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Reference Information
6
DIFFERENTIAL MODE AND COMMON MODE
Differential amplifiers sense the voltage difference which appears between the + input and –
input. This voltage is referred to as the Differential Mode or Normal Mode voltage. The volt-
age component which is referenced to earth and is identical on both inputs is rejected by the
amplifier. This voltage is referred to as the Common Mode voltage and can be expressed as:
V
DIFFERENTIAL MODE RANGE AND COMMON MODE RANGE
Differential Mode range is the maximum signal which can be applied between the + and -
inputs without overloading the amplifier/amplifier, which otherwise would result in clipping or
distorting of the waveform measured by the oscilloscope.
The Common Mode Range is the maximum voltage with respect to earth ground which can
be applied to either input. Exceeding the common mode range can result in unpredictable
measurements. Because the Common Mode signal is normally rejected and not displayed
on the oscilloscope, the user needs to be careful to avoid accidentally exceeding the com-
mon mode range.
Because the input signal of a differential amplifier is not referenced to ground, the concept of
"V
" versus "V
" may be confusing.
peak
peak-peak
With a ground referenced signal, V
signal will have with respect to ground. In a differential system, there is no ground reference.
Therefore the Differential Mode Range refers to the maximum instantaneous amplitude of
the signal difference between the positive input and the negative input. Since most amplifi-
ers have symmetrical bipolar inputs, the value is generally expressed as an absolute value,
and can have either polarity.
For example, an amplifier with a differential mode rating of ±1 V can have a maximum volt-
age difference appearing at any instant in time of 1 V between the inputs. The polarity could
be either positive or negative. This does not imply that the number can be doubled to 2 volts,
however. For clarity, consider the following table of absolute voltages applied to the inputs of
a differential amplifier that has a differential mode range or ±1 V and a common mode range
of ±5 V:
DA18000AC-OM-E Rev A
Reference Information
V
+
V
+input
–input
------------------------------------- -
=
CM
2
is the maximum instantaneous voltage amplitude the
peak
6-1
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