Datasheet

+
20 kΩ 380 kΩ
380 kΩ
380 kΩ
19 kΩ
V− V+
4 7
2
3
1
6
5
+
V
OUT
= (+IN) – (–IN)
–IN
+IN
R
1
R
2
C
2
C
1
(1)
INA149
SBOS579B –SEPTEMBER 2011–REVISED JULY 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 47 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 47. Output Filter for Noise Reduction
Table 1. Components Values for Different Filter Bandwidths
BUTTERWORTH OUTPUT NOISE
LOW-PASS (f
–3 dB
) (mV
PP
) R
1
R
2
C
1
C
2
200 kHz 1.8 No filter
100 kHz 1.1 11 kΩ 11.3 kΩ 100 pF 200 pF
10 kHz 0.35 11 kΩ 11.3 kΩ 1 nF 2 nF
1 kHz 0.11 11 kΩ 11.3 kΩ 10 nF 20 nF
100 Hz 0.05 11 kΩ 11.3 kΩ 0.1 µF 0.2 µF
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