Texas Instruments THS4631DGNEVM Evaluation Module THS4631DGNEVM THS4631DGNEVM Datenbogen
Produktcode
THS4631DGNEVM
C
F
+
1
p
R
F
G B P
)
ǒ
1
p
R
F
G B P
Ǔ
2
)
4C
S
p
R
F
G B P
Ǹ
2
F
*
3dB
+
G B P
2
p
R
F
ǒ
C
S
)
C
F
Ǔ
Ǹ
Gain
A
OL
20 dB/Decade
Rate-of-Closure
Rate-of-Closure
GBP
20 dB/
Decade
Decade
Noise Gain
−20 dB/
Decade
Decade
f
Pole
Zero
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
SLOS451B
–
DECEMBER 2004
–
REVISED AUGUST 2011
(1)
Once the optimal feedback capacitor has been
selected, the transimpedance bandwidth can be
calculated with
selected, the transimpedance bandwidth can be
calculated with
(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
a variety of source capacitances. The achievable
bandwidths of the various circuit configurations are
summarized numerically in
. The frequency
responses are presented in
, and
.
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
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.
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
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
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
10
Copyright
©
2004
–
2011, Texas Instruments Incorporated