Datasheet

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SBOS291G − NOVEMBER 2003 − REVISED SEPTEMBER 2007
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13
110
80
50
20
−
10
Frequency (Hz)
Transimpedance Gain (dB)
100 10k1k 1M 10M100k 100M
−
3dB BW at 1MHz
SeeFigure6a
C
DIODE
= 10pF
SeeFigure6c
SeeFigure6b
Figure 7. Transimpedance Gains for Circuits in
Figure 6
The effect of these circuit configurations on output noise
is shown in Figure 8 and on integrated output noise in
Figure 9. A 2-pole Butterworth filter (maximally flat in
passband) is created by selecting the filter values using
the equation:
C
F
R
F
+ 2C
FILTER
R
FILTER
with:
f
*3dB
+
1
2p R
F
R
FILTER
C
F
C
FILTER
Ǹ
The circuit in Figure 6b rolls off at 20dB/decade. The
circuit with the additional filter shown in Figure 6c rolls
off at 40dB/decade, resulting in improved noise
performance.
300
200
100
0
Frequency (Hz)
Output Noise (nV/
√
Hz)
C
DIODE
= 10pF
SeeFigure6a
See Figure 6b
SeeFigure6c
110010 10k1k 1M 10M100k 100M
Figure 8. Output Noise for Circuits in Figure 6
500
400
300
200
100
0
Frequency (Hz)
110010 10k1k 1M 10M100k 100M
419µV
30µV
86µV
C
DIODE
= 10pF
See Figure 6a
See Figure 6b
SeeFigure6c
Integrated Output Noise (µV
rms
)
Figure 9. Integrated Output Noise for Circuits in
Figure 6
Figure 10 shows the effect of diode capacitance on
integrated output noise, using the circuit in Figure 6c.
For additional information, refer to Noise Analysis of
FET Transimpedance Amplifiers (SBOA060), and
Noise Analysis for High-Speed Op Amps (SBOA066),
available for download from the TI web site.
80
60
0
20
0
Frequency (Hz)
1 10010 10k1k 1M 10M100k 100M
C
DIODE
= 100pF
C
DIODE
= 10pF
C
DIODE
= 1pF
See Figure 6c
C
DIODE
= 50pF
50µV
35µV
30µV
27
µ
V
79µV
C
DIODE
= 20pF
Integrated Output Noise (µV
rms
)
Figure 10. Integrated Output Noise for Various
Values of C
DIODE
for Circuit in Figure 6c
(4)
(5)