The load
The output impedance of the WMA-300 is
50
Ω
. The amplifier is generally used for
high-impedance applications where the
load is mainly capacitive. This is true for
MEMS devices, EO-modulators and PZT's
(piezo
’
s) alike. It should be noted that a
coaxial cable also presents a capacitive
load of approximately 100pF/m. The cables
that are connected may well limit the
maximum
usable
current
at
high
frequencies.
Matched loading with a 50Ω circuit is
possible by connecting a 50Ω resistor in
series with the output to ground, but is not
usually recommended. The advantage is
that excessively long cables will not distort
the waveforms anymore. The disadvantage
is a clearly reduced voltage range (300mA
in 50Ω gives 15V instead of 150V).
With high-frequency measurements, cable
length should be minimized or the cables
themselves will cause overshoot and
current limiting. Although the amplifier
remains fully stable, more than 1.5 meter of
output cable is not recommended for the
WMA-300 for optimal results.
If output monitoring is required, it is
recommended
to
connect
a
10x
oscilloscope probe to the output. A special
BNC to probe tip connector is usually
supplied with the probe (Fig. 1). However,
the user can choose a different way of
connecting the oscilloscope, as long as
care is taken with the high output voltage.
Pieces of non-coaxial cable in the
connection can cause overshoot in the
oscilloscope reading.
Figure 1. The 10x probe connected for monitoring the output signal
The amplifier should not be used for
telecommunication as described in the
R&TTE directive 95/5/EC. To prevent
unwanted radio emission, the TDK ferrite
ring which is supplied with the instrument
should be mounted on the output cable
close to the WMA-300 housing as
indicated in Fig. 2, when square wave
output signals above 100kHz and 50V are
generated. In this case, the length of the
output cable should be no more than 1.5
meter. The measurement is not influenced
by this ring; it is only required to comply with
European EMC legislation.
Figure 2. The ferrite ring (type TDK ZCAT3035-1330) should be placed on the output cable as
indicated when a square wave of more than 100kHz and 50V is generated to prevent
unwanted radio emission in the 30MHz region.
