THS4601

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

10 k

Ω TRANSIMPEDANCE BANDWIDTH

FOR VARIOUS SOURCE CAPACITANCES

CS = 18 pF, CF = 2.2 pF

CS = 47 pF,

CF = 3.3 pF

CS = 100 pF,

CF = 3.9 pF

CS = 220 pF,

CF = 5.6 pF

Figure 30

60

65

70

75

80

85

90

95

100

105

10 k

100 k

1 M

10 M

CS = 18 pF,

CF = 0.6 pF

CS = 47 pF,

CF = 0.6 pF

CS = 100 pF,

CF = 1.5 pF

CS = 220 pF,

CF = 1.8 pF

Frequency – Hz

100 k

Ω TRANSIMPEDANCE BANDWIDTH

FOR VARIOUS SOURCE CAPACITANCES

Figure 31

90

95

100

105

110

115

120

125

130

10 k

100 k

1 M

10 M

CS = 18 pF, CF = 0

CS = 47 pF,

CF = 0

CS = 100 pF,

CF = 0

CS = 220 pF,

CF = 0.4 pF

Frequency – Hz

1 M

Ω TRANSIMPEDANCE BANDWIDTH

FOR VARIOUS SOURCE CAPACITANCES

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.