PAVG = PP-P (1 / 2)[(r" />
參數(shù)資料
型號(hào): MAX3665EUA
廠商: Maxim Integrated Products
文件頁(yè)數(shù): 7/10頁(yè)
文件大小: 0K
描述: IC PREAMP 622MBPS TRANS 8-UMAX
產(chǎn)品培訓(xùn)模塊: Lead (SnPb) Finish for COTS
Obsolescence Mitigation Program
標(biāo)準(zhǔn)包裝: 50
類(lèi)型: 轉(zhuǎn)阻前置放大器
應(yīng)用: 光纖學(xué)網(wǎng)絡(luò)
安裝類(lèi)型: 表面貼裝
封裝/外殼: 8-TSSOP,8-MSOP(0.118",3.00mm 寬)
供應(yīng)商設(shè)備封裝: 8-uMAX
包裝: 管件
MAX3665
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
450AP-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 = 450A, 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):
I
VC
RC
NOISE
PHOTO
FILT
= ()(
)
()(
)
622Mbps, Ultra-Low-Power, 3.3V
Transimpedance Preamplifier for SDH/SONET
6
_______________________________________________________________________________________
MAX3665
C1
5pF
22nH
RL
100
a) SIMPLE, 1-POLE, 530MHz FILTER
50
50
MAX3665
4pF
5pF
RL
100
b) 3-POLE, 515MHz FILTER
50
50
1.2pF
REFER TO THE MAX3665 EV KIT DATA SHEET
FOR THE FILTER LAYOUT EXAMPLE.
Figure 3. Filter Design Examples
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