microDXP Technical Reference Manual
Version 3.15
October 7, 2019
37
3.2.1 Dynamic Range Reduction
In many cases, and particularly for reset-type preamplifiers, the full-scale output voltage
range is much greater than the voltage step produced by a single x-ray event (see Figure
4-2). A high sampling rate is necessary to provide good pulse pileup detection, as described
in §4.7, and sufficient ADC resolution is required to accurately sample the noise prior to
the digital filters. For high count rates, pulse-pair resolution less than 50 ns is desirable,
which implies a sampling rate of 40 MSPS or more. In order to reduce the noise
in
measuring Vx (see Figure 4-1 and Figure 4-3), experience shows that
must be at least 4
times the ADC’s single bit resolution
V
1
. This effectively sets the gain of the amplifier
stages preceding the ADC. Then, if the preamplifier’s full scale voltage range is V
max
, it
must digitize to N
effective
bits, where N is given by:
N = log
10
(V
max
/
V
1
) / log
10
(2)
Equation 3-1
For a typical high-resolution spectrometer, N might be 16 or more. An ADC with 16
effective bits resolution that supports 40 MSPS is quite expensive (note that a typical 16-
bit ADC has 13 or 14 effective bits). The alternative approach is to first reduce the dynamic
range of the preamplifier output signal such that a moderately priced ADC can be used.
3.2.1.1
Reset-Type Preamplifiers
For reset-type preamplifiers the dynamic range reduction is accomplished using a novel
technology, for which XIA has received US and international patents, and which is
indicated in Figure 3-2. Here a preamplifier output is shown which cycles between roughly
-3.0 Vand -0.25 V. We observe that it is not the overall function which is of interest, but
rather the individual steps, such as shown in Figure 4-2
of the next chapter, that carry the
x-ray amplitude information. Thus, if we know the average slope of the preamp output, we
can generate a saw tooth function that has this average slope and restarts each time the
preamplifier is reset, as shown in Figure 3-2. If we then subtract this saw tooth from the
preamplifier signal, we can amplify the difference signal to match the ADC’s input range.
The generator required to produce this saw tooth function is quite simple, comprising a
current integrator with an adjustable slope and a reset switch. A DAC (SLOPEDAC)
controls the current, which sets the slope. The DAC input value is set algorithmically to
maintain the ASC output (i.e. the “Amplified Sawtooth Subtracted Data” of Figure 3-2)
within the ASC input range.
In practice, the large-signal dynamic range can be reduced by a factor of 8 to 16, thus
reducing the required number of
effective
bits necessary to achieve the same resolution
from 16 to a more easily achievable 12 or 13.
