Figure 7–6: Typical random noise in chromatography (lower trace)
Looking closely at the lower noise trace, you can recognize both frequencies (and others). This is
typical of noise in chromatography: a collection of more or less random frequencies.
7.2.2 Low pass noise filters
Noise filters work by suppressing certain frequencies in the acquired signal. Low pass filters
typically allow chromatographic peaks (low frequency) to pass, while higher frequency noise is
attenuated. No matter how advanced, it is impossible to use a low pass filter successfully if there
is no difference in frequency between signal and noise.
Analog filters are made of capacitors, resistors, and amplifiers (opamps). Digital filters are
mathematical routines to process an acquired signal. Traditionally, in many detectors for
chromatography an analog low-pass filter (rise time filter) is applied. A “passive” RC filter consists
of resistors and capacitors. An active, higher order filter can be considered as a series of these
RC filters. In a fourth-order filter the signal coming from the first filter is filtered again in a second,
a third, and a fourth. During these steps, loss of signal occurs simply because of all the resistors
that are applied. Operational amplifiers, which are “active” components, are applied in each stage
to restore the signal to its original value.
With the availability of powerful processors, digital signal processing has become an excellent
alternative to hardware filters. In its most simple form, a running average filter takes the average
of n data points to create a new data point. For example, in a 5-point running average filter,
output data point y[80] is calculated from measured data points x[80] – x[84] as:
Each input data point has the same weighting factor of 1/5. In more advanced digital signal
processing, a more complicated equation is used to calculate the output data point y[n]:
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