V

) I

11

= (2a + 1) V R

11

= (2a + 1)

11

GIVEN: I

= I

LMP2234

SNOSAW4D – SEPTEMBER 2007 – REVISED MARCH 2013

www.ti.com

There are two stages in this amplifier. The last stage, the output stage, is a differential amplifier. In an ideal case

the two amplifiers of the first stage, the input stage, would be configured as buffers to isolate the inputs. However

they cannot be connected as followers because of mismatch in amplifiers. That is why there is a balancing

resistor between the two. The product of the two stages of gain will give the gain of the instrumentation amplifier.

Ideally, the CMRR should be infinite. However the output stage has a small non-zero common mode gain which

results from resistor mismatch.

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 LMP2234.

(3)

By Ohm’s Law:

(4)

However:

(5)

So we have:

(6)

Now looking at the output of the instrumentation amplifier:

(7)

Substituting from Equation 6:

(8)

This shows the gain of the instrumentation amplifier to be:

−K(2a+1)

(9)

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 GAUGE 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 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 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.

20

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