Application Information
LMP7731
The LMP7731 is a single, low noise, rail-to-rail input and out-
put, and low voltage amplifier.
The low input voltage noise of only 2.9 nV/
√
Hz with a 1/f cor-
ner at 3 Hz makes the LMP7731 ideal for sensor applications
where DC accuracy is of importance.
The LMP7731 has a high gain bandwidth of 22 MHz. This
wide bandwidth enables use of the amplifier at higher gain
settings while retaining usable bandwidth for the application.
This is particularly beneficial when system designers need to
use sensors with very limited output voltage range as it allows
larger gains in one stage which in turn increases the signal to
noise ratio.
The LMP7731 has proprietary input bias cancellation circuitry
on the input stages. This allows the LMP7731 to have only
about 1.5 nA bias current with a bipolar input stage. This low
input bias current, paired with the inherent lower input voltage
noise of bipolar input stages makes the LMP7731 an excellent
choice for precision applications. The combination of low in-
put bias current, and low input voltage noise enables the user
to achieve unprecedented accuracy and higher signal integri-
ty.
National Semiconductor is heavily committed to precision
amplifiers and the market segment they serve. Technical sup-
port and extensive characterization data are available for
sensitive applications or applications with a constrained error
budget.
The LMP7731 is offered in the space saving 5-Pin SOT-23
and 8-Pin SOIC packages. These small packages are ideal
solutions for area constrained PC boards and portable elec-
tronics.
INPUT BIAS CURRENT CANCELLATION
The LMP7731 has proprietary input bias current cancellation
circuitry on their input stages.
The LMP7731 has rail-to-rail input. This is achieved by having
two input stages in parallel. Figure 1 shows only one of the
input stages as the circuitry is symmetrical for both stages.
Figure 1 shows that as the common mode voltage gets closer
to one of the extreme ends, current I
1
significantly increases.
This increased current shows as an increase in voltage drop
across resistor R
1
equal to I
1
*R
1
on IN
+
of the amplifier. This
voltage contributes to the offset voltage of the amplifier. When
common mode voltage is in the mid-range, the transistors are
operating in the linear region and I
1
is significantly small. The
voltage drop due to I
1
across R
1
can be ignored as it is orders
of magnitude smaller than the amplifier's input offset voltage.
As the common mode voltage gets closer to one of the rails,
the offset voltage generated due to I
1
increases and becomes
comparable to the amplifiers offset voltage.
20175206
FIGURE 1. Input Bias Current Cancellation
INPUT VOLTAGE NOISE MEASUREMENT
The LMP7731 has very low input voltage noise. The peak-to-
peak input voltage noise of the LMP7731 can be measured
using the test circuit shown in Figure 2
20175255
FIGURE 2. 0.1 Hz to 10 Hz Noise Test Circuit
The frequency response of this noise test circuit at the 0.1 Hz
corner is defined by only one zero. The test time for the 0.1
Hz to 10 Hz noise measurement using this configuration
should not exceed 10 seconds, as this time limit acts as an
additional zero to reduce or eliminate the noise contributions
of noise from frequencies below 0.1 Hz.
Figure 3 shows typical peak-to-peak noise for the LMP7731
measured with the circuit in Figure 2 for the LMP7731.
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LMP7731
