IV. Proposed Gas Sensor Interface
4.5 Fully digital temperature sensor
As explained in the previous temperature sensor chapter, there are many temperature sensors based on BJT, and these sensors require a high-performance ADC. However, high-performance ADC was required to digitize it, which results in additional power-consumption and chip area. To overcome these problems, the proposed temperature sensor will be designed by fully digital circuit as shown in Fig. 45.
The circuit is divided into three stages according to the function, and the operation sequence of the circuit is as follows. (1) shows the designed delay cell and the delay change with temperature. When a clock is applied to the input of an inverter-based delay cell, each unit delay outputs a delayed input clock. In addition, the delay pulse width of the unit delay has a characteristic proportional to the temperature, and the proposed temperature sensor is designed using this characteristic. (2) is a circuit that digitizes the delay pulse proportional to temperature. Normally, the delay pulse is digitized through a clock with a high frequency, but the power consumption increases due to high operation
Fig. 121 Schematic of proposed fully digital temperature sensor.
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speed. However, the proposed fully digital temperature sensor digitizes the delay pulse using CLKCAL, a clock that is a little smaller than the input clock through the MCU. Finally, the temperature
information obtained through the 8-bit up down counter is transmitted to the MCU through the SPI READ and level shifter. In the MCU, the mapping table and formulas obtained from process PDK are utilized to correct the reference resistance and current courses as shown in Fig. 46. The temperature sensing range is -30 to 80 degrees.
Fig. 122 Calibration process of gas sensor reference resistance.
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4.5 Chip calibration
(a) (b)
Many of the calibrations introduced were mostly to compensate for mismatch of device and MOSFET characteristics by process. However, circuits that need to operate for a long time, such as gas sensing ROIC, cause the device to age. However, no calibration methods have been introduced to complement these aging characteristics. Fig. 47 introduces gas sensing and proposed chip calibration mode. In the gas sensing mode, resistance and current of the gas sensor are measured by RDAC, comparator and ADC. However, the resistance value of RDAC is changed over time and frequency of use, so it is necessary to compensate for these changes. In the calibration mode, RCAL that has same resistance value with RDAC is used to detect the RDAC and measured resistance data is stored in the MCU. The RCAL
is only used in the calibration mode, so aging is rarely caused. The RDAC resistance is changed as the gas sensing circuit operation time increases. The aged RDAC is measured again in the calibration mode and the measured value is used as the reference resistance in gas sensing. The advantages of proposed chip calibration are no external circuits are required and periodic and automatic calibration is possible through MCU.
Fig. 123 Schematic of gas sensing mode and calibration mode.
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4.6 Wide dynamic range gas detector
(a)
(b)
Fig. 48 shows schematic of proposed coarse gas detector circuit. The proposed coarse gas detector is designed to accommodate both resistance type sensors for hazardous gas and FET based current type sensors for explosive gas, which provides precision sensing mode using two correlated double sampling (CDS) and 18-bit zoom ADC. In step1, SGAS1 and SGAS_EN are turned on to obtain the resistance of hazardous gas sensor through RDAC1 using 8-bit SAR logic, which is coarse detection. For precision detection, switch SGAS1 is turned off, switch SGAS2 and SGAS2_EX are turned on during step 2, and the IDAC1
Fig. 124 schematic of proposed coarse gas detector circuit.
(a) gas detector front-end (b) low power and high-resolution mode
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is determined by 8-bit SAR logic and IDAC2 has the same as IDAC1. The VREF and VSENSE are input to double CDS, and its output is digitized by the zoom ADC and digital results are passed to the MCU, which is shown in Fig. 48(b). In explosive gas sensing, the switches SEXPLO and SEXPLO_EXT are turned on for coarse detection using 8-bit SAR logic during step 1. The RDAC2 and RDAC1 are set to be the same and switch SGAS2_EX is turned on to make IDAC1 close to ISENSOR through the 8-bit SAR logic. The subsequent operation for precision sensing is same as resistance type sensor detection. This proposed gas detector circuits provide resistance range from 8 kilo to 2 megohms and current source from 450 nano to 114 microamps. However, this detection range is insufficient to cover the resistance and current ranges required by recently manufactured gas sensors. Therefore, a gas detection circuit having a wider dynamic range is required.
