MAX3660
RF Output and Input Stage
The differential outputs should be connected to a
balun transformer to produce a single-ended 75Ω out-
put. If the MAX3660 is used to drive a single-ended
postamplifier, the use of a balun is recommended
(refer to Maxim Reference Design HFRD-22.4) to
achieve adequate linearity and noise performance.
With a typical low-cost balun, output return loss (-S22)
is better than 15dB up to 550MHz and is limited by the
balun performance.
When MUTE is logic-low, the transimpedance is less
than 20dBΩ.
Applications Information
Photodiode/TIA Interface
The MAX3660 is designed to provide a 23dBmV/chan-
nel output at 870MHz with excellent CSO, CTB, and
CNR, and its frequency response extends well beyond
1000MHz.
The RF output has 4dB ±1dB of uptilt and ±0.9dB of
flatness (47MHz to 870MHz) when used with a photodi-
ode and assembly having characteristics similar to
those shown in Figure 1, which is consistent with a typi-
cal low-cost FTTH triplexer connected by 5mm leads to
matched vias. The MAX3660’s very low input imped-
ance (approximately 10Ω) also provides tolerance to
variations in photodiode and assembly electrical char-
acteristics.
It is particularly important to provide electrical symme-
try in the anode and cathode connections, including
the triplexer/ROSA lead routing and PCB mounting con-
figuration. Consult the EV kit and Maxim reference
designs for examples of good layout techniques. With
typical optical transmitter characteristics, the MAX3660
achieves CSO and CTB better than -65dBc and
achieves CNR (including amplifier noise, photodiode
shot noise, and transmitter RIN) of 48dB (at -6dBm or
greater with OMI = 3.3%, or at -8dBm or greater with
OMI = 4.2%) between 47MHz and 870MHz. Refer to
the MAX3660 EV kit data sheet for a description of the
setup used for CSO, CTB, and CNR typical operating
characteristics measurements.
To achieve optimum CNR performance, the AGC should
be configured so that the MAX3660’s gain is greatest
(V
VAGC
≤ 0.175V) at the lowest intended optical input
level, typically -6dBm or -8dBm. To maintain CTB and
CSO performance, care should also be exercised when
designing the AGC so that the maximum operating
VAGC level is limited to approximately 1.6V. Operating
with input signal levels greater than 1.6mA
P-P
can result
in a reduction in linearity due to clipping.
Photodiode Bias Network
A combination of resistors and inductors, such as
shown in Figure 3, provides DC bias to the photodi-
ode. The series connection of two inductors and one
resistor is intended to mitigate effects of inductor self-
resonance.
The DC voltage drop across the lower resistor provides
an effective means to measure average optical power
for use as a signal strength indicator and/or feed-for-
ward AGC.
The value of the resistors can be adjusted to vary the
feed-forward gain. Depending on the specific photodi-
ode characteristics and desired frequency response,
between 5V and 12V should normally be used for V
PD
.
Analog CATV Transimpedance Amplifier
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