LMP7731MF NSC [National Semiconductor], LMP7731MF Datasheet - Page 14

• Manufacturer Part Number: LMP7731MF
• Description: 2.9 nV/sqrt(Hz) Low Noise, Precision, RRIO, Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
LMP7731
The LMP7731 is a single, low noise, low offset voltage, rail-
to-rail input and output, and low voltage precision amplifier.
The low input voltage noise of only 2.9 nV/
ner at 3 Hz makes the LMP7731 ideal for sensor applications
where DC accuracy is of importance.
The LMP7731 has a very low guaranteed offset voltage of
only ±40 µV. This low offset voltage along with the very low
input voltage noise allows higher signal integrity and higher
signal to noise ratios as the error contribution by the amplifier
is at a minimum.
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, low input offset voltage, and low input voltage
noise enables the user to achieve unprecedented accuracy
and higher signal integrity.
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 SOT23
package. This small package is an ideal solution for area
constrained PC boards and portable electronics.
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 2. 0.1 Hz to 10 Hz Noise Test Circuit
with a 1/f cor-
14
Figure 1 shows that as the common mode voltage gets closer
to one of the extreme ends, current I
This increased current shows as an increase in voltage drop
across resistor R
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
voltage drop due to I
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
comparable to the amplifiers offset voltage.
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
FIGURE 1. Input Bias Current Cancellation
1
equal to I
1
across R
1
*R
1
1
can be ignored as it is orders
on IN
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1
1
is significantly small. The
1
increases and becomes
significantly increases.
+
of the amplifier. This
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