Teledyne LeCroy ADP300_305 - Manual (Page 31)

Operator’s Manual
Reference Information
Differential Mode and Common Mode
Differential amplifiers amplify the voltage difference that appears between the +
input and – input. This voltage is referred to as the Differential Mode or Normal
Mode voltage. The voltage component that 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:
2
Input Input
CM
V V
V
− +
+
=
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, which otherwise would result
in clipping or distorting 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, you need to be careful
to avoid accidentally exceeding the common mode range.
Common Mode Rejection Ratio
The ideal differential amplifier would amplify only the differential mode voltage
component and reject all of the common mode voltage component. Real
differential amplifiers are not perfect, so a small portion of the common mode
voltage component appears at the output. Common Mode Rejection Ratio
(CMRR) is the measure of how well the amplifier rejects the common mode
voltage component. CMRR is equal to the differential mode gain (or normal gain)
divided by the common mode gain. The 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). The higher the number the greater the rejection the
better the performance.
923079-00 Rev A 27