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Fabricante

MAX3660

Analog CATV Transimpedance Amplifier

Maxim Integrated 

PIN

1, 4, 9, 12

2

3

5

6

7

8, 14, 15, 16

10

11

13

—

Analog CATV Transimpedance Amplifier

NAME

VCC

IN+

IN-

VAGC

MUTE

HYST

GND

OUT-

OUT+

TEST

EP

Pin Description

FUNCTION

+5.0V Supply

Positive Analog Input. Connect to photodiode cathode.

Negative Analog Input. Connect to photodiode anode.

AGC Control Input. See the Gain (ZT) vs. Frequency graph.

Active-Low Mute Control Input. VMUTE < 0.8V to disable output.

AGC Hysteresis Control Input. A resistor from HYST to GND controls the hysteresis level.

Supply Ground

Negative RF Output

Positive RF Output

Reserved for test. Connect to GND for normal operation.

Exposed Pad. The exposed pad must be soldered to the circuit board ground for proper thermal

and electrical performance.

Detailed Description

The MAX3660 variable gain TIA has differential AC-

coupled photocurrent inputs and 75Ω differential RF

output. When used with a low-cost operational amplifi-

er, photodiode assembly, bias network, and balun, the

MAX3660 provides a complete high-performance

BPON/GPON video receiver with a simple and effective

feed-forward AGC. It can also be used with feedback

AGC.

Low-Noise Variable-Gain Amplifier

The low-noise differential input is designed to be AC-

coupled to the anode and cathode of the analog photo-

diode in a PON triplexer. The maximum input current to

achieve rated linearity is 1.675mAP-P.

Very low TIA input impedance provides excellent fre-

quency response with no (internal or external) compen-

sation between photodiode and amplifier, thus

simplifying design, manufacturing, and photodiode

selection.

VAGC and Hysteresis Control

The overall transimpedance is controlled using the VAGC

input pin. See the Typical Operating Characteristics for

descriptions of the transimpedance, OIP2 (CSO), and

OIP3 (CTB) performance for VAGC voltages between 0

and 1.8V.

The MAX3660 has a very flat and stable gain vs. volt-

age characteristic in the range 0.175V ≤ VVAGC ≤ 1.4V,

enabling a simple feed-forward AGC based on average

optical power level as measured by the photodiode DC

current (see Figure 4 for the EV kit schematic).

Feedback AGC can be used to achieve a wider

dynamic range, in which case the VAGC voltage would

be controlled by an external power detector, such as

the MAX2014, typically through a microcontroller inter-

face. In this case, the maximum voltage at VAGC

should be kept below approximately 1.65V to maintain

adequate linearity levels for typical GPON applications.

The forward signal path is implemented with three

switched variable gain stages, each covering one-third

of the total dynamic range. When the voltage input at

VAGC crosses the points on the Gain (ZT) vs. VAGC

curve where a new stage is selected (VVAGC = 350mV

and VVAGC = 700mV), there can be a small (approxi-

mately 50ns) deviation in the output, causing an inter-

ruption to the CATV signal. Hysteresis is provided for

the VAGC input to prevent the output signal from dither-

ing when the average optical input level is very close to

one of these two switching points. The amount of hys-

teresis can be controlled by the value of RHYST, and is

minimum (0.14dB) when RHYST is open.

RF Output and Cable Tilt Compensation

The MAX3660 includes integrated cable compensation

(uptilt). With a photodiode assembly similar to that

described in Figure 1, the output at 870MHz is 4dB

higher compared to the output at 47MHz. About half of

the uptilt is due to the combination of photodiode

capacitance and the inductance of the triplexer leads,

and half is internal to the MAX3660.

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