MAX3665(2000) Просмотр технического описания (PDF) - Maxim Integrated

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MAX3665
(Rev.:2000)

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

Maxim Integrated 

622Mbps, Ultra-Low-Power, 3.3V

Transimpedance Preamplifier for SDH/SONET

Therefore,

PAVG = Pp-p (1 / 2)[(re + 1) / (re - 1)]

Sensitivity is a key specification of the receiver module.

The ITU/Bellcore specifications for SDH/SONET

receivers require a link sensitivity of -27dBm with a bit

error rate (BER) of 10-10. There is an additional 1dB

power penalty to accommodate various system losses;

therefore, the sensitivity of a 622Mbps receiver must be

better than -28dBm.

Although several parameters affect sensitivity (such as

the quantizer sensitivity and preamplifier gain, as previ-

ously discussed), most fiber optic receivers are designed

so that noise is the dominant factor. Noise from the high-

gain transimpedance amplifier, in particular, determines

the sensitivity. The noise generated by the MAX3665 can

be modeled with a Gaussian distribution. In this case, a

BER of 10-10 corresponds to a peak-to-peak signal

amplitude to RMS noise ratio (SNR) of 12.7. The

MAX3665’s typical input-referred noise, in, (bandwidth-

limited to 470MHz) is 55nARMS. Therefore, the minimum

input for a BER of 10-10 is (12.7 · 55nA) = 699nAp-p.

Rearranging the previous equations in these terms

results in the following relationship:

Optical Sensitivity (dBm) =

10log[(in / ρ)(SNR)(1/2)(re + 1) / (re - 1)(1000)]

At room temperature, with re = 10, SNR = 12.7, in =

55nA, and ρ = 0.9A/W, the MAX3665 sensitivity is

-33.2dBm. For worst-case conditions, noise increases

to 72nA and sensitivity decreases to -32.1dBm. The

MAX3665 provides 5.1dB margin over the SDH/SONET

specifications, even at +85°C.

The MAX3665’s overload current (IMAX) is greater than

450µAp-p. The pulse-width distortion and input current

are closely related. If the clock recovery circuit can

accept more pulse-width distortion, a higher input current

might be acceptable. For worst-case responsivity and

extinction ratio, ρ = 1A/W and re = ∞, the input overload

is:

Overload (dBm) = -10log (IMAX)(1 / 2)(1000)

For IMAX = 450µA, the MAX3665 overload is -6.5dBm.

Step 2: Designing Filters

The MAX3665’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. Parasitics on the PC board will affect the filter char-

acteristics. Refer to the MAX3665 EV kit data sheet for a

layout example of the filter shown in Figure 3b.

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 MAX3665, 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 )(CFILT )

a) SIMPLE, 1-POLE, 530MHz FILTER

MAX3665

50Ω

1.2pF

C1

5pF

50Ω

RL

100Ω

b) 3-POLE, 515MHz FILTER

MAX3665

50Ω

1.2pF

50Ω

22nH

4pF

5pF

RL

100Ω

22nH

REFER TO THE MAX3665 EV KIT DATA SHEET

FOR THE FILTER LAYOUT EXAMPLE.

Figure 3. Filter Design Examples

6 _______________________________________________________________________________________

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