Summer Circuit and Calculator
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A common analog mathematical computation is to add
Vo= V1+ V2.. VN.
This is easily done with the following circuit.
Figure 1. Resistive Adder Circuit
Because resistive circuits are linear an
analog voltage summer circuit is constructed from one or more resistors
combined at a node. Because of the law of superposition, the output is a sum of the individual input
voltages scaled by the resistor networks. If the resistors are all
the same value, then the voltage is the sum of the voltages divided by the
number of channels.
The passive circuit results in an attenuation
of each signal, so in common practice this circuit is often followed by an
active gain element such as an OpAmp to restore the signal levels of each
Vo= sum( Vn*(Rpn/(Rn+Rpn)
) for n=1 to N
Rpn is the parallel combination of other
resistors not including Rn
If all resistors are the same value:
Vo= sum( Vn)/N for n=1
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