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CALIBRATION AND REPAIR
20
INPUT VOLTAGE NOISE
The amplifier's input voltage noise
approximates that of a 1000Ω resistor
(about 4 nV/√Hz). For source impedances
below 1000Ω, the output noise will be
dominated by the amplifier's input voltage
noise. A typical amplifier has an input
voltage noise vs. frequency as shown in the
figure below. Notice that the voltage noise
rises at lower frequencies (“1/f" noise).
DYNAMIC RESERVE
The dynamic reserve of the amplifier is a
measure of how large a signal can be
present at the input to the amplifier without
causing an overload condition.
The definition of dynamic reserve is:
• DR (dB) = 20 log (Vin(f) w/o
overload / Vin for full scale)
A full-scale output voltage is 10 Vpp.
Signals at the output (or at any stage) which
exceed 10 Vpp cause an overload. The
dynamic reserve is greater than 0 dB only
when the filters are used to remove
unwanted signals.
The dynamic reserve is a function of
frequency and depends on the amplifier
configuration (gain, filters and dynamic
reserve setting). The figure below shows
the dynamic reserve (and maximum input
signal without overload) for a SR560 set to
a gain of 1000, the high pass filter set to 1
kHz and the low pass filter set to 10 kHz (for
a bandpass from 1 kHz to 10 kHz). The
dynamic reserve characteristic is shown for
both "High Dynamic Reserve" and "Low
Noise" gain modes.
There are several features to note. In the
bandpass region between 1 and 10 kHz the
dynamic reserve is 0 dB. The dynamic
reserve is 3 dB at the filter frequencies of 1
and 10 kHz. The dynamic reserve rises by
6 dB/oct (or 20 dB per decade) as the signal
moves away from the pole frequency, since
each RC filter attenuates the signal. If a
faster roll-off for interfering signals were
required, a 12 dB/octave HP or LP filter
could be used.
The HIGH DR characteristic offers 16 dB
more DR at low frequencies and 26 dB
more at high frequencies. The high
frequency DR is limited only by the
maximum 3 Vpp limit of the input stage.
The maximum DR in the low noise mode is
36 dB. Since there is no gain between the
HP and LP filters in the Low Noise gain
mode, the DR is the same at very high
frequencies and very low frequencies.
The input reference voltage noise for the
High DR gain mode is about 10 nV/√Hz,
compared to 4 nV/√Hz in the Low Noise
gain mode. The table (middle of next page)
summarizes the input referenced noise and
maximum dynamic reserve for all gains.