Biomedical Engineering Reference
In-Depth Information
Fig. 25
(
a
) TWG2.0 with PCB size of 10 × 5 cm² and (
b
) TWG2.3 with PCB
The next TWG-redesign on a PCB of 5 × 3 cm² as shown in Fig.
25b
will be used
for the next PIMMS-generation.
6.7
Current Measurement
The maximum ion currents to be detected, when amplified by an MCP, are in the
range of few hundred picoamperes. The current is negative, since the MCP gener-
ates electrons, and positive, if the ion current is directly measured.
The present setup, therefore, allows capturing currents between −250 and 250 pA.
It incorporates an 18-bit high speed successive approximation analog to digital con-
verter (ADC). Since 1 bit determines the sign, the signal resolution is 17 bits, i.e.,
131,072 distinct values. Two neighbored values differ by 1.9 fA. In order to detect
gas concentrations of 1 ppm and less another order of magnitude is necessary cor-
responding to a >20 bit resolution. This is accomplished by the high speed sampling
capability of the ADC. By oversampling in hardware typically a thousand values are
sampled within 1 ms, which increases the number of interpretable bits to 22.
For currents in the pico- and femtoampere range the signal to noise ratio (SNR)
becomes low. Without post processing of the measured signal absolute noise levels
of about 140 fA were achieved. This would make a ppm resolution impossible. The
described oversampling reduces noise to <25 fA. Further noise reduction to <100 aA is
achieved by averaging in firmware and software (see below). It should be noted that this
way to reduce noise will correspondingly increase the time to capture the spectra. The
analog to digital conversion hardware is shown in Fig.
26
. The size of the PCB is
5 × 3 mm²; it contains the low noise power supplies, the ADC, and the averaging stage,
For lower noise also the input stage of the converter has to be placed as close as
possible to the detector. This input stage is the only electronic component placed
within the vacuum chamber. The assembly of the PIMMS on the PCB with the metal
box containing the shielded input stage of the pA-electronics is shown in Fig.
27
.
