MAX3760 データシートの表示（PDF） - Maxim Integrated

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MAX3760

622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers

Maxim Integrated 

622Mbps, Low-Noise Transimpedance

Preamplifier for LAN and WAN Optical Receivers

Power Supply

The MAX3760 can operate from a power-supply volt-

age (VCC - GND) between 4.5V and 5.5V. GND can be

any stable voltage, including -5.0V, as found in many

systems using ECL interface levels.

Layout Considerations

Use good high-frequency design and layout tech-

niques. The use of a multilayer circuit board with sepa-

rate ground and VCC planes is recommended. Take

care to bypass VCC and to connect the GND pin to the

ground plane with the shortest possible traces.

Designing a Low-Capacitance Input

Noise performance and bandwidth will be adversely

affected by stray capacitance at IN. Make every effort

to minimize capacitance on this node. Select a low-

capacitance photodiode and use good high-frequency

design and layout techniques. The MAX3760 is opti-

mized for 0.75pF of capacitance on the input—approxi-

mately the capacitance of a low-cost photodetector

packaged in a header.

When using the MAX3760 in the SO package, note that

the package capacitance is about 0.3pF. The PC board

between the MAX3760 input and the photodetector will

add parasitic capacitance. Keep the input line short,

and remove power and ground planes beneath it.

For the best possible performance, assemble the

MAX3760 in die form using chip-and-wire technology,

or package the die in a TO header. These techniques

minimize parasitic capacitance, resulting in the lowest

noise.

Designing Filters

The MAX3760’s noise performance is strongly affected

by the circuit’s bandwidth, which changes over temper-

ature and varies from lot to lot. Receiver sensitivity can

be improved by adding filters to limit this bandwidth.

Filter designs range from a one-pole filter using a single

capacitor to more complex filters using inductors. The

simple filter provides a moderate rolloff with minimal

components, while the complex filter provides a sharp-

er rolloff and better transient response. A simple

530MHz filter can be created by placing a 6pF capaci-

tor between the OUT+ and OUT- pins.

Supply-voltage noise at the photodiode cathode pro-

duces a current, I = CPHOTO∆V/∆t, which reduces

receiver sensitivity (CPHOTO is the photodiode capaci-

tance.) The MAX3760’s FILTER resistor, combined with

an external capacitor, can be used to reduce this noise

(see Typical Application Circuit). Current generated by

supply-noise voltage is divided between CFILTER and

CPHOTO. The input noise current due to supply noise is

as follows (assuming the filter capacitor is much larger

than the photodiode capacitance):

(( ))(( )) INOISE =

VNOISE CPHOTO

RFILTER CFILTER

If the amount of tolerable noise is known, the filter

capacitor is easy to select, as follows:

(( )()( )) CFILTER >

VNOISE CPHOTO

RFILTER INOISE

For example, with maximum noise voltage = 10mVRMS,

CPHOTO = 0.75pF, RFILTER = 1kΩ, and INOISE selected

to be 30nA (half the MAX3760’s input noise):

(10mV)(0.75pF)

( )( ) CFILTER = 1000 30E - 9 = 250pF

TOP VIEW OF TO-46 HEADER

GND

OUT-

PHOTODIODE

CFILT

VCC

CVCC

OUT+

MAX3760

PHOTODIODE IS MOUNTED

ON CFILT.

CASE IS GROUND.

Figure 3. Suggested Layout for TO-46 Header

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