46
Noise suppression: ADF™
AZURA® Detector ECD 2.1 Instructions, V6685
Analogue filters are made of hardware, from capacitors, resistors and
amplifiers (opamps). Digital filters are mathematical routines to process
an acquired signal.
Traditionally, in many detectors for chromatography an analogue low-
pass filter is applied (rise time filter). A ‘passive’ RC filter consists of resis-
tors and capacitors. An active higher order filter can be considered as a
number of these RC filters in series. In a 4th order filter the signal coming
from the first filter is filtered again in a second, third and fourth filter. Du-
ring 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 for 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-points 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]:
In contrast to the previous equation, each data point has a different
weighting factor a. Sum of these weighting factors a0…n will always be 1.
Characteristic of noise filters is that processing the signal will result in a
delay. This is inevitable, as the mathematics of digital signal processing
requires a number of previous data points to process a new data point.
The filter characteristic in DSP is often named after the scientist who
‘invented’ the mathematics behind the signal processing routine. Well-
known names in this field are Bessel, Chebychev, Savitsky, Golay, Ham-
ming and many others.
7.4 Amplitude response plot
Fig. 32
Amplitude response plot of an ideal low pass filter with a cut-off fre-
quency of 1 Hz.
Frequency (Hz)
A
m
p
lit
u
de
0
1
2
3
4
5
0
0.2
0.4
0.6
0.8
1
1.2
pass
filter block
Summary of Contents for Azura ECD 2.1
Page 1: ...HPLC Detector ECD 2 1 Instructions Document No V6685 ...
Page 32: ...28 ECD 2 1 controller AZURA Detector ECD 2 1 Instructions V6685 Pulse mode ...
Page 33: ...29 ECD 2 1 controller AZURA Detector ECD 2 1 Instructions V6685 Scan mode CONFIC menu ...
Page 34: ...30 ECD 2 1 controller AZURA Detector ECD 2 1 Instructions V6685 DIAG menu ...