Input protection
A high-speed amplifier like the WMA-320 can never be
made fully insensitive to input overload conditions, as this
would limit the performance of the amplifier to an
unacceptably low level. However, compared to other
circuits, the input is protected extremely well by a
combination of a high-power 50
Ω
impedance and diode
protection for extreme overload conditions. It can even
withstand connecting the output to the input for some time,
which may accidentally happen in a lab when cables are
going everywhere.
For normal operation, input voltages should remain in the
–2V to +2V range, resulting with an amplification of 50x in
an output voltage swing of –100V to +100V. Above 3V (-
3V), the input protection diodes will limit the voltage fed to
the amplifier.
Above 30V (-30V) at the input, the amplifier may be
permanently damaged if the current of the source is not
limited. The output can be connected to the input because
the current limit of the amplifier will cut back the voltage to
a level that can be managed by the input.
Never apply more than +30V (-30V) to the amplifier input!
Output protection
The WMA-320 has been designed to be fully stable with all
capacitive loads. It has been optimized for its step-
response, but is also a very good linear and sine-wave
amplifier.
Instability under capacitive loading conditions is a common
problem of high-speed negative feedback amplifiers on the
market, often resulting in unwanted overshoot voltages,
and, in extreme cases, oscillations (check the respective
manuals and datasheets for details). In the WMA-320, this
problem has been solved by a clever dual feedback
system. No significant overshoot occurs at any capacitive
load.
Overloading or short-circuiting this amplifier will not break
down the amplifier, due to the extremely fast current
limiting circuit that has been employed.
Although the amplifier cannot be damaged by a short-
circuit condition or capacitive loading, two situations
should be avoided:
-
Connecting a charged capacitor
-
Using high inductance values (coils)
The load
The output impedance of the WMA-320 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 50
Ω
circuit is possible by
connecting a 50
Ω
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 50
Ω
gives 15V instead of 100V).
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-320 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-320 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.
