Datasheets : Difference Amplifier (or Subtractor) - Process Unit

What does it do?

The difference amplifier is used with two analogue input signals. It gives an analogue output signal which is proportional to the difference between the two input voltages.

How does it operate?

When the two input signals are the same, the output signal will remain steady at half the supply voltage. The output signal will go above or below this steady voltage when one input signal is greater than the other. The output signal is measured relative to a reference signal, which is fixed at half the power supply voltage. (An alternative is to use a pair of power supplies, +Vs and –Vs, and then the reference signal is 0V.)
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.
The difference amplifier circuit uses an operational amplifier, or op amp, to subtract the input signals. Operational amplifiers have two inputs, an inverting input ('-') and a non-inverting input ('+'). The amount of amplification, or gain, can be varied by changing R1, R2, R3 and R4. If (as in the circuit diagram) all four resistors are equal then: R1 must always be the same as R2 and R3 must always be the same as R4. But if all four resistors are not equal then:

The LM324 is a suitable inexpensive IC, contains four operational amplifiers and can work from a d.c. power supply with a voltage anywhere between 3 and 32V.

Possible applications

• Monitoring the difference between two analogue sensors, such as two temperature sensors.

Making

The PCB shows the basic circuit. Three operational amplifiers in the IC are not used in this simple design; they can be applied in other subsystems.

Build and test the unit that will provide the input signals before building the difference amplifier.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 11 is low (0V);
• the voltage on pin 4 is high (the supply voltage);
• the voltages on pins 2 and 3 (connected to the blue PCB tracks) vary in response to the units that provides the input signal.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) is proportional to the difference between the voltages of the input signals.

Fault finding

If there is a fault, check that:

• The voltage on pin 11 is low (0V)
• The voltage on pin 4 is high (the supply voltage)
• The values of all the resistors is correct

If there is a fault, check the tracks and solder joints.

Alternatives

• The Comparator gives a digital output signal in response to the difference between two analogue voltages
• A PIC can respond to the difference between two analogue voltages provided it has two analogue to digital converter input pins. PICs are more flexible but more expensive.

Web links

• Manufacturer’s data sheet for LM324

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