MAX3762EEP-T Maxim Integrated, MAX3762EEP-T Datasheet - Page 7

• Manufacturer Part Number: MAX3762EEP-T
• Description: Limiting Amplifiers 622Mbps w/Chatter Free Power Detect
• Manufacturer: Maxim Integrated

Specifications of MAX3762EEP-T

• Operating Supply Voltage: 5 V
• Supply Current: 30 mA
• Operating Temperature Range: + 85 C
• Package / Case: QSOP-20
• Available Set Gain: 12.5 dB
• Mounting Style: SMD/SMT
• Number Of Channels: 1
• Power Dissipation: 591 mW

The MAX3761/MAX3762 includes a loss-of-signal moni-
tor with a programmable assert threshold and a hys-
teresis comparator. Internally, one comparator input is
tied to the RSSI output signal and the other is tied to the
threshold-voltage (VTH) pin, which provides a threshold
for the LOS indication. An op amp referenced to an
internal bandgap voltage (1.18V) is supplied for pro-
gramming a supply-independent threshold voltage.
Only two external resistors are needed to program the
LOS assert level. VTH is programmable from 1.18V to
2.4V, providing adequate coverage of the RSSI output’s
useful range. The op amp runs on very low supply cur-
rent and provides an accurate, temperature-stable
threshold, but can source only 20μA of current. For
proper operation, resistor R1 (see the Typical
Operating Circuit ) should have a value ≥ 100kΩ. The
input bias current at INV is < 50nA.
To ensure chatter-free LOS operation, the internal LOS
comparator contains approximately 90mV of hysteresis.
The RSSI signal output has a slope of 25mV/dB.
Therefore, the overall circuit hysteresis is approximately
3.6dB[90mV / (25mV/dB)]. The LOS assert threshold is
45mV below V
45mV above V
The DISABLE pin can be used to disable the data-
output buffer. When DISABLE is high, the differential
output signal at OUT+ and OUT- is approximately zero.
In the disabled state, the common-mode voltage of
each output is approximately V
Figure 2. Loss-of-Signal Definitions
V
VOLTAGE
DEASSERT
V
ASSERT
RSSI
1.2V
V
TH
TH
TH
.
V
, while the LOS deassert threshold is
45mV
ASSERT
_______________________________________________________________________________________
45mV
Low-Power, 622Mbps Limiting Amplifiers
INPUT SIGNAL AMPLITUDE
(min)
with Chatter-Free Power Detect for LANs
Loss-of-Signal Indicator
3.6dB
typical
CC
V
DEASSERT
- 0.8V. Connecting
Output Buffers
25mV/dB
(max)
DISABLE to LOS+ implements a squelch function. When
using the squelch function, the output signal is disabled
whenever the input signal is too small to be reliably
detected (as determined by the voltage at VTH). Use of
the disable function is recommended at all times.
The data outputs (OUT+ and OUT-) are implemented
with emitter followers that have output impedance of
approximately 2Ω. The MAX3762’s PECL LOS outputs
also are implemented with emitter followers that have
output impedance of approximately 2Ω.
The MAX3761 TTL LOS output buffers are open-collec-
tor transistors with 6kΩ internal pull-up resistors.
The MAX3761/MAX3762 can be operated with a single
+5V or -5V power supply.
First determine the receiver system’s sensitivity in dBm
either by estimating or from prototyping results.
Estimate the total gain of the preamplifier and photodi-
ode, then use Figure 3 to select resistor R2, placing the
LOS assert 3dB to 4dB below the receiver sensitivity.
Alternatively, use the Typical Operating Characteristics
to select the V
program V
Select R1 ≥ 100kΩ.
Figure 3. Using TIA Gain and Photodiode Responsivity to
Select LOS Programming Resistor
__________________Design Procedure
GAIN IS PHOTODIODE RESPONSIVITY x TRANSIMPEDANCE GAIN.
EXTINCTION RATIO OF 10 IS ASSUMED.
Programming the LOS Assert Level
TH
80
70
60
50
40
30
20
10
0
with the following relation:
V
-38
TH
TH
R1 = 100kΩ
= 1.18(1 + R2 / R1)
-36
value needed for LOS assert, then
-34
GAIN = 4000
INPUT SIGNAL (dBm)
-32
GAIN = 6000
-30
-28
GAIN = 2000
Supply Voltage
-26
-24
-22
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