Datasheet

LMP2021/
LMP2022
R
IN
/R
F
IN
+
-
R
IN
R
F
R
FILT
LMV771
-
+
R
FILT
OUT
C
FILT
IN
+
-
R
IN
//
RF
R
F
R
IN
C
F
LMP2021, LMP2022
www.ti.com SNOSAY9E –SEPTEMBER 2008–REVISED MARGH 2013
Figure 41. Noise Reducing Filter for Lower Gains
In order to achieve lower noise floors for even more noise stringent applications, a simple filter can be added to
the op amp’s output after the amplification stage. Figure 42 shows the schematic of a simple circuit which
achieves this objective. Low noise amplifiers such as the LMV771 can be used to create a single pole low pass
filter on the output of the LMP2021/LMP2022. The noise performance of the filtering amplifier, LMV771 in this
circuit, will not be dominant as the input signal on LMP2021/LMP2022 has already been significantly gained up
and as a result the effect of the input voltage noise of the LMV771 is effectively not noticeable.
Figure 42. Enhanced Filter to Further Reduce Noise at Higher Gains
Using the circuit in Figure 42 has the advantage of removing the non-linear filter bandwidth dependency which is
seen when the circuit in Figure 41 is used. The difference in noise performance of the circuits in Figure 41, 8
becomes apparent only at higher gains. At voltage gains of 10 V/V or less, there is no difference between the
noise performance of the two circuits.
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