Fig. 49 shows the schematic of proposed wide dynamic range gas detection circuit for low resistance mode. In step 1, RDAC1 and gas sensor are connected in series, and the generated voltage VSENSE and common mode voltage are compared through a comparator, which is same with basic gas detection operation. If the coarse 8-bit code result is all '0', it means that resistance of gas sensor is smaller than the reference resistance used in RDAC1. In step 2, the gas sensor and the resistor RDAC2_S are connected
Fig. 125 Schematic of proposed wide dynamic range gas detection circuit for low resistance mode.
Small RDAC
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in series through the switches SEXPLO_S and SGAS_EN to compareStep 2 also consists of 8 cycles to get 8- bit information. A high-resolution mode implemented by double CDS and zoom ADC is also provided using the voltage VSENSE generated in step 2. In the proposed wide dynamic range gas detector, the range of low resistance mode is from 100 Ω to 10.2 kΩ. The gas sensor resistance can be obtained by the formula below.
𝑅 = _
. (15) Fig. 50 presents the schematic of proposed wide dynamic range gas detection circuit for high resistance mode. Step 1 is operated the same as the low resistance mode. If the coarse 8-bit code result is all '1', it means that resistance of gas sensor is larger than the max resistance used in RDAC1. Step 2 is operated as a high resistance mode and the switches SGAS2_S, SGAS_EN, and SEXPLO are closed. The current source IDAC_S provides a small current and flows its current to the gas sensor and resistor RDAC2. IDAC_S
has a current range from 8 nA to about 2 μA, which allows for large gas sensor resistance measurements.
Fig. 126 Schematic of proposed wide dynamic range gas detection circuit for high resistance mode.
Small IDAC
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In the high resistance mode, IDAC_S current can be passed only to the gas sensor to measure the gas sensor. However, if a very large sensor resistance is connected, the VSENSE voltage increases in the first comparison cycle, which means that the MOSFET used in the IDAC_S can be operated in the triode region. This means that the amount of IDAC_S current can be changed by VSENSE, and there is a possibility of incorrect conversion. To solve this problem, the gas sensor and the resistor RDAC2 are connected in parallel to reduce the voltage of VSENSE, thereby enabling the current source to operate in a stable saturation region. In the proposed wide dynamic range gas detector, the range of high resistance mode is from 2 MΩ to 112.5 MΩ. The gas sensor resistance can be obtained by the formula below.
𝑅 =
_ (16) The operation of proposed wide dynamic range gas detection circuit for low current mode is shown in Fig. 51. Step 1 is a stage in which the gas sensor current is connected in series with RDAC2 through the switch SEXPLO, and generated voltage VSENSE is compared with the common mode voltage to determine the current. . If the coarse 8-bit code result is all '0', it means that current of gas sensor is smaller than the current generated by common mode voltage and reference resistance used in RDAC2. In
Fig. 127 Schematic of proposed wide dynamic range gas detection circuit for low current mode.
Small IDAC
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this case, step 2 is operated in a low current mode, and a small current source, IDAC1_S, flows additionally to the resistor RDAC2. Since the current flows additionally to RDAC2, the result of the coarse 8-bit code of step 2 is not '0' and a VSENSE voltage is generated. The current measurement range of the low current mode is 8 nA to 2.04 μA. The gas sensor’s current in low current mode can be obtained by the formula below.
𝐼 = − 𝐼 _ (17)
Fig. 52 shows the circuit operation of the high current mode. As in the low current mode, step 1 operates, and when the coarse 8-bit code result obtained through the comparator is all '1', step 2 operates in the high current mode. In order to measure the high current of the gas sensor, it is measured in series with the resistor RDAC2_S. The current measurement range of the high current mode is 102 μA to 22.5 mA. The gas sensor’s current in high current mode can be obtained by the formula below.
