7

SY88773V

Micrel, Inc.

M9999-110905

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

DETAILED DESCRIPTION

The SY88773V low-power, limiting post amplifier operates

from a single +3.3V or +5V power supply, over temperatures

from –40

°C to +85°C. Signals with data rates up to 3.2Gbps

the allowed input voltage swing. The SY88773V generates

an LOS output, providing feedback to /EN for output stability.

LOSLVL sets the sensitivity of the input amplitude detection.

Input Amplifier/Buffer

DC bias point. If AC-coupling to the SY88773V, REF can

be used as the DC bias point by externally connecting the

inputs through appropriate termination resistors to REF. If

DC-coupling to the SY88773V, ensure the upstream device’s

output swing meets the SY88773V’s common mode range.

Figure 2 shows a simplified schematic of the input structure.

The high-sensitivity of the input amplifier detects and

are linearly amplified with a typically 38dB differential voltage

gain. Since it is a limiting amplifier, the SY88773V outputs

SY88773V to operate with high-gain should have the

upstream TIA placed as close as possible to the SY88773V’s

input pins to ensure the device’s best performance.

Output Buffer

The SY88773V’s CML output buffer is designed to drive

50

Ω lines. The output buffer requires appropriate termination

for proper operation. An external 50

Ω resistor to VCC or

equivalent for each output pin provides appropriate output

buffer termination. Figure 3 shows a simplified schematic of

the output structure and includes an appropriate termination

method. Of course, driving a downstream device with a

CML input that is internally terminated with 50

Ω to VCC

eliminates the need for external termination. As noted in

waveforms across 25

Ω total loads. The output buffer, thus,

switches typically 16mA tail-current. Figure 4 shows the

power supply current measurement which excludes the

16mA tail-current.

Loss-of-Signal

The SY88773V incorporates a chatter-free, LOS open-

collector TTL output with internal 4.75k

Ω pull-up resistor as

shown in Figure 5. LOS 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 LOSLVL and de-asserts low otherwise. LOS can be

fed back to the enable (/EN) input to maintain output stability

under a loss-of-signal condition. /EN de-asserts low the

true output signal without removing the input signals.

Typically, 4.6dB LOS hysteresis is provided to prevent

chattering.

Loss-of-Signal Level Set

A programmable, loss-of-signal level set pin sets the

threshold of the input amplitude detection. Connecting an

external resistor between VCC and LOSLVL sets the voltage

at LOSLVL. This voltage ranges from V

external resistor creates a voltage divider between VCC

and REF as shown in Figure 6. If desired, an appropriate

external voltage may be applied rather than using a resistor.

VV

–

R

R+

LOSLVL

CC

LOSLVL

LOSLVL

=

13

28

.

.

where voltages are in volts and resistances are in k

Ω.

The smaller the external resistor, which implies a smaller

voltage difference from LOSLVL to VCC, the lower the LOS

sensitivity. Hence, larger input amplitude is required to de-

assert LOS. The “Typical Operating Characteristics” section

contains graphs showing the relationship between the input

Hysteresis

The SY88773V provides typically 4.6dB LOS electrical

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

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,

hence, the ratios change linearly. Therefore, the optical

hysteresis in dB is half the electrical hysteresis in dB given

in the datasheet. The SY88773V provides typically 2.3dB

LOS optical hysteresis. As the SY88773V is an electrical

device, this datasheet refers to hysteresis in electrical terms.

With 4.6dB LOS hysteresis, a voltage factor of 1.7 is required

to de-assert LOS.

Hysteresis and Sensitivity Improvement

If increased LOS sensitivity and hysteresis are required,

an application note entitled “Notes on Sensitivity and

Hysteresis in Micrel Post Amplifiers” is available at http://

www.micrel.com/product-info/app_hints+notes.shtml.