Rainbow Electronics MAX3745 User Manual
Page 9
Layout Considerations
Noise performance and bandwidth are adversely affect-
ed by capacitance at the IN pad. Minimize capacitance
on this pad and select a low-capacitance photodiode.
Assembling the MAX3744/MAX3745 in die form using
chip and wire technology provides the best possible
performance. Figure 7 shows a suggested layout for a
TO header for the MAX3744/MAX3745. The placement
of the filter cap to minimize the ground loop of the pho-
todiode is required to achieve the specified bandwidth.
The OUT+ and OUT- bond wire lengths should also be
minimized to meet the bandwidth specification. Special
care should be taken to ensure that ESD at IN does not
exceed 500V.
Photodiode Filter
Supply voltage noise at the cathode of the photodiode
produces a current I = C
PD
∆V/∆t, which reduces the
receiver sensitivity (C
PD
is the photodiode capaci-
tance.) The filter resistor of the MAX3744/MAX3745,
combined with an external capacitor, can be used to
reduce this noise (see the Typical Application Circuit).
Current generated by supply noise voltage is divided
between C
FILTER
and C
PD
. The input noise current due
to supply noise is (assuming the filter capacitor is much
larger than the photodiode capacitance):
I
NOISE
= (V
NOISE
)(C
PD
) / (R
FILTER
)(C
FILTER
)
If the amount of tolerable noise is known, the filter
capacitor can be easily selected:
C
FILTER
= (V
NOISE
)(C
PD
) / (R
FILTER
)(I
NOISE
)
For example, with maximum noise voltage = 100mV
P-P
,
C
PD
= 0.85pF, R
FILTER
= 600
Ω, and I
NOISE
selected to
be 350nA:
C
FILTER
= (100mV)(0.85pF) / (600
Ω)(350nA) = 405pF
Wire Bonding
For high-current density and reliable operation, the
MAX3744/MAX3745 use gold metalization. Connections
to the die should be made with gold wire only, using ball-
bonding techniques. Die thickness is typically 14 mils
(0.4mm).
MAX3744/MAX3745
2.7Gbps SFP Transimpedance
Amplifiers with RSSI
_______________________________________________________________________________________
9
PO
PI
P
AVG
TIME
OPTICAL POWER
Figure 6. Optical Power Relations
TOP VIEW OF TO-46 HEADER
V
CC
OUT+
OUT-
CASE
GROUND
400pF TO
1000pF
PHOTODIODE
400pF TO
1000pF
MAX3744
MAX3745
Figure 7. Suggested Layout for TO-46 Header