The gas sensor section of the ASIC used a common reference voltage point split between the gas sensor circuit and an offset circuit (figure 3.2). Two non-inverting amplifier circuits (i.e. the gas sensor circuit and the offset circuit) were connected to each other through an instrumentation amplifier circuit. The amplified output of the instrumentation amplifier was filtered through a fourth order Bessel filter before being output to the data acquisition system.
Figure 3.2 Schematics of the gas sensor section of the ASIC
Both the offset circuit as well as the gas sensor circuit were internally arranged in a non-inverting amplifier formation. Volatile Organic Compound (VOC) sensitive chemoresistors replaced conventional static resistors in the gas sensor circuit. The amplifier gain was determined by the ratio of these two VOC sensitive chemoresistors. Under ideal conditions this ratio is kept as close to unity as possible. However, a slight variation in matching this ratio was always present. In addition to this the resistance value of the chemoresistors was observed to drift randomly over long periods of time (over a year). The chemoresistors also responded to changing ambient conditions such as temperature, light and humidity.
The ratiometric arrangement cancelled many of the ageing and temperature variation effects. However, perfect cancellation of these effects was not achieved. Several configurations of these two resistors in the gas sensor circuit were tested to verify equations 2.9, 2.10 and 2.11, as described in chapter 2. Equation 2.9 reflected the
test situation where the chemoresistors which formed the Ra resistor in the amplifier circuit varied as it responded to analyte concentrations. This is represented in figure 3.3.
Figure 3.3 Noninverting circuit for gas sensor with Ra as a chemoresistive
sensor
The test circuit where the chemoresistive sensor was represented by Rb in the non- inverting amplifier was similar to the one used in the offset circuit in the ASIC and is shown in figure 3.4.
The corresponding equation for the circuit shown in figure 3.3 is
1 ∆ 2 ∆ (2.9)
One of the two resistors in the offset circuit was an externally programmable potentiometer which could be digitally set from within LabVIEW software. The advantage of this design was that it allowed setting the baseline output voltage prior to testing under non-exposure conditions. The circuit schematic for the offset circuit was the same as in the case of Rb forming a variable chemoresistor in a non-inverting amplifier circuit as in figure 3.4. For the offset circuit the offset resistor (Rb) is built into the ASIC (figure 3.4). The corresponding equation for that circuit is
1 ∆ (2.10)
A special case of the gas sensor circuit is also tested in this study where both the chemoresistors are exposed to the test vapour. In this arrangement the resistances of both the chemoresistors vary with the concentration of the vapours to which the chemoresistive sensors along with the ASICs are exposed. Two variations of this arrangement were used over the course of this study. The first one used both the sensors of the same basic material; however, the chemoresistor corresponding to Rb was coated with a silicone gel. The silicone gel is expected to act similar to a partitioning layer and expected to enhance the resulting resistance change due to exposure to some vapours.
The other variation of this setup used two chemoresistors based on different chemoresistive film materials. An enhanced signal output, for vapours with non-similar sorption properties with both the chemoresistors, and cancellation of the output for VOCs with similar sorption properties was expected as a result of using this setup. The circuit schematic of this arrangement is given in figure 3.5 below.
Figure 3.5 Noninverting circuit for gas sensor with Ra and Rb representing
the variable ratiometric arrangement
Both the offset circuit as well as the gas sensor circuit were supplied with the same 1.2 V reference voltage. Externally programmable potentiometers in the offset circuit set the baseline output voltage. The output from the gas sensor circuit and the offset circuit were fed into an instrumentation amplifier. This part of the circuit amplified any differences between the outputs of the two. The gain of the instrumentation amplifier (shown in figure 3.6) is given by equation 3.1. Here Rgain can
be set through a programmable potentiometer. The value of this resistor was normally kept at 50 kΩ. The value of R1 was 100 kΩ and was fabricated into the ASIC. Thus, a
minimum gain of 5 was achievable through this setup. Due to slight variations in the actual fabricated devices, a gain of 7 was more commonly observed using LabVIEW software.
1
(3.1)
The fabricated resistance values for the differential amplifier stage of the instrumentation amplifier were 100 kΩ for the feedback resistors (R1) and 10 kΩ for the
forward paths (R2 and R3). A circuit diagram of the instrumentation amplifier is given in
Figure 3.6 Instrumentation Amplifier
The ASIC circuit applies a fourth order Bessel low pass filter to the output of the instrumentation amplifier. This should remove any high frequency noise present in the output. The filter uses the values of capacitors and resistors given in table 3.1 for a
cut-off frequency of fc = 50 kHz as mentioned by García-Guzmán (García-Guzmán
2005). A circuit diagram of a fourth order Bessel low pass filter is given in figure 3.7.
Figure 3.7 Fourth Order Bessel Low Pass Filter
The output from the Bessel filter is Vout which is the final device output. This voltage
output is received by National Instruments Data Acquisition Card (NI-DAQ) and
Component Capacitance (pF) Component Resistance (Ω)
Ca1 20.0 Ra1 402 k
Cb1 14.7 Rb1 1.05 M
Ca2 20.0 Ra2 687 k
Cb2 7.5 Rb2 956 k
Table 3.1 Resistance and Capacitance used for the Bessel Low Pas Filter
(GarcíaGuzmán 2005)