MAX3760 Maxim, MAX3760 Datasheet - Page 7

• Manufacturer Part Number: MAX3760
• Description: 622Mbps / Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers
• Manufacturer: Maxim

The MAX3760 can operate from a power-supply volt-
age (V
any stable voltage, including -5.0V, as found in many
systems using ECL interface levels.
Use good high-frequency design and layout tech-
niques. The use of a multilayer circuit board with sepa-
rate ground and V
care to bypass V
ground plane with the shortest possible traces.
Noise performance and bandwidth will be adversely
affected by stray capacitance at IN. Make every effort
to minimize capacitance on this node. Select a low-
capacitance photodiode and use good high-frequency
design and layout techniques. The MAX3760 is opti-
mized for 0.75pF of capacitance on the input—approxi-
mately the capacitance of a low-cost photodetector
packaged in a header.
When using the MAX3760 in the SO package, note that
the package capacitance is about 0.3pF. The PC board
between the MAX3760 input and the photodetector will
add parasitic capacitance. Keep the input line short,
and remove power and ground planes beneath it.
For the best possible performance, assemble the
MAX3760 in die form using chip-and-wire technology,
or package the die in a TO header. These techniques
minimize parasitic capacitance, resulting in the lowest
noise.
The MAX3760’s noise performance is strongly affected
by the circuit’s bandwidth, which changes over temper-
ature and varies from lot to lot. Receiver sensitivity can
be improved by adding filters to limit this bandwidth.
Filter designs range from a one-pole filter using a single
capacitor to more complex filters using inductors. The
simple filter provides a moderate rolloff with minimal
components, while the complex filter provides a sharp-
er rolloff and better transient response. A simple
530MHz filter can be created by placing a 6pF capaci-
tor between the OUT+ and OUT- pins.
Supply-voltage noise at the photodiode cathode pro-
duces a current, I = C
receiver sensitivity (C
tance.) The MAX3760’s FILTER resistor, combined with
an external capacitor, can be used to reduce this noise
(see Typical Application Circuit ). Current generated by
Preamplifier for LAN and WAN Optical Receivers
CC
Designing a Low-Capacitance Input
- GND) between 4.5V and 5.5V. GND can be
CC
_______________________________________________________________________________________
CC
and to connect the GND pin to the
PHOTO
planes is recommended. Take
PHOTO
622Mbps, Low-Noise Transimpedance
Layout Considerations
is the photodiode capaci-
V/ t, which reduces
Designing Filters
Power Supply
supply-noise voltage is divided between C
C
as follows (assuming the filter capacitor is much larger
than the photodiode capacitance):
If the amount of tolerable noise is known, the filter
capacitor is easy to select, as follows:
For example, with maximum noise voltage = 10mV
C
to be 30nA (half the MAX3760’s input noise):
Figure 3. Suggested Layout for TO-56 Header
PHOTO
PHOTO
. The input noise current due to supply noise is
C
= 0.75pF, R
FILTER
C
I
NOISE
FILTER
C
OUT-
FILTER
=
TOP VIEW OF TO-56 HEADER
=
FILTER
>
10mV
1000 30
CASE IS GROUND
R
V
PHOTODIODE
NOISE
V
FILTER
R
NOISE
FILTER
= 1kΩ, and I
0 75
.
V
E
CC
C
-
pF
C
C
9
PHOTO
FILTER
I
PHOTO
NOISE
OUT+
NOISE
250
FILTER
pF
selected
RMS
and
7
,