Stanford Research Systems SR570 User Manual page 36

noise greater. So, when making a measurement,
it is important to keep in mind the tradeoff
between high dynamic reserve (bandwidth) and
low noise performance.
The dynamic reserve is a function of frequency
and depends on the amplifier configuration
(sensitivity, filters and gain mode setting). The
figure below shows the dynamic reserve for a
SR570 set to sensitivities of 20 µA/V, 50 µA/V,
and 100 µA/V, with the high pass filter set to
100 Hz and the low pass filter set to 1 kHz (for a
bandpass from 100 Hz to 1 kHz). The dynamic
reserve characteristics are shown for both "High
Bandwidth" and "Low Noise" gain modes. The
Low Drift mode has the same dynamic reserve
as the Low Noise mode.
There are several features to note. In the
bandpass region between 100 Hz and 1 kHz the
dynamic reserve is near 0 dB. The dynamic
40
30
20
Low Noise
20 µA/V
10
0
1
Dynamic Reserve vs. Frequency
50 µA/V
100 µA/V
10 100
reserve is 3 dB at the filter frequencies of 100 Hz
and 1 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 rolloff for
interfering signals was required, a 12 dB/octave HP
or LP filter could be used.
The DR rises to a maximum which depends on the
gain distribution in the amplifier circuit. The plot
gives the DR for a 1-2-5 sequence of sensitivities on
two different gain modes. It turns out that between
20 pA/V and 100 µA/V, the curves are exactly the
same for any other 1-2-5 sequence. Below 20 pA/V,
the maximum DR increases by 20 dB for all gain
modes over those in the plot. Above 100 µA/V the
Low Noise curves are the same as in the plot and the
High Bandwidth curves are the same as the Low
Noise. Keep in mind that the amplifier bandwidth
may limit the DR at the higher frequencies on some
sensitivities.
Bandpass
Filter
100 Hz - 1 kHz
6 dB/octave
20 dB/decade
(due to filters)
1000
Frequency (Hz)
B-2
High Bandwidth
20 µA/V
50 µA/V
100 µA/V
4
10
5
10