𝐼 =
_ // (18) Fig. 128 Schematic of proposed wide dynamic range gas detection circuit for
high current mode.
Small RDAC
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4.6 Dual mode IADC
The proposed dual mode IADC is divided into step 1 supporting low-power operation based on SAR operation and step 2, 3, and 4 for high-resolution mode. When no gas is detected, only 8-bit SAR operation is performed to reduce the power of the gas sensor system. If gas is detected, steps 2, 3, and 4 are operated to obtain high-resolution gas sensor information. This enables efficient gas sensor system operation.
The high-resolution mode is divided into step 2 for incremental ADC operation and steps 3 and 4 for binary extended counting (BEC). The incremental ADC of step 2 is a zoom ADC structure, which receives information from the previous 8-bit SAR ADC to obtain additional resolution. Then, binary extended counting is performed in steps 3 and 4 using the output voltage VRES remaining in the integrator of step 2 as an input.
Fig. 129 Conceptual block diagram of proposed dual mode IADC.
Fig. 130 Detail operations of proposed reconfigurable IADC.
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Fig. 54 shows the detail operations of proposed reconfigurable IADC. In Step 1, a voltage close to the input VIN is generated. In step 2 of the incremental ADC operation, VINT_IN that is the difference between the input VIN and the obtained voltage in step 1 is generated and this is converted. Finally, the BEC corresponding to step 3 and 4 converts the VRES to 5-bit conversion in a similar manner to the SAR ADC, respectively.
Fig. 55 shows a block diagram of proposed four-step IADC. When switch SSAR_EN is turned on, the SAR ADC implements 8-bit conversion on input VIN. In step 2, a reference feedback K of the IADC is determined through 7-bit most significant bits (MSBs) of the SAR conversion, X and its least significant bits (LSB), LSB 1b. This operation allows for a 7-bit DAC instead of an 8-bit DAC to reduce the difference between the input and the reference feedback K.
𝟏 𝐳 − 𝟏
Fig. 131 Block diagram of proposed four-step IADC.
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An operational transconductance amplifier (OTA) used in ADC is designed as two-stage structure as shown in Fig. 56. Since only one OTA is used in ADC, incremental ADC can operate with low power.
In addition, chopper stabilization technique is used to remove flicker noise.
VBP0
VINP
VBN0
VSS VDD
VCMFB
VINN
VCMFB
VCM
VCMFB
VOUTP VOUTN
Fig. 132 Circuit diagram of amplifier and common mode feedback (CMFB) in the integrator.
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Chapter Ⅴ
Fabrication and Measurement Results
This chapter shows the manufactured gas sensor module and Gas ROIC. In addition, the proposed gas response self-calibration results, ROIC’s performance and measurement results are presented.
5.1 Fabrication results
5.1.1 Gas ROIC sensor module
Fig. 57 shows the fabricated gas ROIC sensor module. The gas ROIC sensor module consists of a gas sensor module at the top, a ROIC module at the middle, and an LTE module at the bottom. Each layer was connected using male and female FH01(2.54)-SS20P. The ROIC module includes KLDX- 0202-A so that it can be driven through the SMPS adapter connection. Each module is designed to fit
Fig. 133 Gas ROIC sensor module.
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the size of the sensor module and is designed to be as small as possible. The size is 12 cm wide, 7 cm long, and 7 cm high.
Fig. 58 shows the explosion-proof packaged gas ROIC sensor module. Explosion-proof packaging is designed to prevent explosion when explosive gas at high concentrations and high temperature gas sensors encounters. The upper part of the explosion-proof packaging has a hole to accept gas, and an antenna for LTE communication is connected to the left.
Fig. 134 Explosion-proof packaged gas ROIC sensor module.
