TEXAS INSTRUMENTS ONET2591TA Technical data

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2.5-Gbps Transimpedance Amplifier With AGC and RSSI

FEATURES APPLICATIONS

• 1.8-GHz Bandwidth

• 2.6-k Ω Differential Transimpedance

• Automatic Gain Control (AGC)

• 6.6-pA/ √ Hz Typical Input Referred Noise

• 2-mA

• Received Signal Strength Indication (RSSI)

• CML Data Outputs With On-Chip 50- Ω

Back-Termination

• On-Chip Supply Filter Capacitor

• Single 3.3-V Supply

• Die Size: 0,78 mm × 1,18 mm

Maximum Input Current

p-p

ONET2591TA

SLLS669 – SEPTEMBER 2005

• SONET/SDH Transmission Systems at OC24

and OC48

• 2.125-Gbps and 1.0625-Gbps Fibre-Channel

Receivers

• Gigabit Ethernet Receivers

• PIN Preamplifier-Receivers

DESCRIPTION

The ONET2591TA is a high-speed transimpedance amplifier used in optical receivers with data rates up to 2.5
Gbps.

It features a low input referred noise, 1.8-GHz bandwidth, automatic gain control (AGC), 2.6-k Ω transimpedance,
and received signal strength indication (RSSI).

The ONET2591TA is available in die form and is optimized for use in a TO can.
The ONET2591TA requires a single 3.3-V supply, and its power-efficient design typically dissipates less than 53

mW. The device is characterized for operation from –40 ° C to 85 ° C ambient temperature.

AVAILABLE OPTIONS

T

A

–40 ° C to 85 ° C ONET2591TAY

DIE

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas
Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.

Copyright © 2005, Texas Instruments Incorporated

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B0066-01

OUT+

OUT–

RSSI

V

CC

FILTER

220 W

200 pF

275 pF

DC Input Current

Cancellation,

AGC, and RSSI

Band-Gap Voltage

Reference and

Bias Current

Generation

GND

R

F

IN

Voltage Amplifier

CML Output BufferTransimpedance Amplifier

ONET2591TA

SLLS669 – SEPTEMBER 2005

BLOCK DIAGRAM

The ONET2591TA is a high-performance, 2.5-Gbps transimpedance amplifier consisting of the signal path,
supply filter, a control block for dc input current cancellation, automatic gain control (AGC), received signal
strength indication (RSSI), and a band-gap voltage reference and bias current generation block.

The signal path comprises a transimpedance amplifier stage, a voltage amplifier, and a CML output buffer.
The on-chip filter circuit provides filtered V

input current cancellation and AGC use internal low-pass filters to cancel the dc current on the input and to
adjust the transimpedance amplifier gain. Furthermore, circuitry to monitor the received signal strength is
provided.

A simplified block diagram of the ONET2591TA is shown in Figure 1 .

for the photodiode and for the transimpedance amplifier. The dc

CC

Figure 1. Simplified Block Diagram of the ONET2591TA

SIGNAL PATH

The first stage of the signal path is a transimpedance amplifier that takes the photodiode current and converts it
into a voltage signal.

If the input signal current exceeds a certain value, the transimpedance gain is reduced by means of AGC
circuitry.

The second stage is a voltage amplifier that provides additional gain and converts its single-ended input voltage
into a differential data signal.

The third signal-path stage is the output buffer, which provides CML outputs with on-chip, 50- Ω back-termination
to V

.

CC

2

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ONET2591TA

SLLS669 – SEPTEMBER 2005

FILTER CIRCUITRY

The filter pin provides filtered V
implemented using a filter resistor of 220 Ω and an internal 200-pF capacitor. The corresponding corner
frequency is below 4 MHz.

If a lower cutoff frequency is required for the intended application, an external capacitor can be connected to one
of the FILTER pins.

The supply voltage for the whole amplifier is filtered by means of an on-chip, 275-pF capacitor as well, thus
avoiding the necessity to use an external supply-filter capacitor.

DC INPUT CURRENT CANCELLATION, AGC, AND RSSI

The voltage drop across the internal photodiode supply-filter resistor is monitored by means of a dc input current
cancellation, AGC, and RSSI control circuit block.

If the dc input current exceeds a certain level, it is partially cancelled by means of a controlled current source.
This measure keeps the transimpedance amplifier stage within sufficient operating point limits for optimum
performance. Furthermore, disabling the dc input cancellation at low input currents leads to superior noise
performance.

The AGC circuitry lowers the effective transimpedance feedback resistor R
acting as a controlled shunt. This prevents the transimpedance amplifier from being overdriven at high input
currents, which leads to improved jitter behavior within the complete input-current dynamic range. Because the
voltage drop across the supply-filter resistor is sensed and used by the AGC circuit, the photodiode must be
connected to a FILTER pad for the AGC to function correctly.

