MAX3665EUA Maxim Integrated Products, MAX3665EUA Datasheet - Page 6

• Manufacturer Part Number: MAX3665EUA
• Description: IC PREAMP 622MBPS TRANS 8-UMAX
• Manufacturer: Maxim Integrated Products
• Type: Transimpedance Preamplifierr

Specifications of MAX3665EUA

• Applications: Optical Networks
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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
error rate (BER) of 10
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
amplitude to RMS noise ratio (SNR) of 12.7. The
MAX3665’s typical input-referred noise, i
limited to 470MHz) is 55nA
input for a BER of 10
Rearranging the previous equations in these terms
results in the following relationship:
At room temperature, with r
55nA, and
-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 (I
450µA
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, = 1A/W and r
For I
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-
622Mbps, Ultra-Low-Power, 3.3V
Transimpedance Preamplifier for SDH/SONET
6
_______________________________________________________________________________________
MAX
Overload (dBm) = -10log (I
Optical Sensitivity (dBm) =
10log[(i
P-P
. The pulse-width distortion and input current are
= 450µA, the MAX3665 overload is -6.5dBm.
P
-10
AVG
n
= 0.9A/W, the MAX3665 sensitivity is
corresponds to a peak-to-peak signal
/ )(SNR)(1/2)(r
= P
P-P
e
= , the input overload is:
-10
(1 / 2)[(r
-10
. There is an additional 1dB
is (12.7 · 55nA) = 699nA
RMS
e
e
. Therefore, the minimum
e
= 10, SNR = 12.7, i
+ 1) / (r
MAX
+ 1) / (r
MAX
)(1 / 2)(1000)
) is greater than
e
e
n
- 1)(1000)]
- 1)]
, (bandwidth-
P-P
n
=
.
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 = C
the receiver sensitivity. C
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
(C
Current generated by supply noise is divided between
C
ply noise is (assuming the filter capacitor is much larger
than the photodiode capacitance):
Figure 3. Filter Design Examples
FILT
b)
a)
FILT
SIMPLE, 1-POLE, 530MHz FILTER
3-POLE, 515MHz FILTER
MAX3665
MAX3665
and C
) is placed in parallel with the photodiode.
I
NOISE
PHOTO
50
50
50
50
1.2pF
1.2pF
. The input noise current due to sup-
V
NOISE
R
REFER TO THE MAX3665 EV KIT DATA SHEET
FOR THE FILTER LAYOUT EXAMPLE.
FILT
PHOTO
4pF
PHOTO
C1
5pF
C
C
22nH
22nH
PHOTO
FILT
( V/ t), which reduces
is the photodiode
R
100
L
5pF
R
100
L