VO = -K (2a + 1) (V1 - V2)

VO =

KR2

R2

(VO2 - VO1)

= -K (VO1 - VO2)

R11 = V1 - V2

V

VO1 - VO2 = (2R1 +

) I

R11 R

11

= (2a + 1) V R

11

= (2a + 1)

R11 x IR

11

GIVEN: I

R1

= I

R11

LMP2231

SNOSB01E – JANUARY 2008 – REVISED MARCH 2013

www.ti.com

In the input stage of the circuit, current is the same across all resistors. This is due to the high input impedance

and low input bias current of the LMP2231.

(1)

By Ohm’s Law:

(2)

However:

(3)

So we have:

VO1–VO2 = (2a+1)(V1–V2)

(4)

Now looking at the output of the instrumentation amplifier:

(5)

Substituting from Equation 4:

(6)

This shows the gain of the instrumentation amplifier to be:

−K(2a+1)

(7)

Typical values for this circuit can be obtained by setting: a = 12 and K= 4. This results in an overall gain of

−100.

SINGLE SUPPLY STRAIN GAGE BRIDGE AMPLIFIER

Strain gauges are popular electrical elements used to measure force or pressure. Strain gauges are subjected to

an unknown force which is measured as a the deflection on a previously calibrated scale. Pressure is often

measured using the same technique; however this pressure needs to be converted into force using an

appropriate transducer. Strain gauges are often resistors which are sensitive to pressure or to flexing. 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 voltage detector across the other diagonal

measures the output voltage.

Bridges are mainly used as null circuits or to measure a differential voltages. Bridges will have no output voltage

if the ratios 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 element 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.

20

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