MAX3664 データシートの表示（PDF） - Maxim Integrated

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MAX3664

622Mbps, Ultra-Low-Power, 3.3V Transimpedance Preamplifier for SDH/SONET

Maxim Integrated 

622Mbps, Ultra-Low-Power, 3.3V

Transimpedance Preamplifier for SDH/SONET

Step 3: Designing Filters

The MAX3664’s noise performance is a strong function

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

ature and varies from lot to lot. The receiver sensitivity

can be improved by adding filters to limit this band-

width. Filter designs can range from a one-pole filter

using a single capacitor, to more complex filters using

inductors. Figure 3 illustrates two examples: the simple

filter provides moderate roll-off with minimal compo-

nents, while the complex filter provides a sharper roll-

off and better transient response.

Supply voltage noise at the cathode of the photodiode

produces a current I = CPHOTO (∆V/∆t), which reduces

the receiver sensitivity. CPHOTO is the photodiode

capacitance.

The FILT resistor of the MAX3664, combined with an

external capacitor (see Typical Operating Circuit) can

be used to reduce this noise. The external capacitor

(CFILT) is placed in parallel with the photodiode.

Current generated by supply noise is divided between

CFILT and CPHOTO. The input noise current due to sup-

ply noise is (assuming the filter capacitor is much larger

than the photodiode capacitance):

INOISE

=

(VNOISE )(CPHOTO )

(RFILT )(CFILTER )

If the amount of tolerable noise is known, then the filter

capacitor can be easily selected:

( ( ))(( ) ) CFILT =

VNOISE CPHOTO

RFILT INOISE

For example, with maximum noise voltage = 100mVp-p,

CPHOTO = 0.5pF, RFILT = 1kΩ, and INOISE selected to

be 5nA (1/10 of MAX3664 input-referred noise):

[ ] CFILT = (0.1)(0.5E −12) / (1000)(5E − 9) = 10nF

Step 4: Designing a Low-Capacitance Input

Noise performance and bandwidth are adversely

affected by stray capacitance on the input node.

Select a low-capacitance photodiode and use good

high-frequency design and layout techniques to mini-

mize capacitance on this pin. The MAX3664 is

optimized for 0.5pF of capacitance on the input—

approximately the capacitance of a photodetector

diode sharing a common header with the MAX3664 in

die form.

a) SIMPLE, 1-POLE, 530MHz FILTER

MAX3664

60Ω

60Ω

C1

5pF

RL

100Ω

b) 3-POLE, 470MHz BESSEL FILTER

MAX3664

60Ω

60Ω

15.5nF

1.2pF

15.5nF

7.3pF

RL

100Ω

Figure 3. Filter Design Examples

Photodiode capacitance changes significantly with bias

voltage. With a 3.3V supply voltage, the reverse voltage

on the PIN diode is only 2.5V. If a higher voltage supply

is available, apply it to the diode to significantly reduce

capacitance.

Take great care to reduce input capacitance. With the

SO and µMAX versions of the MAX3664, the package

capacitance is about 0.3pF, and the PC board between

the MAX3664 input and the photodiode can add para-

sitic capacitance. Keep the input line short, and remove

power and ground planes beneath it. Packaging the

MAX3664 into a header with the photodiode provides

the best possible performance. It reduces parasitic

capacitance to a minimum, resulting in the lowest noise

and the best bandwidth.

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