MAX3724-MAX3725 Datasheet, PDF (8/10 Pages) Maxim Integrated Products – 3.2Gbps SFP Transimpedance Amplifiers with RSSI

3.2Gbps SFP Transimpedance
Amplifiers with RSSI
The MAX3724 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 MAX3724 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 to be 6µA at the low
end, and 500µA at the high end of the required range.
Optical Power Relations
Many of the MAX3724/MAX3725 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 MAX3724/MAX3725. (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
MAX3724/MAX3725 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:
Sensitivity
=
10log^{}
14.1 × I_N(r_e
2ρ(r_e −
+
1)
1) ₁₀₀₀_dBm
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 MAX3724/
MAX3725 can accept while meeting deterministic jitter
specifications. The optical overload can be estimated
in terms of average power with the following equation:
Overload
=
10log^{}
2mA_RMS
2ρ(r_e
( r_e
−
+
1)
1) ₁₀₀₀_dBm
Optical Linear Range
The MAX3724/MAX3725 have high gain, which limits
the output when the input signal exceeds 50µA_P-P. The
MAX3724/MAX3725 operate in a linear range (10% lin-
earity) for inputs not exceeding:
Linear
Range
=

10log_
50µA_RMS(r_e +
2ρ(r_e − 1)
1)

1000_
dBm
Table 1. Optical Power Relations
PARAMETER
SYMBOL
Average power
P_AVG
Extinction ratio
r_e
Optical power of a 1
P₁
Optical power of a zero
P₀
Signal amplitude
P_IN
Note: Assuming 50% average duty cycle and mark density.
RELATION
P_AVG = (P₀ + P₁) / 2
r_e = P₁ / P₀
P₁ = 2P_AVG(r_e) / (r_e + 1)
P₀ = 2P_AVG / (r_e + 1)
P_IN = P₁ - P₀; P_IN = 2P_AVG(r_e - 1) / (r_e + 1)
8 _______________________________________________________________________________________