Fig. 135 Gas sensor module and ROIC module.
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Fig. 59 shows the gas sensor module and ROIC module. The gas sensor module is packaged like a commercial product and the gas sensor is connected to the PCB through wiring bonding. The sensors on the right are designed to be disposable, and if the sensor fails, only the new sensor is replaced without discarding the entire sensor board. The gas ROIC module on the right in figure is composed of low dropout (LDO) for power supply, MCU and gas ROIC. The gas ROIC was connected to the PCB through a chip on board (COB).
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5.1.2 Gas ROIC sensor module
Fig. 60 shows the gas ROIC chip micrograph. The designed gas ROIC chip is consists of ROIC with wide dynamic range, SAR ADC to support low power, IADC for high resolution sensing, heater controller that provides various heater temperatures, temperature sensor to measure chip temperature, and SPI for chip control and gas data reading. A prototype of the proposed gas ROIC was fabricated in a 180-nm BCD CMOS process, and its chip area is 3.42 mm2.
Fig. 136 Gas ROIC chip micrograph.
Fig. 137 Chip micrograph of dual mode zoom IADC.
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Fig. 61 presents the chip micrograph of dual mode zoom IADC. This dual mode zoom IADC integrates functional component such as the 8-bit SAR ADC, the IADC, and the, excluding a decimation filter. The chip was manufactured using a 180-nm conventional CMOS process, and its average power consumption is 176 μW.
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5.1.3 Monitoring system
Fig. 62 shows the installed gas sensor module and monitoring system. The fabricated gas sensor module is installed to be less affected by environmental variables. Gas concentration can be checked in real time through the cloud monitoring system.(http://220.119.254.195)
Fig. 138 The installed gas sensor module and monitoring system.
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5.2 Measurement results
5.2.1 Self-calibration results
Fig. 139 Gas measurement setup.
Fig. 63 shows gas sensing measurement setup. A 5 V voltage was applied to the gas sensor module through the power supply, and the sensor operation was confirmed. The gas sensor data is obtained through interworking with a laptop computer using the module's Bluetooth. Timing issues that may occur due to multi-channel operation were removed, and a concentration equation was generated by analyzing the response of each sensor. When the gas was injected and the gas concentration was sufficiently saturated, the sensing operation was stopped, and the recovery step was performed for 2 hours.
Fig. 140 multi-channel gas sensor operation (CO 1ppm).
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Fig. 64 shows multi-channel gas sensor operation (CO 1ppm). The manufactured gas sensor module is designed to measure 10 types of gas sensors at the same time. It is confirmed that CO 1 ppm gas is injected and finally the corresponding concentration is expressed. In addition, the selectivity of the gas sensor is secured, and it seems that gas sensor only reacts to target gas. The measurement time is about 600 seconds, which depends on the gas type.
Fig. 65 shows edge computing-based pattern recognition (CO, NO2, H2). Channels 0, 1, and 6 represent the patterns of CO, NO2, and H2 sensors, respectively. When the pattern recognition algorithm determines that a specific gas has leaked, the value of the corresponding channel is changed to '1'. When CO gas is injected, it is confirmed that channel 0 becomes '1' through a pattern recognition algorithm. In the same way, it can be confirmed that NO2 and H2 are also well distinguished. In order to recognize patterns for other gases, training through ANN must be performed based on measured data for target gases.
Fig. 141 Edge Computing-based pattern recognition (CO, NO2, H2).
Fig. 142 Gas response change over time corresponding to gas concentration.
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Fig. 66 illustrates gas response change over time corresponding to gas concentration. To verify the methodology of gas response self-calibration, commercial gas sensor response data according to CO gas concentration was obtained through an experiment. The experiment was conducted by setting the gas concentration to 10, 20, 35, and 50 ppm, and CO gas was injected for 400 seconds to secure a sufficient response time.
Fig. 67 depicts CO gas response according to gas concentration and its logarithmic fitting response.
