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SY88993AL

3.3V 3.2Gbps High-Speed Limiting Post Amplifier with High Input Sensitivity

Micrel 

Micrel, Inc.

Block Diagram

SY88993AL

Detailed Description

The SY88993AL is a high-sensitivity limiting post

amplifier that operates from a single +3.3V power

supply, over temperatures from –40oC to +85oC. Signals

with data rates up to 3.2Gbps and as small as 4mVPP

can be amplified. Figure 2 shows the allowed input

voltage swing. The SY88993AL generates a LOS output.

LOSLVL sets the sensitivity of the input amplitude

detection.

Input Amplifier/Buffer

Figure 3 shows a simplified schematic of the

SY88993AL’s input stage. The high-sensitivity of the

input amplifier allows signals as small as 4mVPP to be

detected and amplified. The input amplifier also allows

input signals as large as 1800mVPP. Input signals below

6mVpp are linearly amplified with a typical 38dB

differential voltage gain. Since it is a limiting amplifier,

the SY88993AL outputs typically 800mVPP voltage-

limited waveforms for input signals that are greater than

10mVPP. Applications requiring the SY88993AL to

operate with high-gain should have the upstream TIA

placed as close as possible to the SY88993AL’s input

pins to ensure the best performance of the device.

Output Buffer

The SY88993AL’s CML output buffer is designed to

drive 50Ω lines. The output buffer requires appropriate

termination for proper operation. An external 50Ω

resistor to VCC for each output pin provides this. Figure 4

shows a simplified schematic of the output stage.

Loss-of-Signal

The SY88993AL generates a chatter-free LOS open-

collector TTL output which requires an external pull-up

resistor, as shown in Figure 1. LOS is used to determine

that the input amplitude is large enough to be considered

a valid input. LOS asserts high if the input amplitude falls

below the threshold sets by LOSLVL and de-asserts low

otherwise. LOS can be fed back to the enable bar (/EN)

input to maintain output stability under a loss of signal

condition. /EN de-asserts the true output signal without

removing the input signals. Typically, 5.6dB LOS

hysteresis is provided to prevent chattering.

Loss-of-Signal Level Set

The SY88993AL incorporates a programmable level

detect function to identify when the input signal has been

lost. This information can be fed back to the /EN input of

the device to maintain stability under loss of signal

conditions. Using LOSLVL pin the sensitivity of the level

detect can be adjusted. The LOSLVL voltage can be set

by connection a resistor divider between VCC and VREF,

Figure 5.

Hysteresis

The SY88993AL typically provides 5.6dB LOS electrical

hysteresis. By definition, a power ratio measured in dB is

10log (power ratio). Power is calculated as V2IN/R for an

electrical signal. Hence, the same ratio can be stated as

20log (voltage ratio). While in linear mode, the electrical

voltage input changes linearly with the optical power and

therefore, the ratios change linearly. Thus, the optical

hysteresis in dB is half the electrical hysteresis in dB

given in the data sheet. Since the SY88993AL is an

electrical device, this data sheet refers to hysteresis in

electrical terms. With 5.6dB LOS hysteresis, a voltage

factor of about 2 is required to assert or de-assert LOS.

September 2008

2

M9999-093008-A

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

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