Dewetron DEWE-30-16, Technical Reference Manual

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Main System
DE-M070505E • DEWE-30-16 • Technical Reference Manual • Printing version 1.1.1 • April 5, 2011
Bandwidth and cable length
Although the bandwidth of the 30-16-DIFF-OUT amplifier itself is above 1 MHz the total reached bandwidth
is depending on the used cable length. The longer the cable the lower the efficient bandwidth. The reason is
the parasite cable capacitance which is around 30-200 pF per meter.
Shielded twisted pair cable
DEWE-30-xx output
with 30-xx-DIFF-OUT option Data acquisition input
Parasitic capacitance
Therefore a 100 m cable can have a capacitance of around 10 nF. This capacitive load have to be driven by
the 30-16-DIFF-OUT amplifier. The needed current for driving this load depends on the signal level but also
of the signal frequency since
So current needed for driving a 100 m cable at 5 Vpp and 100 kHz is already around 30 mA. The maximum
output current of the 30-16-DIFF-OUT is specified with ±60 mA. If the signal frequency increase or the cable
is longer it is possible to get out of this specification.
Cable termination
Like in any higher frequency application (RS-485, CAN, IEEE1394 … ) the cable has to be terminated at the
end to avoid signal reflections at the signal line. The value of this resistor should have to be the same value
like the characteristic impedance of the used cable. This resistor causes on the one hand additional load for
the 30-16-DIFF-OUT amplifier but on the other hand also errors because of the voltage dropout at cable.
U
R-LINE
U
R-LINE
U
MEAS
Termination Resistor
Line Resistors
For example a 100 meter 2 x 0.25 mm² cable has a line resistance of around 7 Ohm. If the termination
resistor is 120 Ohm (typical value for twisted pair cable) this error is already around 5%. This influence
should be recognized in the measurement result or should be rescaled in the measurement setup.