The 1kΩ resistors will form a voltage
divider with the output resistance of the
signal source which is used to drive the
WMA-200 amplifier. If this impedance is
very low, no change in gain will be
observable, but if this source has e.g. a
(commonly encountered) 50Ω output
resistance, the apparent gain will change
from 20x to a lower but very well-defined
19.05x due to this voltage divider effect.
The 1kΩ resistors also add to the output
offset voltage because the offset current of
the input amplifier generates a voltage
over these resistors. This output offset
voltage is ~3mV if the BNC input is used
with a 50Ω signal generator. When both
inputs are shorted together or connected
to a low-impedance source, the offset is
reduced to ~700 μV.
Using the BNC connector, the amplifier will
amplify 20.0x, up to the maximum output
voltage. To obtain the lowest possible
noise performance possible, it is highly
recommended to use the differential input
instead of the BNC. The BNC input is
internally connected to the positive input of
the differential connector (see Fig. 1 for
the connections). The negative input of the
differential connector should be supplied
with the same input signal as the positive
input, but this signal should have the
opposite polarity. Both of the differential
inputs amplify 20.0x, which means that the
absolute amplitude of either input should
be only half of the value that would be
used to drive the BNC input.
Figure 1. Signals to be supplied to the BNC and differential connector
A low noise amplifier like the WMA-200
model can never be made fully insensitive
to input overload conditions, as this would
limit the performance of the amplifier
significantly. For normal operation, input
voltages should remain in the –8.75V to
+8.75V range when using the BNC input,
and half this value when using the
differential connector. This will result, with
an amplification of 20.0x, in an output
voltage swing of –175V to +175V. Below -
15V and above +15V, the input protection
circuitry will limit the voltage fed to the
amplifier, but the amplifier (or the signal
source) may be permanently damaged if
the current of the source is not limited.
Outputs
The WMA-200 model has a 50Ω output
resistance, and has been designed to be
fully stable with all capacitive loads. It has
been optimized for a perfect step-
response, but it is also a very good linear
and sine-wave amplifier.
The WMA-200 amplifier has a clever
feedback system, which ensures that no
significant overshoot occurs at any
capacitive load. The bandwidth of the
amplifier is automatically reduced to
ensure stability. Instability under capacitive
loading conditions is a common problem of
other high-speed negative feedback
amplifiers, often resulting in unwanted
output overshoot voltages, and, in extreme
cases, oscillations.
Short-circuiting this amplifier will not break
down the amplifier, due to the extremely
fast current limiting circuit that has been
employed.
The output impedance of the WMA-200
model is 50Ω, to ensure stability with all
Inverting (-)
Non-inverting (+)
Ground
