tza3043b NXP Semiconductors, tza3043b Datasheet - Page 6

• Manufacturer Part Number: tza3043b
• Description: Gigabit Ethernet/fibre Channel Transimpedance Amplifier
• Manufacturer: NXP Semiconductors

Philips Semiconductors
PIN diode bias voltage DREF
The transimpedance amplifier together with the PIN diode
determines the performance of an optical receiver for a
large extent. Especially how the PIN diode is connected to
the input and the layout around the input pin influence the
key parameters like sensitivity, the bandwidth and the
Power Supply Rejection Ratio (PSRR) of a
transimpedance amplifier. The total capacitance at the
input pin is critical to obtain the highest sensitivity. It should
be kept to a minimum by reducing the capacitance of the
PIN diode and the parasitics around the input pin. The
PIN diode should be placed very close to the IC to reduce
the parasitics. Because the capacitance of the PIN diode
depends on the reverse voltage across it, the reverse
voltage should be chosen as high as possible.
The PIN diode can be connected to the input in two ways
as shown in Figs 7 and 8. In Fig.7 the PIN diode is
connected between pins DREF and IPhoto. Pin DREF
provides an easy bias voltage for the PIN diode. The
voltage at DREF is derived from V
The low-pass filter consisting of the internal resistors
R1, R2, C1 and the external capacitor C2 rejects the
supply voltage noise. The external capacitor C2 should be
equal or larger then 1 nF for a high PSRR.
2000 Mar 28
Gigabit Ethernet/Fibre Channel
transimpedance amplifier
Fig.7
1 nF
C2
The PIN diode connected between the input
and pin DREF.
I i
IPhoto
DREF
1
3
TZA3043
125
R2
10 pF
125
C1
R1
CC
V CC
by a low-pass filter.
8
MGU103
6
The reverse voltage across the PIN diode is 4.18 V
(5
3.3 V supply.
It is preferable to connect the cathode of the PIN diode to
a higher voltage then V
available on the board. In this case pin DREF can be left
unconnected. When a negative supply voltage is available,
the configuration in Fig.8 can be used. It should be noted
that in this case the direction of the signal current is
reversed compared to the Fig.7. Proper filtering of the bias
voltage for the PIN diode is essential to achieve the
highest sensitivity level.
Fig.8
0.82 V) for 5 V supply or 2.48 V (3.3
negative supply voltage
The PIN diode connected between the input
and a negative supply voltage.
I i
IPhoto
DREF
1
3
TZA3043
125
TZA3043; TZA3043B
CC
R2
when such a voltage source is
10 pF
125
C1
R1
V CC
Product specification
8
MGU104
0.82 V) for