Lecroy AP034-OM-E-04 User Manual

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DIFFERENTIAL MODE AND COMMON MODE

Differential probes amplify the voltage difference that appears
between the + and – inputs. This voltage is referred to as the
Differential Mode or Normal Mode voltage. The voltage
component that is refer enced to earth ground, and is identic al on
both inputs, is rejected by the amplifier. This voltage is referred to
as the Common Mode voltage, because it is common to both
inputs. The common mode voltage can be expressed as:

V

VCM =

DIFFERENTIAL MODE RANGE AND COMMON MODE RANGE

The Differential Mode Range is the max imum signal that can be
applied between the + and – inputs without overloading the probe
amplifier, resulting in “clipping” or distortion of the waveform
measured by the oscilloscope.

+Input

+ V

2

-Input

The Common Mode Range is the max imum voltage with respect
to earth ground that can be applied to either input. Exceeding the
common mode range can result in unpredictable results.
Because the Common Mode s ignal is normally rejected, and is
not displayed on the oscilloscope, you need to be careful to avoid
accidentally exceeding the common mode range.

AP034-OM-E Rev D ISSUED: January 2000 ²

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Common Mode Range

Maximum voltage from
either input to ground

Figure 6. Common Mode and Different i al Mode Range

COMMON MODE REJECTION RATIO

The ideal differential probe or diff erential amplifier would amplif y
only the differential mode voltage component and reject all of the
common m ode voltage component. Real differential probes and
amplifiers are not perfect, and a small portion of the common
mode voltage component does appear in the output. Common
Mode Rejection Ratio (CMRR) is the measure of how m uch the
probe or amplifier rejects the comm on m ode voltage com ponent.
CMRR is equal to the differential mode gain (or normal gain)
divided by the common mode gain. Com m on mode gain is equal
to the output voltage divided by the input voltage when both
inputs are driven by only the common mode signal. CMRR can
be expressed as a ratio (for exam ple, 10
(for example, 80 dB). Higher numbers indicate greater rejection
(better performance).

Differential Mode Range

Maximum voltage

between inputs

000:1) or implic itly in dB

The first-order term that determines the CMRR is the relative
gain matching between the + and – input paths. To obtain high
CMRR values, the input attenuators in a differential probe are
precisely matched to each other. The m atching includes the DC
attenuation as well as the capacitance that determines the AC
attenuation. As the frequency of the common m ode components
increases, the effects of stray parasitic capacitance and
inductance in determining the AC attenuation become more
pronounced. The CMRR becomes smaller as the frequency
increases. Hence, CMRR is usually specified as a plot versus
common mode frequency.

The common mode frequency in these plots is assum ed to be
sinusoidal. In real life applications, the common mode signal is

² ISSUED: January 2000 AP034-OM-E Rev D