tza3043b NXP Semiconductors, tza3043b Datasheet - Page 7

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

Philips Semiconductors
AGC
The TZA3043 transimpedance amplifier can handle input
currents from 1 A to 1.5 mA. This means a dynamic
range of 63 dB. At low input currents, the transimpedance
must be high to get enough output voltage, and the noise
should be low enough to guaranty minimum bit error rate.
At high input currents however, the transimpedance
should be low to avoid pulse width distortion. This means
that the gain of the amplifier has to vary depending on the
input signal level to handle such a wide dynamic range.
This is achieved in the TZA3043 by implementing an
Automatic Gain Control (AGC) loop. The AGC loop
consists of a peak detector, a hold capacitor and a gain
control circuit.
The peak amplitude of the signal is detected by the peak
detector and it is stored on the hold capacitor. The voltage
over the hold capacitor is compared to a threshold level.
The threshold level is set to 25 A (p-p) input current. AGC
becomes active only for input signals larger than the
threshold level.
2000 Mar 28
handbook, full pagewidth
Gigabit Ethernet/Fibre Channel
transimpedance amplifier
V o(dif)
V
(1) V
(2) V
(3) V
(mV)
V o
(V)
o(dif)
600
400
200
3.9
3.7
3.5
3.3
3.1
= V
CC
CC
CC
0
1
= 3 V.
= 3.3 V.
= 5 V.
OUT
V
OUTQ
.
10
Fig.9 AGC characteristics.
10
7
2
minimum and can not be reduced further but the front-end
remains linear until input currents of 1.5 mA.
The upper part of Fig.9 shows the output voltages of the
TZA3043 (OUT and OUTQ) as a function of the DC input
current. In the lower part, the difference of both voltages is
shown. It can be seen from the figure that the output
changes linearly up to 25 A input current where AGC
becomes active. From this point on, AGC tries to keep the
differential output voltage constant around 200 mV for
medium range input currents (input currents <200 A).
The AGC can not regulate any more above 500 A input
current and the output voltage rises again with the input
current.
It is disabled for smaller signals. The transimpedance is
then at its maximum value (8.3 k differential).
When AGC is active, the feedback resistor of the
transimpedance amplifier is reduced to keep the output
voltage constant. The transimpedance is regulated from
8.3 k at low currents (I < 30 A) to 1 k at high currents
(I < 500 A). Above 500 A the transimpedance is at its
(1)
(2)
(3)
10
TZA3043; TZA3043B
3
V CC = 5 V
V OUTQ
V OUT
I i ( A)
Product specification
MGU105
10
4