DEWE-43A
TECHNICAL REFERENCE MANUAL
3.1.3. CMRR:
All 8 analog channels of the DEWE-43A are fully differential inputs with resistance of 10MΩ||10pF. The
input voltage range is ±10V, ±1V, ±100mV and ±10mV. Because of the differential input structure, the
difference of the input (Ch x(+) – Ch x(-)) will be shown as the result of the measurement. Although the
input is protected for input voltages to ±70V, the common voltage range of each input is limited to
about ±13V. If the input voltage exceeds this range, the result is not valid even when the difference input
voltage is lower than current input range. These voltage ranges will be clipped and introduced as large
errors that can be easily identified in the frequency spectrum. The image below shows the allowable
common-mode input voltages for various input voltages and measurement ranges.
Image 9: Common-mode input voltages
Example: Many signal sources (function generators) and power supplies are floating sources. That
means that they are isolated from each other and from AC power line. If we connect a sensor with
differential output and floating power supply to measurement device, then GND of sensor and
measurement device can have different voltage potential. This is what the measurement device sees as
common mode voltage. This common-mode voltage can range from few volts to few hundred volts, but
in almost all cases this renders the measurement. To prevent this effect, GND signals of the sensor and
measurement device need to be directly connected. That way we eliminate common-mode voltage. On
DEWE-43A this connection is possible over connector GND wire or over “Common GND” receptacle on
the housing.
DEWE-43A V20-2
13/38
