MCA527
over which the values are averaged is often found as rise time. This rise time is in effect
comparable to the shaping time of an analog shaping amplifier, and comparable results
are achieved if the rise time is twice the shaping time. For compatibility reasons, the
MCA527 uses still the term “shaping time” and the rise time is just twice as high as that.
Further the values near the voltage step are omitted from calculation. This parameter is
called flattop. The above mentioned very simple filter would have a flattop of 0.8µs and a
shaping time of 0.05µs.
But there are more things to optimize a digital filter for and to be taken into account:
●
A voltage step sitting on top of a previous step should be calculated to the same
amplitude as if it was starting from the baseline. This correction is adjusted with the
pole zero setting.
●
The positioning of the filter has typically an accuracy not better than the sample
rate, here 100ns. So the filter should be adjusted such that the result is invariant to
a shift of 100ns forward or backward. This correction is also derivated from the pole
zero setting.
3.2 Adjustments and Settings
3.2.1 Input Polarity
Has to be chosen whether the detector preamplifier delivers positive voltage steps or
negative.
3.2.2 Coarse Gain
Can be adjusted between 2 and 1000, but only settings between 2 and 100 affect the
signal input amplifier / attenuator. It defines which signals can be processed.
Table 10: Input signal acceptance range depending on coarse gain
Coarse
Gain
Full Input
Range
Useable Input Range
Positive Polarity
Useable Input Range
Negative Polarity
2
12.50V
-1.25V...11.25V
-11.25V…1.25V
5
5.00V
-0.5V...4.5V
-4.5V…0.5V
10
2.50V
-0.25V...2.25V
-2.25V…0.25V
20
1.25V
-125mV...1125mV
-1125mV…125mV
50
0.50V
-50mV...450mV
-450mV…50mV
100
0.25V
-25mV...225mV
-225mV…-25mV
For best resolution, the coarse gain setting should be chosen as high as possible but still
fits into the input acceptance range. However, above a coarse gain of 20 the
improvements are marginal, and coarse gain settings above 100 have been omitted as
there is no benefit for this and are just calculated.
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