OPA380

OPA2380

SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004

www.ti.com

13

110

80

50

20

−10

Frequency (Hz)

100

10k

1k

1M

10M

100k

100M

−3dB BW at 1MHz

SeeFigure6a

C

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

+ 2C

with:

1

2

p R

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)

C

SeeFigure6a

See Figure 6b

SeeFigure6c

1100

10

10k

1k

1M

10M

100k

100M

Figure 8. Output noise for circuits in Figure 6

500

400

300

200

100

0

Frequency (Hz)

1100

10

10k

1k

1M

10M

100k

100M

419

µV

30

µV

86

µV

C

See Figure 6a

See Figure 6b

SeeFigure6c

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

100

10

10k

1k

1M

10M

100k

100M

C

DIODE

= 100pF

C

DIODE

= 10pF

C

DIODE

= 1pF

See Figure 6c

C

DIODE

= 50pF

50

µV

35

µV

30

µV

27

µV

79

µV

C

DIODE

= 20pF

Figure 10. Integrated output noise for various

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