Datasheet

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SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004
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12
120
100
80
60
40
20
0
−
20
Frequency (Hz)
Transimpedance Gain (dB)
100 10k1k 1M 10M100k 100M
−
3dB at 200kHz
SeeFigure6a
C
DIODE
= 10pF
SeeFigure6c
SeeFigure6b
Figure 7. Transimpedance Gains for Circuits in
Figure 6
The effects of these circuit configurations on output
noise are 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
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.
400
300
200
100
0
Frequency (Hz)
Output Noise (
µ
V/
√
Hz)
100 1k 10k 1M 10M100k 100M
SeeFigure6a
SeeFigure6b
See Figure 6c
C
DIODE
= 10pF
Figure 8. Output Noise for Circuits in Figure 6
500
400
300
200
100
0
Frequency (Hz)
Integrated Output Noise (
µ
Vrms)
100 10k1k 1M 10M100k 100M
C
DIODE
= 10pF
SeeFigure6a
SeeFigure6b
See Figure 6c
310
µ
Vrms
68
µ
Vrms
25
µ
Vrms
Figure 9. Integrated Output Noise for Circuits in
Figure 6
Figure 10 shows the effects 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.
60
50
40
30
20
10
0
Frequency (Hz)
Integrated Output Noise (
µ
Vrms)
1 10010 10k1k 1M 10M100k 100M
See Figure 6c
C
DIODE
= 100pF
C
DIODE
=50pF
C
DIODE
= 20pF
C
DIODE
=1pF
C
DIODE
= 10pF
37
µ
Vrms
28
µ
Vrms
25
µ
Vrms
23
µ
Vrms
56
µ
Vrms
Figure 10. Integrated Output Noise for Various
Values of C
DIODE
for Circuit in Figure 6c
(4)