Datasheet

C
F
+
1
p R
F
GBP
)
ǒ
1
p R
F
GBP
Ǔ
2
)
4C
S
p R
F
GB P
Ǹ
2
F
*3dB
+
GBP
2p R
F
ǒ
C
S
) C
F
Ǔ
Ǹ
Gain
A
OL
20 dB/Decade
Rate-of-Closure
GBP
20 dB/
Decade
Noise Gain
−20 dB/
Decade
f
PoleZero
0
_
+
C
I(DIFF)
R
F
C
F
C
P
C
D
I(
DIODE)
C
I(CM)
C
S
= C
I(CM)
+ C
I(DIFF)
+ C
P
+ C
D
THS4631
SLOS451B –DECEMBER 2004–REVISED AUGUST 2011 www.ti.com
(1)
Once the optimal feedback capacitor has been
selected, the transimpedance bandwidth can be
calculated with Equation 2.
(2)
Figure 34. Transimpedance Circuit Bode Plot
The performance of the THS4631 has been
measured for a variety of transimpedance gains with
a variety of source capacitances. The achievable
bandwidths of the various circuit configurations are
summarized numerically in Table 1. The frequency
responses are presented in Figure 35, Figure 36, and
Figure 37.
A. The total source capacitance is the sum of
Note that the feedback capacitances do not
several distinct capacitances.
correspond exactly with the values predicted by the
Figure 33. Transimpedance Analysis Circuit equation. They have been tuned to account for the
parasitic capacitance of the feedback resistor
(typically 0.2 pF for 0805 surface mount devices) as
Where:
well as the additional capacitance associated with the
C
I(CM)
is the common-mode input capacitance.
PC board. The equation should be used as a starting
C
I(DIFF)
is the differential input capacitance.
point for the design, with final values for CF optimized
in the laboratory.
C
D
is the diode capacitance.
C
P
is the parasitic capacitance at the inverting
node.
Table 1. Transimpedance Performance Summary
The feedback capacitor provides a pole in the noise
for Various Configurations
gain of the circuit, counteracting the zero in the noise
gain caused by the source capacitance. The pole is
SOURCE TRANS- FEEDBACK -3 dB
CAPACITANCE IMPEDANCE CAPACITANCE FREQUENCY
set such that the noise gain achieves a 20-dB per
(PF) GAIN (Ω) (PF) (MHZ)
decade rate-of-closure with the open-loop gain
18 10 k 2 15.8
response of the amplifier, resulting in a stable circuit.
18 100 k 0.5 3
As indicated, the formula given provides the feedback
18 1 M 0 1.2
capacitance for maximized flat bandwidth. Reduction
in the value of the feedback capacitor can increase 47 10 k 2.2 8.4
the signal bandwidth, but this occurs at the expense
47 100 k 0.7 2.1
of peaking in the ac response.
47 1 M 0.2 0.52
100 10 k 3 5.5
100 100 k 1 1.4
100 1 M 0.2 0.37
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