The response with the gas concentration is similar to the exponential graph, and the first logarithmic fitting was performed for easy analysis, which is Res_slope.
Fig. 74 shows Ro1 and Ro2 measurements to obtain the Ro slope. Ro1 was obtained by applying a voltage of 5 V to the heater of the CO sensor and measuring the resistance of the sensor when it reached a sufficiently saturated state. After obtaining Ro1, a 4V heater voltage corresponding to a heater
(a) (b)
Fig. 143. (a) CO gas response according to gas concentration and (b) its logarithmic fitting.
Fig. 144 Ro1 and Ro2 measurements to obtain the Ro slope.
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temperature of 380 degrees was applied to the sensor. Sufficient time had elapsed, Ro2 was obtained.
Ro_slope is obtained through two values, Ro1 and Ro2, and each heater temperature. The obtained Ro_slope and Res_slope are shown in Table Ⅶ below.
Ro_slope Res_slope
Sample 1 -0.0114 -0.2052
Sample 2 -0.0141 -0.1918
Sample 3 -0.0137 -0.2128
Sample 4 -0.0221 -0.1984
Sample 5 -0.0173 -0.2136
t1 t2 t3
Ro_slope1 Res_slope1 Ro_slope2 Res_slope2 Ro_slope3 Res_slope3 Sample 1 -0.0045 -0.2261 -0.0084 -0.2151 -0.0114 -0.2052 Sample 2 -0.0055 -0.2229 -0.0091 -0.2045 -0.0141 -0.1918 Sample 3 -0.0032 -0.2368 -0.0081 -0.2256 -0.0137 -0.2128 Sample 4 -0.0090 -0.2398 -0.0160 -0.2139 -0.0221 -0.1984 Sample 5 -0.0084 -0.2321 -0.0135 -0.2222 -0.0173 -0.2136
Correlation 0.3522 0.0039 0.1550
Table VIII shows the obtained Ro_slope and Res_slope for different times. The time point corresponding to the initial state is defined as t1, and after a week has elapsed, the aging process is performed by overheating and this is defined as t2. The time point after 5 months was defined as t3, and Ro_slope and Res_slope were measured, respectively. As the aging progressed, it was confirmed that Ro_slope increased and Res_slope decreased. To roughly confirm the methodology of gas response self-calibration, the correlation between Ro_slope and Res_slope was calculated at each time point, and
Table Ⅶ
Obtained Ro_slope and Res_slope.
Table ⅤIII
Obtained Ro_slope and Res_slope for different time.
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the result is shown in the Fig. 69. All the results had a low correlation. The results of this experiment were different from previous analyzes, so the theoretical analysis was again conducted.
In the process of expressing the gas sensor Ro slope as an equation, S1 represents an initial value at which aging does not proceed. However, since the sensor has already started aging from the point of manufacture, S1 cannot be obtained. In addition, the relationship between S1 and sensor response is not constant due to sample-to-sample variation. For this reason, the correlation between the gas sensor Ro slope and Res slope is very low. Instead of the existing S1, the aging S1' was introduced. If S1' is set to t1 corresponding to the initial time point, it can be expressed by the following equation.
𝑆1 = 𝑆1 +
( ) − (19) Also, the Ro slope S2 at t2, which is the time of aging due to overheating, is
𝑆2 = 𝑆1 +
( ) ( − ) (20)
Where t2 = t1+Δt. S2 is expressed as S1' and Δt as follows.
𝑆2 = 𝑆1′ + Δ
( ) ( − ) (21)
The Δt is expressed as follows by the equation (1).
𝛥𝑡 = τ exp (𝑙𝑛(𝑅𝑒𝑠𝑡2) − 𝑙𝑛(𝑅𝑒𝑠𝑡1)) (22)
Substituting the obtained Δt into the equation (21)
Fig. 145 Ro_slope and Res_slope fitting model and correlation for t1, t2, and t3.