+

20 kΩ

380 kΩ

380 kΩ

380 kΩ

19 kΩ

V−

V+

4

7

2

3

1

6

5

+

–IN

+IN

(1)

INA149-EP

SBOS608A – MARCH 2012 – REVISED APRIL 2012

www.ti.com

NOISE PERFORMANCE

The wideband noise performane of the INA149 is dominated by the internal resistor network. The thermal or

Johnson noise of these resistors measures approximately 550 nV/

√Hz. The internal op amp contributes virtually

no excess noise at frequencies above 100 Hz.

Many applications may be satisfied with less than the full 500-kHz bandwidth of the INA149. In these cases, the

noise can be reduced with a low-pass filter on the output. The two-pole filter shown in Figure 48 limits bandwidth

and reduces noise. Because the INA149 has a 1/f noise corner frequency of approximately 100 Hz, a cutoff

frequency below 100 Hz does not further reduce noise.

Component values for different filter frequencies are shown in Table 1.

(1) For most applications, the OPA171 can be used as an operational amplifier. For directly driving successive-approximation register (SAR)

data converters, the OPA140 is a good choice.

Figure 48. Output Filter for Noise Reduction

Table 1. Components Values for Different Filter Bandwidths

BUTTERWORTH

OUTPUT NOISE

200 kHz

1.8

No filter

100 kHz

1.1

11 k

Ω

11.3 k

Ω

100 pF

200 pF

10 kHz

0.35

11 k

Ω

11.3 k

Ω

1 nF

2 nF

1 kHz

0.11

11 k

Ω

11.3 k

Ω

10 nF

20 nF

100 Hz

0.05

11 k

Ω

11.3 k

Ω

0.1 µF

0.2 µF

18

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