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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
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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.
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