.0V
Continuous Output Current (OUT+, OUT-) .......-25mA to +25mA
Input Voltage (IN+, IN-) ..............................-0.5V to (VCC + 0.5V)
Operating Ambient Temperature Range
(UBL, UTB)......................................................... 0°C to +85°C
Operating Ambient Temperature Range (ITT).....-20°C to +85°C
Storage Ambient Temperature Range...............-55°C to +150°C
Continuous Power Dissipation (TA = +70°C)
6-Pin TDFN (derate 24.4mW above +70°C)..................1.95W
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
(Typical values measured at VCC = 1.8V and TA= +25°C. Specifications guaranteed over specified operating conditions.)
(See Operating Conditions table.)
PARAMETER
CONDITIONS
MIN TYP MAX UNITS
Supply Current
35 55 mA
Input Swing (IN)
Measured differentially at data source before
encountering loss (Point A in Figure 1) (Note 1)
400
1600 mVP-P
Input Common-Mode Voltage Range (Note 1)
Input Return Loss
Differential Input Resistance
100MHz to 3.2GHz, power off
IN+ and IN-
VCC -
(INMAX/4)
VCC -
(INMIN/4)
15
85 100 115
V
dB

Output Swing
Output Resistance
Output Return Loss
Output Transition Time (tr, tf)
Measured differentially at OUT+ and OUT- with 50
±1% load at each side
450
800 mVP-P
OUT+ or OUT-
100MHz to 3.2GHz, IN+ = high
20% to 80% (Note 2)
42 50 58

14 dB
30 40 55 ps
2.5Gbps, 3.2Gbps, 5.0Gbps; 0in to 30in FR-4
400mVP-P  IN  1600mVP-P
0.10 0.15
Residual Deterministic Jitter
(Notes 1, 3, 4)
2.5Gbps, 3.2Gbps; 40in FR-4
400mVP-P  IN  1600mVP-P
6.25Gbps; 0in to 30in FR-4
600mVP-P  IN  1600mVP-P
0.15 0.20
0.15 0.25
UI
Output Random Jitter
Low-Frequency Cutoff Frequency
Latency
Maximum Bit Rate
Minimum Bit Rate
6.25Gbps; 0in to 30in FR-4
IN = 400mVP-P
(Notes 1, 2)
(Note 1)
(Note 1)
6.25
0.20
0.75
50
200
6.4
1.0
0.30
1.0
2.5
psRMS
kHz
ps
Gbps
Gbps
Note 1: Guaranteed by design and characterization.
Note 2: Using input pattern 0000011111 at 6.25Gbps.
Note 3: Difference in deterministic jitter between data source and equalizer output, evaluated at 2.5Gbps, 3.2Gbps, 5Gbps, and
6.25Gbps. Pattern used: PRBS (27), ninety-six 0s, 1, 0, 1, 0, PRBS (27), ninety-six 1s, 0, 1, 0, 1.
Note 4: Signal is applied differentially at input to a 6-mil wide, loosely coupled stripline. Deterministic jitter at the output of the
transmission line is from media-induced loss, not from clock source modulation (see Figure 1).
2 _______________________________________________________________________________________

