DEWE-43
Page 18
Dewe-43 Technical reference manual
CMRR:
All 8 analog channels of the DEWE-43 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 frequency spectrum. The figure bellow show the allowable
common-mode input voltages for various input voltages and measurement
ranges.
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 see 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
