LMV841MGX NSC [National Semiconductor], LMV841MGX Datasheet - Page 13

• Manufacturer Part Number: LMV841MGX
• Description: CMOS Input, RRIO, Wide Supply Range Operational Amplifiers
• Manufacturer: NSC [National Semiconductor]

However, using low resistor values will increase the power
consumption of the application. This is not always acceptable
for portable applications.
To determine if the noise is acceptable for the application, use
the following formula for resistor noise :
where:
e
k = Boltzmann constant (1.38 x 10–23 J/K)
T = Absolute temperature (K)
R = Resistance (Ω)
B = Noise bandwidth (Hz), fmax - fmin
Given in an example with a resistor of 1MΩ at 25°C (298 K)
over a frequency range of 100 kHz:
To keep the noise of the application low it might be necessary
to decrease the resistors to 100k, which will decrease the
noise to –97.8 dBV (12.8 uV).
The op amp's input-referred noise of 20 nV/
equivalent to the noise of a 24 kΩ resistor.
ACTIVE FILTER
The rail-to-rail input and output of the LMV841/LMV844 and
the wide supply voltage range make these amplifiers ideal to
use in numerous applications. One of the typical applications
is an active filter as shown in Figure 3. This example is a band-
pass filter, for which the pass band is widened. This is
achieved by cascading two band-pass filters, with slightly dif-
ferent center frequencies.
The center frequency of the separate band-pass filters can be
calculated by:
In this example a filter was designed with its pass band at 10
kHz. The two separate band-pass filters are designed to have
th
= Thermal noise voltage (Vrms)
FIGURE 3. Active Filter
at 1 kHz is
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a center frequency of approximately 10% from the frequency
of the total filter:
C = 33 nF
R1 = 2 kΩ
R2 = 6.2 kΩ
R3 = 45 Ω
This will give for filter A:
And for filter B with C = 27 nF:
Bandwidth can be calculated by:
For filter A this will give
and for filter B:
The response of the two filters and the combined filter is
shown in Figure 4.
FIGURE 4. Active Filter Curve
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