Datasheet

R
EQ
+ R
F1
ǒ
1 )
R
F2
R
F3
Ǔ
_
+
R
F2
C
F
λ
−V
(Bias)
R
L
R
F1
R
F3
1
C
FEQ
+
1
C
F1
ǒ
1 )
C
F3
C
F2
Ǔ
_
+
R
F2
C
F
λ
−V
(Bias)
R
L
R
F1
_
+
R
F
C
F2
λ
−V
(Bias)
R
L
C
F1
C
F3
THS4631
SLOS451B –DECEMBER 2004–REVISED AUGUST 2011 www.ti.com
such that the corner frequency of the
transconductance network is much less than the
transimpedance bandwidth of the circuit. Choosing
(5)
this corner frequency properly leads to more accurate
measurements of the transimpedance bandwidth. If
the interface circuit corner frequency is too close to
the bandwidth of the circuit, determining the power
level in the flatband is difficult. A decade or more of
flat bandwidth provides a good basis for determining
the proper transimpedance bandwidth.
ALTERNATIVE TRANSIMPEDANCE
CONFIGURATIONS
Other transimpedance configurations are possible.
Three possibilities are shown below.
A. A resistive T-network enables high
The first configuration is a slight modification of the
transimpedance gain with reasonable
basic transimpedance circuit. By splitting the resistor values.
feedback resistor, the feedback capacitor value
Figure 40. Alternative Transimpedance
becomes more manageable and easier to control.
Configuration 2
This type of compensation scheme is useful when the
feedback capacitor required in the basic configuration
The third configuration uses a capacitive T-network to
becomes so small that the parasitic effects of the
achieve fine control of the compensation capacitance.
board and components begin to dominate the total
The capacitor CF3 can be used to tune the total
feedback capacitance. By reducing the resistance
effective feedback capacitance to a fine degree. This
across the capacitor, the capacitor value can be
circuit behaves the same as the basic
increased. This mitigates the dominance of the
transimpedance configuration, with the effective CF
parasitic effects.
given by Equation 6.
(6)
A. Splitting the feedback resistor enables use
of a larger, more manageable feedback
capacitor.
Figure 39. Alternative Transimpedance
Configuration 1
The second configuration uses a resistive T-network
A. A capacitive T-network enables fine control
to achieve high transimpedance gains using relatively
of the effective feedback capacitance using
small resistor values. This topology can be useful
relatively large capacitor values.
when the desired transimpedance gain exceeds the
Figure 41. Alternative Transimpedance
value of available resistors. The transimpedance gain
Configuration 3
is given by Equation 5.
12 Copyright © 2004–2011, Texas Instruments Incorporated