Loss-of-Signal Indicator

The MAX3761/MAX3762 includes a loss-of-signal moni-

tor with a programmable assert threshold and a hys-

teresis comparator. Internally, one comparator input is

tied to the RSSI output signal and the other is tied to the

threshold-voltage (VTH) pin, which provides a threshold

for the LOS indication. An op amp referenced to an

internal bandgap voltage (1.18V) is supplied for pro-

gramming a supply-independent threshold voltage.

Only two external resistors are needed to program the

LOS assert level. VTH is programmable from 1.18V to

2.4V, providing adequate coverage of the RSSI output’s

useful range. The op amp runs on very low supply cur-

rent and provides an accurate, temperature-stable

threshold, but can source only 20µA of current. For

proper operation, resistor R1 (see the Typical

Operating Circuit) should have a value

≥ 100kΩ. The

input bias current at INV is < 50nA.

To ensure chatter-free LOS operation, the internal LOS

comparator contains approximately 90mV of hysteresis.

The RSSI signal output has a slope of 25mV/dB.

Therefore, the overall circuit hysteresis is approximately

3.6dB[90mV / (25mV/dB)]. The LOS assert threshold is

Output Buffers

The DISABLE pin can be used to disable the data-

output buffer. When DISABLE is high, the differential

output signal at OUT+ and OUT- is approximately zero.

In the disabled state, the common-mode voltage of

DISABLE to LOS+ implements a squelch function. When

using the squelch function, the output signal is disabled

whenever the input signal is too small to be reliably

detected (as determined by the voltage at VTH). Use of

the disable function is recommended at all times.

The data outputs (OUT+ and OUT-) are implemented

with emitter followers that have output impedance of

approximately 2

Ω. The MAX3762’s PECL LOS outputs

also are implemented with emitter followers that have

output impedance of approximately 2

Ω.

The MAX3761 TTL LOS output buffers are open-collec-

tor transistors with 6k

Ω internal pull-up resistors.

__________________Design Procedure

Supply Voltage

The MAX3761/MAX3762 can be operated with a single

+5V or -5V power supply.

Programming the LOS Assert Level

First determine the receiver system’s sensitivity in dBm

either by estimating or from prototyping results.

Estimate the total gain of the preamplifier and photodi-

ode, then use Figure 3 to select resistor R2, placing the

LOS assert 3dB to 4dB below the receiver sensitivity.

Alternatively, use the Typical Operating Characteristics

Select R1

≥ 100kΩ.

Low-Power, 622Mbps Limiting Amplifiers

with Chatter-Free Power Detect for LANs

_______________________________________________________________________________________

7

1.2V

45mV

25mV/dB

45mV

INPUT SIGNAL AMPLITUDE

RSSI

VOLTAGE

3.6dB

typical

Figure 2. Loss-of-Signal Definitions

0

-38

-36

-28

-22

10

60

70

INPUT SIGNAL (dBm)

-32

-34

-26

-30

-24

40

20

80

50

30

GAIN = 2000

GAIN = 6000

R1 = 100k

Ω

GAIN = 4000

Figure 3. Using TIA Gain and Photodiode Responsivity to

Select LOS Programming Resistor

GAIN IS PHOTODIODE RESPONSIVITY x TRANSIMPEDANCE GAIN.

EXTINCTION RATIO OF 10 IS ASSUMED.