Datasheet

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SLOS388B − OCTOBER 2001 − REVISED JUNE 2002
15
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APPLICATION INFORMATION
designing the transimpedance circuit (continued)
Table 1. Transimpedance Performance Summary for Various Configurations
SOURCE CAPACITANCE
(pF)
TRANSIMPEDANCE GAIN
(Ω)
FEEDBACK CAPACITANCE
(pF)
−3 dB FREQUENCY
(MHz)
18 10k 2.2 10.4
18 100k 0.6 3.3
18 1M 0 1.1
47 10k 3.3 7.6
47 100k 0.6 2.8
47 1M 0 0.88
100 10k 3.9 5.9
100 100k 1.5 1.3
100 1M 0 0.62
220 10k 5.6 3.8
220 100k 1.8 1.1
220 1M 0.4 0.36
Figure 29
65
70
75
80
85
90
10 k 100 k 1 M 10 M 100 M
Frequency − Hz
Transimpedance Gain − dB
10 kΩ TRANSIMPEDANCE BANDWIDTH
FOR VARIOUS SOURCE CAPACITANCES
C
S
= 18 pF, C
F
= 2.2 pF
C
S
= 47 pF,
C
F
= 3.3 pF
C
S
= 100 pF,
C
F
= 3.9 pF
C
S
= 220 pF,
C
F
= 5.6 pF
Figure 30
60
65
70
75
80
85
90
95
100
105
10 k 100 k 1 M 10 M
C
S
= 18 pF,
C
F
= 0.6 pF
C
S
= 47 pF,
C
F
= 0.6 pF
C
S
= 100 pF,
C
F
= 1.5 pF
C
S
= 220 pF,
C
F
= 1.8 pF
Frequency − Hz
Transimpedance Gain − dB
100 kΩ TRANSIMPEDANCE BANDWIDTH
FOR VARIOUS SOURCE CAPACITANCE
S
Figure 31
90
95
100
105
110
115
120
125
130
10 k 100 k 1 M 10 M
C
S
= 18 pF, C
F
= 0
C
S
= 47 pF,
C
F
= 0
C
S
= 100 pF,
C
F
= 0
C
S
= 220 pF,
C
F
= 0.4 pF
Frequency − Hz
Transimpedance Gain − dB
1 MΩ TRANSIMPEDANCE BANDWIDTH
FOR VARIOUS SOURCE CAPACITANCE
S
measuring transimpedance bandwidth
While there is no substitute for measuring the performance of a particular circuit under the exact conditions that
are used in the application, the complete system environment often makes measurements harder. For
transimpedance circuits, it is difficult to measure the frequency response with traditional laboratory equipment
because the circuit requires a current as an input rather than a voltage. Also, the capacitance of the current
source has a direct effect on the frequency response. A simple interface circuit can be used to emulate a
capacitive current source with a network analyzer. With this circuit, transimpedance bandwidth measurements
are simplified, making amplifier evaluation easier and faster.