SA5211 Datasheet, PDF (15/20 Pages) NXP Semiconductors – Transimpedance amplifier 180MHz

Philips Semiconductors
Transimpedance amplifier (180MHz)
Product specification
SA5211
V_CC1
R₁
R₃
R₁₂
V_CC2
R₁₃
INPUT
Q₂
Q₄
Q₁
Q₃
+
PHOTODIODE
R₂
GND₁
Q₁₅
R₁₄
R₇
R₅
R₄
Q₁₆
R₁₅
VB2
Q₁₁
Q₁₂
OUT–
+
OUT+
GND₂
SD00328
Figure 12. Transimpedance Amplifier
R1
I_C1
INPUT
I_B
I_IN
Q1
V_IN
I_F
V_CC
R3
Q2
R2
Q3
V_EQ3
R_F
R4
SD00329
Figure 13. Shunt-Series Input Stage
APPLICATION INFORMATION
Package parasitics, particularly ground lead inductances and
parasitic capacitances, can significantly degrade the frequency
response. Since the SA5211 has differential outputs which can feed
back signals to the input by parasitic package or board layout
capacitances, both peaking and attenuating type frequency
response shaping is possible. Constructing the board layout so that
Ground 1 and Ground 2 have very low impedance paths has
produced the best results. This was accomplished by adding a
ground-plane stripe underneath the device connecting Ground 1,
Pins 8–11, and Ground 2, Pins 1 and 2 on opposite ends of the
SO14 package. This ground-plane stripe also provides isolation
between the output return currents flowing to either V_CC2 or Ground
2 and the input photodiode currents to flowing to Ground 1. Without
this ground-plane stripe and with large lead inductances on the
board, the part may be unstable and oscillate near 800MHz. The
easiest way to realize that the part is not functioning normally is to
measure the DC voltages at the outputs. If they are not close to their
quiescent values of 3.3V (for a 5V supply), then the circuit may be
oscillating. Input pin layout necessitates that the photodiode be
physically very close to the input and Ground 1. Connecting Pins 3
and 5 to Ground 1 will tend to shield the input but it will also tend to
increase the capacitance on the input and slightly reduce the
bandwidth.
As with any high-frequency device, some precautions must be
observed in order to enjoy reliable performance. The first of these is
the use of a well-regulated power supply. The supply must be
capable of providing varying amounts of current without significantly
changing the voltage level. Proper supply bypassing requires that a
good quality 0.1µF high-frequency capacitor be inserted between
V_CC1 and V_CC2, preferably a chip capacitor, as close to the package
pins as possible. Also, the parallel combination of 0.1µF capacitors
with 10µF tantalum capacitors from each supply, V_CC1 and V_CC2, to
the ground plane should provide adequate decoupling. Some
applications may require an RF choke in series with the power
supply line. Separate analog and digital ground leads must be
maintained and printed circuit board ground plane should be
employed whenever possible.
Figure 14 depicts a 50Mb/s TTL fiber-optic receiver using the
BPF31, 850nm LED, the SA5211 and the SA5214 post amplifier.
1998 Oct 07
15