microDXP Technical Reference Manual
Version 3.15
October 7, 2019
49
4.1.2 RC-Type Preamplifiers
Figure 4-3a is a simplified schematic of an RC-type preamplifier, wherein C
f
is discharged
continuously through feedback resistor R
f
. The output of an RC-type preamplifier
following the absorption of an x-ray of energy Ex in detector D is, again, a voltage step of
amplitude V
x
. The continuous discharge of C
f
through R
f
results in an exponential voltage
decay after the x-ray step, with decay constant
, where:
= R
f
C
f
Equation 4-1
In practice the decay time may depend on subsequent circuitry, i.e. if a pole-zero
cancellation circuit is used, thus
may not be directly related to the feedback elements of
the front-end. The point of this simplified model is that the resulting waveform is a step
with a single-pole RC decay, as depicted in Figure 4-3b. The discussion in §4.2 through
§4.6 assumes a reset-type preamplifier, but is also mostly applicable to RC-type
preamplifiers.
Figure 4-3:
a)
RC-type charge sensitive preamplifier with a positively biased detector;
b)
Output
on absorption of an x-ray. Note that the step has a falling edge, thus the signal
polarity is negative.
4.2 X-ray Energy Measurement & Noise Filtering
Reducing noise in an electrical measurement is accomplished by filtering. Traditional
analog filters use combinations of a differentiation stage and multiple integration stages to
convert the preamp output steps, such as shown in Figure 4-1b, into either triangular or
semi-Gaussian pulses whose amplitudes (with respect to their baselines) are then
proportional to V
x
and thus to the x-ray’s energy.
Digital filtering proceeds from a slightly different perspective. Here the signal has been
digitized and is no longer continuous, but is instead a string of discrete values, such as
shown in Figure 4-4. Given this data set, and some kind of arithmetic processor, the
obvious approach to determining V
x
is to take some sort of average over the points before
the step and subtract it from the value of the average over the points after the step. That is,
