Therefore,

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

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

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

Rearranging the previous equations in these terms

results in the following relationship:

Optical Sensitivity (dBm) =

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.

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,

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

∆V/∆t), which reduces

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

Current generated by supply noise is divided between

ply noise is (assuming the filter capacitor is much larger

than the photodiode capacitance):

I

VC

RC

NOISE

NOISE

PHOTO

FILT

FILT

)

()(

)

622Mbps, Ultra-Low-Power, 3.3V

Transimpedance Preamplifier for SDH/SONET

6

_______________________________________________________________________________________

MAX3665

C1

5pF

22nH

22nH

100

Ω

50

Ω

50

Ω

MAX3665

4pF

5pF

100

Ω

50

Ω

50

Ω

1.2pF

1.2pF

REFER TO THE MAX3665 EV KIT DATA SHEET

FOR THE FILTER LAYOUT EXAMPLE.

Figure 3. Filter Design Examples