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MAX3660

Analog CATV Transimpedance Amplifier

Maxim Integrated 

Analog CATV Transimpedance Amplifier

Gain vs. VAGC Voltage

The overall transimpedance at 47MHz is related to the

voltage at VAGC by the relation:

( ) ZT(dBΩ)

=

66dBΩ

+

20

log

⎡

⎢

⎣⎢

175mV

VVAGC (mV)

⎤

⎥,

⎦⎥

0.175V ≤ VVAGC ≤ 1.4V

The gain at 870MHz is 4dB greater (70dBΩ at VVAGC =

0.175V) because of the uptilt, although the amount of

uptilt can be modified as described above.

Between 0 and 0.175V the gain is constant, and above

1.5V it falls off relatively quickly. Operation above

VVAGC = 1.6V should be avoided to obtain adequate

linearity performance.

The high-impedance VAGC input should be driven by a

source (op amp, DAC, etc.) capable of sinking up to

200µA.

Feed-Forward AGC

With a feed-forward circuit like that of the EV kit, the

MAX3660 provides a constant (±1dB) output of

19dBmV/channel at 47MHz and 23dBmV/channel at

870MHz, for optical input levels ranging between

-8dBm and +2dBm at OMI = 4.2%.

Feedback AGC

The VAGC voltage can also be controlled from a power

detector, such as the MAX2014 or MAX9933, for feed-

back AGC.

It is important to note that the Gain (ZT) vs. VAGC char-

acteristic includes hysteresis at the two points where

the input stage switches gain (350mV and 700mV),

which can cause problems such as limit-cycle oscilla-

tion with continuous analog feedback implementations.

The feedback circuit should be designed to avoid oscil-

lation or dithering.

Uptilt

The integrated uptilt results in equal input levels pro-

ducing an output voltage that is 4dB greater at 870MHz

compared to 47MHz, eliminating the loss normally

associated with an external passive tilt network. The

amount of uptilt can be varied by adjusting the triplexer

lead length, or by adding small inductors in series with

the anode and cathode, to compensate for photodi-

odes/triplexers that differ significantly from Figure 1.

Equivalent Input Noise

The typical equivalent input referred noise (EIN) of the

MAX3660 with a photodiode connected at the input is

5.5pA/Hz1/2, yielding 48dB or better CNR under normal

BPON/GPON conditions. Without a photodiode con-

nected, the typical EIN is 4.5pA/Hz1/2.

RF Output

The RF output should be connected to the MAX3660

using AC-coupling capacitors and a balun transformer

to achieve the desired noise and linearity performance.

Without the capacitors, shorting OUT+ and OUT-

together, or shorting OUT+ or OUT- to ground, can

draw sufficient current to damage the output stage.

EV Kit Circuit

The MAX3660 EV kit circuit shown in Figure 4 was used

to collect the data in the Typical Operating

Characteristics figures. When connected to a photodi-

ode-equipped triplexer, the EV kit circuit provides a

complete receiver, including photodiode bias, feed-for-

ward AGC, and output transformer.

Jumper JU1 controls the MUTE input, JU3 sets the

amount of hysteresis, and JU2 controls the input of the

op amp driving the VAGC input. Install JU2 to enable

feed-forward VAGC, or alternatively, the gain can be

controlled by TP6 with JU2 removed.

_______________________________________________________________________________________ 7

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