Falco Systems WMA-200 User Manual page 5

connector together wile leaving the ground
pin in the middle open (Fig. 2).
The output noise will be lowered further
when the bandwidth of the amplifier is
reduced, which happens
significant capacitive load is connected to
the output. This will happen automatically
when a high-capacitance
connected to the output. For all other
cases, a shielded 1uF load capacitor box
is supplied with the amplifier, which is
connected parallel to the load (Fig 4). This
will bring the noise down from its value of
~50µV (with a DC - 100kHz bandwith) to
rms
Figure 4. When the bandwidth- and noise reducing capacitor box is connected to the load, the
noise reduces to ~20µV
The load
The output impedance of the WMA-200
model is 50Ω, to ensure stability with all
capacitive loads. The amplifier is generally
used for high-impedance
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
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 (275mA in 50Ω
gives 13,75V maximum instead of 175V
maximum).
With sensitive and/or
measurements, coaxial or shielded twisted
pair (STP) cable should be used 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
when a
piezo is
, while the bandwidth is simultaneously reduced to DC – 500Hz.
rms
applications
also presents a
high-frequency
only ~20µV (with a DC – 500Hz
rms
bandwidth). If used, always connect the
capacitor box directly next to the load, so
that the box also
interference picked up by the output cable
before this interference reaches the load.
For a comprehensive treatment on the
subject of noise and interference, the
reader is referred to the Falco Systems
application note 'High voltage amplifiers:
so you think you have noise!', online at
www.falco-systems.com.
Amplifier
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
required.
Transmitter mode
This amplifier can generate a significant
amount of power at frequencies used for
radio transmission and reception. The
amplifier should
telecommunication as described in the
R&TTE directive 95/5/EC. Also for this
reason always use coaxial cables.
Amplifier characteristics
In the following pages, several amplifier
characteristics are illustrated:
- Frequency response as a function of
capacitive load (Fig. 5, 6)
filters away the
Capacitor box
Load
performance is not
not be used for