capacitive loads. The amplifier is generally
used for high-impedance applications
where the load is capacitive. This is the
case for MEMS devices, EO-modulators
and piezo’s alike. It should be noted that a
coaxial cable itself also presents a
capacitive load of approximately 100pF/m.
Matched loading with a 50Ω load circuit is
possible by connecting a 50Ω resistor in
series with the output to ground, but is not
recommended. The advantage is that
excessively long cables will not distort the
waveforms. The disadvantage is a highly
reduced voltage range (150mA in 50Ω
gives 7.5V maximum instead of 175V
maximum).
A monitor out signal is available on a
separate BNC connector, which provides a
1:100 replica of the output signal as seen
by the load.
Cabling
The WMA-200 high voltage amplifier has
been designed for use in the most
demanding low noise experiments. As
such, the noise generated by the amplifier
is so low, that the actual performance of
the set-up in which it is used can easily be
dominated by interference picked up from
the surroundings. For optimal performance
use the differential input, with a high-
quality differential cable, and keep the
cable as short as possible.
For sensitive and/or high-frequency
measurements, coaxial or shielded twisted
pair (STP) cable is recommended for the
input, and a coaxial cable for the output.
The length of the cables should be
minimized. Otherwise, the cables will
cause overshoot due to cable reflections
(an effect related to the finite speed of
light), and current limiting due to the cable
capacitance. Although the amplifier itself
remains fully stable, using less than 3
meter of input and output cable is
recommended for the WMA-200 amplifier
to obtain optimal results without cable
reflections, even if obtaining the lowest
possible noise performance is not
required.
As the output connector is a single-ended
BNC, all interference picked up by the
output cable will be directly appear across
the
connected
load.
The
noise
performance of the amplifier has been
assessed with the inputs shorted, and the
output of the amplifier connected with a
direct coaxial connection to a low-noise
oscilloscope front end. Fig. 2 shows the
arrangement. Even a 10 centimeter output
cable increases the attainable noise level
by a few microvolts in a typical setup due
to interference pick-up. But this may not
be a problem in many applications; the
performance
is
still
excellent
by
conventional standards when a longer
coaxial cable is used. It is recommended
not to make a ground connection to the
load in any other way than via the WMA-
200 amplifier output connector ground, to
prevent ground loops. The optimal
arrangement of connections is given in
Fig. 3.
Figure 2. Assessment of the ultimate attainable output noise level of the WMA-200 high
voltage amplifier requires care not to beak the coaxiality of the connection. Cables should be
as short as possible (or even absent, as in this example) not to pick up any interference,
which will appear directly across the load. The differential inputs are shorted together at the
amplifier input in this example.
