LMP2234AMT/NOPB National Semiconductor, LMP2234AMT/NOPB Datasheet - Page 17

• Manufacturer Part Number: LMP2234AMT/NOPB
• Description: IC AMP PREC QUAD LO PWR 14TSSOP
• Manufacturer: National Semiconductor
• Series: LMP®, PowerWise®r

Specifications of LMP2234AMT/NOPB

• Amplifier Type: General Purpose
• Number Of Circuits: 4
• Output Type: Rail-to-Rail
• Slew Rate: 0.058 V/µs
• Gain Bandwidth Product: 130kHz
• Current - Input Bias: 0.02pA
• Voltage - Input Offset: 10µV
• Current - Supply: 36µA
• Current - Output / Channel: 30mA
• Voltage - Supply, Single/dual (±): 1.6 V ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -
• Other names: *LMP2234AMT; *LMP2234AMT/NOPB; LMP2234AMT

SINGLE SUPPLY STRAIN GAUGE BRIDGE AMPLIFIER
Strain gauges are popular electrical elements used to mea-
sure force or pressure. Strain gauges are subjected to an
unknown force which is measured as the deflection on a pre-
viously calibrated scale. Pressure is often measured using the
same technique; however this pressure needs to be convert-
ed into force using an appropriate transducer. Strain gauges
are often resistors which are sensitive to pressure or to flex-
ing. Sense resistor values range from tens of ohms to several
hundred kilo-ohms. The resistance change which is a result
of applied force across the strain gauge might be 1% of its
total value. An accurate and reliable system is needed to
measure this small resistance change. Bridge configurations
offer a reliable method for this measurement.
Bridge sensors are formed of four resistors, connected as a
quadrilateral. A voltage source or a current source is used
across one of the diagonals to excite the bridge while a volt-
age detector across the other diagonal measures the output
voltage.
Bridges are mainly used as null circuits or to measure differ-
ential voltages. Bridges will have no output voltage if the ratios
Instrumentation amplifiers are great for interfacing with bridge
sensors. Bridge sensors often sense a very small differential
signal in the presence of a larger common mode voltage. In-
strumentation amplifiers reject this common mode signal.
Figure 5 shows a strain gauge bridge amplifier. In this appli-
cation one of the LMP2234 amplifiers is used to buffer the
LM4140A's precision output voltage. The LM4140A is a pre-
cision voltage reference. The other three amplifiers in the
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FIGURE 4. Bridge Sensor
17
of two adjacent resistor values are equal. This fact is used in
null circuit measurements. These are particularly used in
feedback systems which involve electrochemical elements or
human interfaces. Null systems force an active resistor, such
as a strain gauge, to balance the bridge by influencing the
measured parameter.
Often in sensor applications at lease one of the resistors is a
variable resistor, or a sensor. The deviation of this active el-
ement from its initial value is measured as an indication of
change in the measured quantity. A change in output voltage
represents the sensor value change. Since the sensor value
change is often very small, the resulting output voltage is very
small in magnitude as well. This requires an extensive and
very precise amplification circuitry so that signal fidelity does
not change after amplification.
Sensitivity of a bridge is the ratio of its maximum expected
output change to the excitation voltage change.
Figure 4(a) shows a typical bridge sensor and Figure 4(b)
shows the bridge with four sensors. R in Figure 4(b) is the
nominal value of the sense resistor and the deviations from R
are proportional to the quantity being measured.
LMP2234 are used to form an instrumentation amplifier. This
instrumentation amplifier uses the LMP2234's high CMRR
and low V
ferential signal generated by the output of the bridge sensor.
This amplified signal is then fed into the ADC121S021 which
is a 12-bit analog to digital converter. This circuit works on a
single supply voltage of 5V.
OS
and TCV
OS
to accurately amplify the small dif-
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