RF Output and Input Stage

The differential outputs should be connected to a

balun transformer to produce a single-ended 75

Ω 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 20dB

Ω.

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 10

Ω) 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

≤ 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

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,

Analog CATV Transimpedance Amplifier

6

_______________________________________________________________________________________

TIA IN+

TIA IN-

1k

Ω

1k

Ω

1.8k

Ω BEAD

10

μH

10

μH

1.8k

Ω BEAD

0.001

μF

0.001

μF

0.1

μF

Figure 3. Photodiode Bias Network