Teledyne LeCroy WaveLink Differential Probe Series (8-13 GHz) - Manual (Page 66)

WaveLink Medium-Bandwidth (8-13 GHz) Differential Probe
Differential Mode and Common Mode
Differential probes sense the voltage difference that appears between the + and – inputs. This is
referred to as the Differential or Normal Mode voltage. The voltage component that is referenced to
earth and is identical on both inputs is rejected by the amplifier. This is referred to as the Common
Mode voltage and can be expressed as:
Differential Mode Range and Common Mode Range
Differential Mode range is the maximum signal that 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 that 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 common mode range.
Common Mode Rejection Ratio
Common Mode Rejection Ratio (CMRR) is the measure of how much the amplifier rejects the
common mode voltage component that appears at the output. CMRR is equal to the differential
mode gain (or normal gain) divided by the common mode gain. Common 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 (e.g., 10,000:1) or implicitly in dB (e.g., 80 dB). Higher
numbers indicate greater rejection (better performance).
The first order term determining the CMRR is the relative gain matching between the + and – input
paths. Obtain high CMRR values by precisely matching the input attenuators in a differential
amplifier. The matching includes the DC attenuation and the capacitance which determines the AC
attenuation. As the frequency of the common mode component increases, the effects of stray
parasitic capacitance and inductance in determining the AC component become more pronounced.
The CMRR becomes smaller as the frequency increases. Therefore, the CMRR is usually specified in
a graph of CMRR versus common mode frequency.
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