Micrel, Inc.

SY88149HL

June 2010

7

M9999-061110-I

hbwhelp@micrel.com or (408) 955-1690

Functional Block Diagram

Detailed Description

The SY88149HL is a high-sensitivity limiting post amplifier

which operates on a +3.3V power supply over the

industrial temperature range. Signals with data rates up to

1.25Gbps and as small as 4mVpp can be amplified. Figure

1 shows the allowed input voltage swing. Depending upon

the LOS/SDSEL option, the SY88149HL can generate an

SD or LOS output, and allow feedback to the JAM input for

amplitude detection.

To satisfy the stringent timing requirements of the GPON

specifications, the signal detect circuit offers 5ns SD

assert (LOS de-assert) time and the option to de-assert

SD (assert LOS) using the /AUTORESET or manual

RESET function. When /AUTORESET is enabled, SD

de-asserts/LOS asserts automatically within 100ns after

the last high-to-low transition of the input burst. When

the /AUTORESET function is disabled, the SD De-

assert/LOS Assert time can be reset by using the

provided RESET pin.

Input Buffer

Figure 2 shows a simplified schematic of the input stage.

The high sensitivity of the input amplifier allows signals as

small as 4mVpp to be detected and amplified. The input

signals are linearly amplified with a typically 48dB

(typ). Applications requiring the SY88149HL to operate

with high-gain should have the upstream TIA placed as

close as possible to the SY88149HL’s input pins. This

ensures the best performance of the device.

Output Buffer

The SY88149HL’s LVPECL output buffer is designed to

drive 50

Ω lines. The output buffer requires appropriate

termination for proper operation. An external 50

Ω

Figure 3 shows a simplified schematic of the output

stage.

Loss of Signal/Signal Detect

The SY88149HL generates a chatter-free Signal-Detect

(SD) or LOS LVTTL output, as shown in Figure 4. A highly

sensitive signal detect circuit is used to determine that the

input amplitude is too small to be considered a valid input.

LOS asserts high if the input amplitude falls below the

threshold

set

by

LOS/SDLVL

and

de-asserts

low

otherwise. SD asserts high if the input amplitude rises

above the threshold set by LOS/SDLVL and de-asserts

low otherwise. LOS/SD can be fed back to the JAM input

to maintain output stability under the absence of an invalid

signal condition. Typically, a 3 dB hysteresis is provided to

prevent chattering.

LOS/SD Level Set

the threshold of the input amplitude detection. Connecting

inputs.