Datasheet

LMP7721
SNOSAW6D –JANUARY 2008–REVISED MARCH 2013 www.ti.com
APPLICATION INFORMATION
ADVANTAGES OF THE LMP7721
Ultra Low Input Bias Current
The LMP7721 has the industry’s lowest specified input bias current. The ultra low input bias current is typically 3
fA, with a specified limit of ±20 fA at 25°C, ±900 fA at 85°C and ±5 pA at 125°C when V
CM
= 1V with a 5V or a
2.5V power supply.
Wide Bandwidth at Low Supply Current
The LMP7721 is a high performance amplifier that provides a 17 MHz unity gain bandwidth while drawing only
1.3 mA of current. This makes the LMP7721 ideal for wideband amplification in portable applications.
Low Input Referred Noise
The LMP7721 has a low input referred voltage noise density (6.5 nV at 1 kHz with 5V supply). Its MOS input
stage ensures a very low input referred current noise density (0.01 pA/ ).
The low input referred noise and the ultra low input bias current make the LMP7721 stand out in maintaining
signal fidelity. This quality makes the LMP7721 a suitable candidate for sensor based applications.
Low Supply Voltage
The LMP7721 has performance specified at 2.5V and 5V power supplies. The LMP7721 is ensured to be
functional at all supply voltages between 2.0V to 5.5V, for ambient temperatures ranging from −40°C to 125°C.
This means that the LMP7721 has a long operational span over the battery's lifetime. The LMP7721 is also
specified to be functional at 1.8V supply voltage, for ambient temperatures ranging from 0°C to 125°C. This
makes the LMP7721 ideal for use in low voltage commercial applications.
RRO and Ground Sensing
Rail-to-rail output swing provides the maximum possible output dynamic range. This is particularly important
when operating at low supply voltages. An innovative positive feedback scheme is created to boost the
LMP7721’s output current drive capability. This allows the LMP7721 to source 30 mA to 40 mA of current at 1.8V
power supply.
The LMP7721’s input common mode range includes the negative supply rail which makes direct sensing at
ground possible in single supply operation.
Unique Pinout
The LMP7721 has been designed with the IN+ and IN−, V
+
and V
−
pins on opposite sides of the package. There
are isolation pins between IN+ and V
−
, IN− and V+. This unique pinout makes it easy to guard the LMP7721’s
input. This pinout design reduces the input bias current’s dependence on common mode or supply bias.
The SOIC package features low leakage and it has large pin spacing. This lowers the probability of dust particles
settling down between two pins thus reducing the resistance between the pins which can be a problem.
Input Protection
The LMP7721 input stage is protected from seeing excessive differential input voltage by a pair of back-to-back
diodes attached between the inputs. This limits the differential voltage and hence prevents phase inversion as
well as any performance drift. These diodes can conduct current when the input signal has a really fast edge,
and, if necessary, should be isolated (using a resistor or a current follower) in such cases.
SYSTEM DESIGN TECHNIQUES WITH THE LMP7721
In order to take full advantage of the LMP7721’s ultra low input bias current, a triaxial cable/connector is
recommended when designing application systems.
A triaxial cable/connector is similar to a coaxial cable/connector and is often referred to as “triax”. Figure 46
shows the structure of the triax.
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