Datasheet

V
+
R
1
R
2
IN
+
IN
-
I
1
INPUT STAGE
I
BIAS
CANCELLATION CIRCUIT
Q1 Q2
V
+
R
C
1
R
C
2
LMP7731
SNOSAT6E –JULY 2007–REVISED MARCH 2013 www.ti.com
APPLICATION INFORMATION
LMP7731
The LMP7731 is a single, low noise, rail-to-rail input and output, and low voltage amplifier.
The low input voltage noise of only 2.9 nV/√Hz with a 1/f corner 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 input bias current, and low input voltage noise enables the user to achieve
unprecedented accuracy and higher signal integrity.
Texas Instruments is heavily committed to precision amplifiers and the market segment they serve. Technical
support 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 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 44 shows only
one of the input stages as the circuitry is symmetrical for both stages.
Figure 44 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.
Figure 44. Input Bias Current Cancellation
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