Micrel, Inc.

SY88343BL

February 2007

7

M9999-021707-B

Functional Block Diagram

Detailed Description

The SY88343BL low-power limiting post amplifier

operates from a single +3.3V power supply, over

temperatures from –40

amplified. Figure 1 shows the allowed input voltage

swing. The SY88343BL generates a LOS output

the sensitivity of the input amplitude detection.

Input Amplifier/Buffer

Figure 2 shows a simplified schematic of the

SY88343BL’s input stage. The high-sensitivity of the

detected and amplified. The input amplifier also allows

linearly amplified with a typical 38dB differential voltage

gain. Since it is a limiting amplifier, the SY88343BL

requiring the SY88343BL to operate with high-gain

should have the upstream TIA placed as close as

possible to the SY88343BL’s input pins. This ensures

the best performance of the device.

Output Buffer

The SY88343BL’s CML output buffer is designed to

drive 50Ω lines. The output buffer requires appropriate

termination for proper operation. An external 50Ω

shows a simplified schematic of the output stage.

Loss-of-Signal

The SY88343BL generates a chatter-free LOS open-

collector TTL output with an internal 4.75kΩ pull-up

resistor, as shown in Figure 4. LOS is used to determine

that the input amplitude is large enough to be

considered a valid input. LOS asserts high if the input

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. Typical, 3.5dB

LOS hysteresis is provided to prevent chattering.

Loss-of-Signal Level Set

threshold of the input amplitude detection. Connecting

The external resistor creates a voltage divider between

Hysteresis

The SY88343BL provides typically 3.5dB LOS electrical

hysteresis. By definition, a power ratio measured in dB

is 10log (power ratio). Power is calculated as V

2

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

SY88343BL is an electrical device; this data sheet refers

to hysteresis in electrical terms. With 3.5dB LOS

hysteresis, a voltage factor of 1.5 is required to assert or

de-assert LOS.