![](http://datasheet.mmic.net.cn/390000/MAX3657_datasheet_16817945/MAX3657_9.png)
M
155Mbps Low-Noise Transimpedance
Amplifier
_______________________________________________________________________________________
9
Understanding Bonding Coordinates and Physical Die
Size
for more information on bond-pad coordinates.
Applications Information
Optical Power Relations
Many of the MAX3657 specifications relate to the input-
signal amplitude. When working with optical receivers,
the input is sometimes expressed in terms of average
optical power and extinction ratio. Figure 4 and Table 1
show relations that are helpful for converting optical
power to input signal when designing with the MAX3657.
Optical Sensitivity Calculation
The input-referred RMS noise current (i
n
) of the
MAX3657 generally determines the receiver sensitivity.
To obtain a system bit-error rate (BER) of 1E-10, the
signal-to-noise ratio must always exceed 12.7. The
input sensitivity, expressed in average power, can be
estimated as:
where
ρ
is the photodiode responsivity in A/W and i
n
is
the RMS noise current in amps. For example, with pho-
todiode responsivity of 0.9A/W, an extinction ratio of 10
and 15nA input-referred noise, the sensitivity of the
MAX3657 is:
Actual results may vary depending on supply noise, out-
put filter, limiting amplifier sensitivity, and other factors
(refer to Maxim Application Note HFAN-3.0.0:
Accurately
Estimating Optical Receiver Sensitivity
).
Input Optical Overload
Overload is the largest input the MAX3657 accepts
while meeting the pulse-width distortion specification.
Optical overload can be estimated in terms of average
power with the following equation:
For example, if photodiode responsivity is 1.0A/W, the
input overload is 0dBm.
Optical Linear Range
The MAX3657 has high gain, which limits the output for
large input signals. The MAX3657 operates in a linear
range for inputs not exceeding:
For example, with photodiode responsivity of 0.9A/W
and an extinction ratio of 10 the linear range is:
Linear Range
A x
μ
2
0 9
x
x
x
dBm
dBm
=
=
10
11
2
9
1000
28
log
.
Linear Range
A r
x
ρ
r
x
dBm
e
e
=
μ
+
10
2
1
2
1
1000
log
(
)
(
)
Overload
mA
x
x
dBm
=
10
2
2
1000
log
ρ
Sensitivity
x
0 9
.
nA x
A W x
/
x
x
dBm
dBm
=
=
10
12 7
2
15
11
9
1000
38
log
.
Sensitivity
x i
x r
x r
(
x
x
dBm
n
e
e
=
+
1
10
12 7
2
1
1000
log
.
(
)
)
ρ
P
P
P
r
r
AVG
2
e
+
e
1
1
2
1
r
e
=
P
P
P
P
P
r
r
IN
IN
AVG
=
e
e
=
=
+
1
1
0
2
1
1
P
*
Assuming a 50% average mark density.
PARAMETER
Average power
Extinction ratio
SYMBOL
P
AVG
r
e
RELATION
P
AVG
= (P0 + P1)/2
r
e
= P1/P0
Optical power
of a 1
P1
Optical power
of a 0
P0
P0 = 2P
AVG
/(r
e
+ 1)
Optical modulation
amplitude
P
IN
Table 1. Optical Power Relations*
Figure 4. Optical Power Relations
P0
P1
P
AVG
TIME
O
r
+
AVG
e
1
=
P
P
r
+
r
AVG
e
e
=
0
2
1