Datasheet

10
100 1k 10k
3 dB FILTER BANDWIDTH (Hz)
0.01
0.1
1
10
RMS NOISE (PV)
A
V
= 60 dB
A
V
= 40 dB
2-Stage Filter
LMP2021, LMP2022
SNOSAY9E –SEPTEMBER 2008–REVISED MARGH 2013 www.ti.com
Figure 43. RMS Input Referred Noise vs. Frequency
Figure 43 shows the total input referred noise vs. 3 dB corner of both filters of Figure 41 and Figure 42 at gains
of 100V/V and 1000V/V. For these measurements and using Figure 41's circuit, R
F
= 49.7 kΩ and R
IN
= 497Ω.
Value of C
F
has been changed to achieve the desired 3 dB filter corner frequency. In the case of Figure 42's
circuit, R
F
= 49.7 kΩ and R
IN
= 497Ω, R
FILT
= 49.7 kΩ, and C
FILT
has been changed to achieve the desired 3 dB
filter corner frequency. Figure 43 compares the RMS noise of these two circuits. As Figure 43 shows, the RMS
noise measured the circuit in Figure 42 has lower values and also depicts a more linear shape.
DIGITAL ACQUISITION SYSTEMS
High resolution ADC’s with 16-bits to 24-bits of resolution can be limited by the noise of the amplifier driving
them. The circuit configuration, the value of the resistors used and the source impedance seen by the amplifier
can affect the noise of the amplifier. The total noise at the output of the amplifier can be dominated by one of
several sources of noises such as: white noise or broad band noise, 1/f noise, thermal noise, and current noise.
In low frequency applications such as medical instrumentation, the source impedance is generally low enough
that the current noise coupled into it does not impact the total noise significantly. However, as the 1/f or flicker
noise is paramount to many application, the use of an auto correcting stabilized amplifier like the
LMP2021/LMP2022 reduces the total noise.
Table 1: RMS Input Noise Performance summarizes the input and output referred RMS noise values for the
LMP2021/LMP2022 compared to that of Competitor A. As described in previous sections, the outstanding noise
performance of the LMP2021/LMP2022 can be even further improved by adding a simple low pass filter following
the amplification stage.
The use of an additional filter, as shown in Figure 42 benefits applications with higher gain. For this reason, at a
gain of 10, only the results of circuit in Figure 41 are shown. The RMS input noise of the LMP2021/LMP2022 are
compared with Competitor A's input noise performance. Competitor A's RMS input noise behaves the same with
or without an additional filter.
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