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6.25Gbps, 1.8V PC Board Equalizer
PARAMETER
CONDITIONS
Supply Voltage (VCC)
Operating Ambient Temperature (UBL, UTT)
10Hz f < 100Hz
Supply Noise Tolerance
100Hz f < 1MHz
1MHz f 1GHz
Bit Rate
NRZ data
Operating Ambient Temperature (ITT)
Operating Conditions
MIN TYP MAX UNITS
1.71 1.8 1.89
V
0 25 85 °C
100
40 mVP-P
10
2.50 6.25 Gbps
-20 25 85 °C
SIGNAL SOURCE
A
<1.0pF SMA
CONNECTOR
PC BOARD
3in L 30in
Figure 1. Conditions of Testing
<1.0pF SMA
CONNECTOR
B
IN
MAX3785
OUT
C
FR-4
4.0 < εr < 4.4
tanδ = 0.022
Typical Operating Characteristics
(VCC = +1.8V, TA = +25°C, unless otherwise noted. Measurements done at 6.25Gbps, 500mVP-P at the source with a test pattern:
PRBS (27), ninety-six 0s, 1, 0, 1, 0, PRBS (27), ninety-six 1s, 0, 1, 0, 1. Deterministic jitter of the MAX3785 and the board was mea-
sured using Tektronix’s FrameScan™. Deterministic jitter of the system was subtracted from the measured value. Eye diagrams were
acquired by FrameScan, which includes system jitter but eliminates random jitter.)
EYE DIAGRAM OF UNEQUALIZED
SIGNAL AFTER 30in OF FR-4
MAX3785 toc01
EYE DIAGRAM OF EQUALIZED
SIGNAL AFTER 30in OF FR-4
MAX3785 toc02
EYE DIAGRAM OF EQUALIZED
SIGNAL AFTER 30in OF FR-4
MAX3785 toc03
70mV/div
70mV/div
70mV/div
30ps/div
TEST PATTERN, 6.25Gbps,
WITHOUT RANDOM JITTER,
INCLUDING 13ps SYSTEM JITTER
30ps/div
TEST PATTERN, 6.25Gbps,
WITHOUT RANDOM JITTER,
INCLUDING 13ps SYSTEM JITTER
30ps/div
210 - 1 PRBS, 6.25Gbps,
WITHOUT RANDOM JITTER,
INCLUDING 13ps SYSTEM JITTER
FrameScan is a trademark of Tektronix.
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6.25Gbps, 1.8V PC Board Equalizer
Typical Operating Characteristics (continued)
(VCC = +1.8V, TA = +25°C, unless otherwise noted. Measurements done at 6.25Gbps, 500mVP-P at the source with a test pattern:
PRBS (27), ninety-six 0s, 1, 0, 1, 0, PRBS (27), ninety-six 1s, 0, 1, 0, 1. Deterministic jitter of the MAX3785 and the board was measured
using Tektronix’s FrameScan. Deterministic jitter of the system was subtracted from the measured value. Eye diagrams were acquired
by FrameScan, which includes system jitter but eliminates random jitter.)
EYE DIAGRAM OF UNEQUALIZED
SIGNAL AFTER 30in OF FR-4
MAX3785 toc04
EYE DIAGRAM OF EQUALIZED
SIGNAL AFTER 30in OF FR-4
MAX3785 toc05
EYE DIAGRAM OF EQUALIZED
SIGNAL AFTER 30in OF FR-4
MAX3785 toc06
70mV/div
70mV/div
70mV/div
50ps/div
TEST PATTERN, 3.125Gbps,
WITHOUT RANDOM JITTER,
INCLUDING 13ps SYSTEM JITTER
EQUALIZER OPERATING CURRENT
vs. TEMPERATURE
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80
TEMPERATURE (°C)
50ps/div
TEST PATTERN, 3.125Gbps,
WITHOUT RANDOM JITTER,
INCLUDING 13ps SYSTEM JITTER
50ps/div
CRPAT, 3.125Gbps,
WITHOUT RANDOM JITTER,
INCLUDING 13ps SYSTEM JITTER
DETERMINISTIC JITTER
vs. BOARD LENGTH (FR-4)
(INPUT LEVEL OF 500mVP-P, TEST PATTERN)
65
60
55
50
45
40
35
30
25 6Gbps
3.125Gbps
20
15
10 2.5Gbps
5
5Gbps
10 15 20 25 30 35 40
BOARD LENGTH (in)
DETERMINISTIC JITTER vs. SIGNAL LEVEL
(TEST PATTERN, 30in OF FR-4 BOARD)
20
15
2.5Gbps
3.125Gbps
10
5 6.4Gbps
0
0.4
5Gbps
0.6 0.8 1.0 1.2 1.4 1.6 1.8
DIFFERENTIAL SIGNAL LEVEL (VP-P)
2.0
4 _______________________________________________________________________________________

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6.25Gbps, 1.8V PC Board Equalizer
Typical Operating Characteristics (continued)
(VCC = +1.8V, TA = +25°C, unless otherwise noted. Measurements done at 6.25Gbps, 500mVP-P at the source with a test pattern:
PRBS (27), ninety-six 0s, 1, 0, 1, 0, PRBS (27), ninety-six 1s, 0, 1, 0, 1. Deterministic jitter of the MAX3785 and the board was measured
using Tektronix’s FrameScan. Deterministic jitter of the system was subtracted from the measured value. Eye diagrams were acquired
by FrameScan, which includes system jitter but eliminates random jitter.)
DETERMINISTIC JITTER vs. DATA RATE
FOR 10in OF FR-4 BOARD
(INPUT LEVEL OF 500mVP-P)
30
25
210 - 1
20
CRPAT
15
10
5
0
2.5
TEST
27 - 1 PATTERN
K28.5
3.5 4.5 5.5
DATA RATE (Gbps)
6.5
DETERMINISTIC JITTER vs. DATA RATE
FOR 20in OF FR-4 BOARD
(INPUT LEVEL OF 500mVP-P)
30
210 - 1
25
CRPAT
20
15
TEST
10 PATTERN
K28.5
5
27 - 1
0
2.5 3.5 4.5 5.5 6.5
DATA RATE (Gbps)
DETERMINISTIC JITTER vs. DATA RATE
FOR 30in OF FR-4 BOARD
(INPUT LEVEL OF 500mVP-P)
30
CRPAT
25
20 TEST PATTERN
15 210 - 1
10
K28.5
27 - 1
5
0
2.5 3.5 4.5 5.5 6.5
DATA RATE (Gbps)
EQUALIZER INPUT RETURN GAIN (SDD11)
(INPUT SIGNAL LEVEL = -40dBm, POWER OFF)
30
20
10
0
-10
-20
-30
-40
100
1000
FREQUENCY (MHz)
10,000
EQUALIZER INPUT RETURN GAIN (SDD22)
(INPUT SIGNAL LEVEL = -40dBm, IN+ HIGH)
30
20
10
0
-10
-20
-30
-40
100
1000
FREQUENCY (MHz)
10,000
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