Finally, this circuit block senses the current through the filter resistor and generates a mirrored current, which is
proportional to the input signal strength. The mirrored current is available at the RSSI output and must be sunk to
ground (GND) using an external resistor. The RSSI gain can be adjusted by choosing the external resistor;
however, for proper operation, ensure that the voltage at the RSSI pad never exceeds V

for the photodiode bias. The on-chip, low-pass filter for the photodiode V

CC

by means of a MOSFET device

F

– 0.65 V.

CC

is

CC

BAND-GAP VOLTAGE AND BIAS GENERATION

The ONET2591TA transimpedance amplifier is supplied by a single, 3.3-V supply voltage connected to the V
pad. This voltage is referred to GND.

On-chip band-gap voltage circuitry generates a supply-voltage-independent reference from which all other
internally required voltages and bias currents are derived.

CC

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7

11

9

8

10

2

3

1

2591TA

4

6

5

M0033-01

GND GND

GND

OUT–

RSSI

GND

OUT+

V

CC

FILTER

FILTER

IN

ONET2591TA

SLLS669 – SEPTEMBER 2005

BOND PAD ASSIGNMENT

The ONET2591TA is available as a bare die. The locations of the bond pads are shown in the following figure.

PAD

NAME NO.

FILTER 5, 6 Analog

GND 1, 2, 10, 11 Supply
IN 7 Analog input Data input to TIA (photodiode anode)

OUT+ 3 Analog output Non-inverted data output. On-chip 50- Ω back-terminated to VCC.
OUT– 9 Analog output Inverted data output. On-chip 50- Ω back-terminated to VCC.

RSSI 8 Analog output gain can be adjusted by choosing the external resistor; however, for proper operation, ensure

V

CC

4 Supply 3.3-V, +10%/–12% supply voltage

BOND PAD DESCRIPTION

TYPE DESCRIPTION

Bias voltage for photodiode (cathode). These pads connect through an internal 220- Ω resistor to
V

and a 200-pF filter capacitor to ground (GND). Both FILTER pads are connected on-chip.

CC

For additional photodiode supply filtering, connect an external capacitor from one of the FILTER
pads to GND. The FILTER pad(s) must be connected to the photodiode for the AGC to function.

Circuit ground. All GND pads are connected on die. Bonding all pads is optional; however, for
optimum performance a good ground connection is mandatory.

Analog output current proportional to the input data amplitude. Indicates the strength of the
received signal (RSSI). Must be sunk through an external resistor to ground (GND). The RSSI

that the voltage at the RSSI pad never exceeds V
this pad must be bonded to ground (GND) to ensure proper operation.

– 0.65 V. If the RSSI feature is not used,

CC

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ONET2591TA

SLLS669 – SEPTEMBER 2005

ABSOLUTE MAXIMUM RATINGS

over operating free-air temperature range (unless otherwise noted)

V

CC

V

, V

OUT–

, V

OUT+

OUT–

FILTER

V

RSSI

I

IN

I

FILTER

I

, I

OUT+

ESD

T

J,max

T

stg

T

A

Supply voltage

, Voltage at FILTER, OUT+, OUT–, RSSI

Current into IN –0.7 mA to 2.5 mA
Current into FILTER – 8 mA to 8 mA
Continuous current at outputs – 8 mA to 8 mA
ESD rating at all pins except IN
ESD rating at IN
Maximum junction temperature 125 ° C
Storage temperature range –65 ° C to 85 ° C
Operating free-air temperature range –40 ° C to 85 ° C

(1) 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 under "recommended operating

conditions" is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
(2) All voltage values are with respect to network ground terminal.
(3) For optimum high-frequency performance, the input pin has reduced ESD protection.

(2)

(3)

(3)

RECOMMENDED OPERATING CONDITIONS

over operating free-air temperature range (unless otherwise noted)

V

CC

T

A

L

FILTER

L

IN

Supply voltage 2.9 3.3 3.6 V
Operating free-air temperature –40 85 ° C

, Wire-bond inductor at pins FILTER and IN 0.8 nH

(1)

–0.3 V to 4 V

(2)

–0.3 V to 4 V

1.5 kV (HBM)
900 V (HBM)

MIN NOM MAX UNIT

DC ELECTRICAL CHARACTERISTICS

over recommended operating conditions (unless otherwise noted). Typical values are at V

PARAMETER TEST CONDITIONS MIN TYP MAX UNIT

V

CC

I

VCC

V

IN

R

OUT

R

FILTER

Supply voltage 2.9 3.3 3.6 V

Average photodiode current IPD= 0 10 14 20

Supply current mA

mA
Average photodiode current IPD= 1 13 17 23

mA
Input bias voltage 0.85 1.05 V
Output resistance Single-ended to V

CC

Photodiode filter resistance 220 Ω

= 3.3 V and TA= 25 ° C.

CC

40 50 60 Ω

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