What does it do?
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The summing amplifier subsystem is used to add two analogue input signals together. |
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How does it operate?
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 In this circuit the input voltages and output voltage are measured relative to a reference voltage, equal to half the supply voltage, and set by the potential divider consisting of R4 and R5.
The output voltage (relative to this reference voltage of +Vs/2) is proportional to the sum of the two input voltages (again, relative to the reference). (An alternative is to use a pair of power supplies, +Vs and –Vs, and then the reference signal is 0V.)
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Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
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The summing amplifier circuit uses an operational amplifier, or op-amp, to add 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 and the feedback resistor VR1.
R1 must always be the same as R2.
Note that the gain of the circuit is negative. So, if the two input voltages VA and VB are above the reference voltage, then Vout will be below the reference voltage. |
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
- Combining the signal from two analogue sensors, such as two temperature sensors.
Making
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Pins of LM324 IC
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How part of the PCB might look |
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 connected to R1 and R2 (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 sum of 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 are correct
If there is a fault, check the tracks and solder joints.
Alternatives
- PICs can respond to the sum of two analogue voltages provided they have two analogue to digital converter input pins. They are more flexible but more expensive.
Web links
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