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Analogue Differential Inputs Graham Elvis, RA Crewe, UK, April 2003 In order to avoid problems with common mode noise (such as ground loops) between source and receiver it is common to use “differential” or “floating” inputs for analogue signals. This note attempts to explain the operation of such an input. Shown below is the simplest form of differential input. Practical circuits are generally more complex but the behaviour is identical so it is instructive to understand the operation of this circuit. R2 (10k) R1 (10k) Vi - In1 R3 (10k) Vi + Op Amp Vo In2 + R4 (10k) Com First, it is essential to understand the how the Operational Amplifier (Op Amp) works : The Op Amp amplifies the voltage difference between In1 and In2 by its “open loop gain”. This gain is very high, typically greater than 100,000. The effect is to adjust its output Vo so that the voltage at In1 is virtually identical to that at In2 (within microvolts). Each input (In1 & In2) has virtually infinite impedance, so takes no significant current. The table below defines a number of operating conditions to illustrate the operation. Each value is the voltage of the specified node with respect to Common (Com). Example 1 : Vi+ = 0V Therefore In2 = 0V Vi- = 0V Op Amp adjusts Vo so that In1 = In2 ( = 0V ) Therefore Vo = 0V Example 2 : Vi+ = 0V Therefore In2 = 0V Vi- = 2V Op Amp adjusts Vo so that In1 = In2 ( = 0V ) Therefore Vo = -2V 582745019 Page 1 of 2 Example 3 : Vi+ = 2V Therefore In2 = 1V Vi- = 2V Op Amp adjusts Vo so that In1 = In2 ( = 1V ) Therefore Vo = 0V Example 4 : Vi+ = 2V Therefore In2 = 1V Vi- = 4V Op Amp adjusts Vo so that In1 = In2 ( = 1V ) Therefore Vo = -2V Example 1 2 3 4 Vi + 0 0 2 2 Vi 0 2 2 4 In1 0 0 1 1 In2 0 0 1 1 Vo 0 -2 0 -2 It can be clearly seen that the output is the inverse of the difference between Vi+ and Viwith a gain of 1. Practical considerations Gain The gain may be increased to 2, for example, by changing resistors R2 and R4 to 20k. Gain = R2/R1 but R4/R3 must also be made equal to the required gain to maintain the differential operation. Constraint In1 and In2 voltages must lie between the positive and negative supply lines to the Op Amp. Assumption The source impedance “seen” by In1 and In2 is much greater than R1 & R2 or the two source impedances are equal. Most practical circuits avoid this limitation by adding unity gain buffers to the In1 & In2 inputs. 582745019 Page 2 of 2