Table 1. optical power relations – Rainbow Electronics MAX3745 User Manual
Page 8
MAX3744/MAX3745
2.7Gbps SFP Transimpedance
Amplifiers with RSSI
8
_______________________________________________________________________________________
The MAX3744 preamp measures the average photodi-
ode current and provides the information to the output
common mode. The MAX3748A RSSI detect block
senses the common-mode DC level of input signals IN+
and IN- and provides a ground-level-referenced output
signal of the photodiode current. The advantage of this
implementation is that it allows the TIA to be packaged
in a low-cost conventional 4-pin TO-46 header.
The MAX3748A RSSI output is connected to an analog
input channel of the DS1858/DS1859 SFP controller to
convert the analog information into a 16-bit word. The
DS1858/DS1859 provide the received power informa-
tion to the host board of the optical receiver through a
2-wire interface. The DS1859 allows for internal calibra-
tion of the receive power monitor.
The MAX3744 and the MAX3748A have been optimized
to achieve RSSI stability of better than 2.5dB within the
6µA to 500µA range of average input photodiode cur-
rent. To achieve the best accuracy, Maxim recom-
mends receive power calibration at the low end (6µA)
and the high end (500µA) of the required range.
Optical Power Relations
Many of the MAX3744/MAX3745 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 6
and Table 1 show relations that are helpful for convert-
ing optical power to input signal when designing with
the MAX3744/MAX3745. (Refer to Application Note
HFAN–3.0.0: Accurately Estimating Optical Receiver
Sensitivity.)
Optical Sensitivity Calculation
The input-referred RMS noise current (I
N
) of the
MAX3744/MAX3745 generally determines the receiver
sensitivity. To obtain a system bit-error rate (BER) of
1E-12, the signal-to-noise ratio must always exceed
14.1. The input sensitivity, expressed in average power,
can be estimated as:
where
ρ is the photodiode responsivity in A/W and I
N
is
RMS current in amps.
Input Optical Overload
The overload is the largest input that the MAX3744/
MAX3745 can accept while meeting deterministic jitter
specifications. The optical overload can be estimated
in terms of average power with the following equation:
Optical Linear Range
The MAX3744/MAX3745 have high gain, which limits
the output when the input signal exceeds 50µA
P-P
. The
MAX3744/MAX3745 operate in a linear range (10% lin-
earity) for inputs not exceeding:
Linear Range
A
r
r
dBm
RMS e
e
log
( )
( )
=
+
−
10
50
1
2
1
1000
µ
ρ
Overload
mA
r
r
dBm
RMS e
e
log
( )
( )
=
+
−
10
2
1
2
1
1000
ρ
Sensitivity
I r
r
dBm
N e
e
log
.
(
)
( )
=
×
+
−
10
14 1
1
2
1
1000
ρ
PARAMETER
SYMBOL
RELATION
Average power
P
AVG
P
AVG
= (P
0
+ P
1
) / 2
Extinction ratio
r
e
r
e
= P
1
/ P
0
Optical power of a 1
P
1
P
1
= 2P
AVG
(r
e
) / (r
e
+ 1)
Optical power of a zero
P
0
P
0
= 2P
AVG
/ (r
e
+ 1)
Signal amplitude
P
IN
P
IN
= P
1
- P
0
; P
IN
= 2P
AVG
(r
e
- 1) / (r
e
+ 1)
Table 1. Optical Power Relations
Note: Assuming 50% average duty cycle and mark density.