LMP2021MAEVAL National Semiconductor, LMP2021MAEVAL Datasheet - Page 13

• Manufacturer Part Number: LMP2021MAEVAL
• Description: BOARD EVAL FOR LMP2021 OP AMP
• Manufacturer: National Semiconductor
• Series: LMP®r

Specifications of LMP2021MAEVAL

• Channels Per Ic: 1 - Single
• Amplifier Type: Instrumentation
• Output Type: Differential
• Slew Rate: 2.6 V/µs
• Current - Output / Channel: 50mA
• Operating Temperature: -40°C ~ 125°C
• Current - Supply (main Ic): 1.1mA
• Voltage - Supply, Single/dual (±): 2.2 V ~ 5.5 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMP2021
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -

ACHIEVING LOWER NOISE WITH FILTERING
The low input voltage noise of the LMP2021/LMP2022, and
no 1/f noise make these suitable for many applications with
noise sensitive designs. Simple filtering can be done on the
LMP2021/LMP2022 to remove high frequency noise. Figure
3 shows a simple circuit that achieves this.
In Figure 3 C
from C
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 4 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 signifi-
cantly gained up and as a result the effect of the input voltage
noise of the LMV771 is effectively not noticeable.
Using the circuit in Figure 4 has the advantage of removing
the non-linear filter bandwidth dependency which is seen
when the circuit in Figure 3 is used. The difference in noise
performance of the circuits in Figures 3, 4 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.
FIGURE 4. Enhanced Filter to Further Reduce Noise at
FIGURE 3. Noise Reducing Filter for Lower Gains
F
and R
F
F
and the corner frequency of the filter resulting
will reduce the total noise.
Higher Gains
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Figure 5 shows the total input referred noise vs. 3 dB corner
of both filters of Figure 3 and Figure 4 at gains of 100V/V and
1000V/V. For these measurements and using Figure 3's cir-
cuit, R
changed to achieve the desired 3 dB filter corner frequency.
In the case of Figure 4's circuit, R
497Ω, R
achieve the desired 3 dB filter corner frequency. Figure 5
compares the RMS noise of these two circuits. As Figure 5
shows, the RMS noise measured the circuit in Figure 4 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 cur-
rent 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 correct-
ing 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 4 benefits
applications with higher gain. For this reason, at a gain of 10,
only the results of circuit in Figure 3 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.
FIGURE 5. RMS Input Referred Noise vs. Frequency
F
= 49.7 kΩ and R
FILT
= 49.7 kΩ, and C
IN
= 497Ω. Value of C
FILT
F
has been changed to
= 49.7 kΩ and R
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F
has been
IN
=