7

SY88843V

Micrel, Inc.

M9999-110905

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

DETAILED DESCRIPTION

The SY88843V low-power, limiting post amplifier operates

from a single +3.3V

±10% or +5V ±10% power supply, over

an industrial temperature range of –40

°C to +85°C. Signals

be amplified. Figure 1 shows the allowed input voltage swing.

The SY88843V generates an SD output, providing feedback

to EN for output stability. SDLVL sets the sensitivity of the

input amplitude detection.

Input Amplifier/Buffer

The SY88843V’s inputs are internally terminated with 50

Ω

to REF. Unless unaffected by this internal termination

scheme, upstream devices need to be AC-coupled to the

SY88843V’s inputs. Figure 2 shows a simplified schematic

of the input structure.

The high sensitivity of the input amplifier detects and

amplified with a typically 38dB differential voltage gain. Since

it is a limiting amplifier, the SY88843V outputs typically

SY88843V to operate with high gain should have the

upstream TIA placed as close as possible to the SY88843V’s

input pins to ensure the device's best performance of the

device.

Output Buffer

The SY88843V’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 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 the previous section,

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.

Signal Detect

The SY88843V incorporates a chatter-free, SD open-

collector TTL output with internal 4.75k

Ω pull-up resistor as

shown in Figure 5. SD is used to determine that the input

amplitude large enough to be considered a valid input. SD

asserts high if the input amplitude rises above the threshold

set by SDLVL and de-asserts low otherwise. SD 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 SD hysteresis is provided to prevent chattering.

Signal Detect-Level Set

A programmable, signal-detect level set pin sets the

threshold of the input amplitude detection. Connecting an

external resistor between VCC and SDLVL sets the voltage

at SDLVL. This voltage ranges from V

external resistor creates a voltage divider between V

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

voltage may be applied rather than using a resistor. The

relationship between V

VV

–

R

R+

SDLVL

CC

SDLVL

SDLVL

=

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 SDLVL to VCC, the lower the SD

sensitivity. Hence, larger input amplitude is required to assert

SD. The “Typical Operating Characteristics” section contains

graphs showing the relationship between the input amplitude

Hysteresis

The SY88843V provides typically 4.6dB SD 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

the ratios change linearly. Therefore, the optical hysteresis

in dB is half the electrical hysteresis in dB given in the

datasheet. The SY88843V provides typically 2.3dB SD

optical hysteresis. As the SY88843V is an electrical device,

this datasheet refers to hysteresis in electrical terms. With

4.6dB SD hysteresis, a voltage factor of 1.7 is required to

assert SD.

Hysteresis and Sensitivity Improvement

If increased SD 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.