In many harsh environmental systems, a growing trend is that high-precision electronics are getting closer to high temperatures. There are several drivers behind this trend, both in energy exploration, aerospace, automotive, heavy industry and other end applications.
For example, in the field of energy exploration, the ambient temperature increase is a function of depth, and the typical operating temperature of the associated equipment is 175 ° C and above. Due to size and power limitations, active cooling is less practical and thermal convection is very limited. In other systems, sensors and signal conditioning nodes need to be placed near high temperature areas, such as engines, braking systems, or high-power energy conversion electronics to increase overall system reliability or reduce cost.
Historically, it has been very difficult for engineers to design reliable high-performance electronics for these applications because there is a lack of manufacturers in the market to produce the specified components for these operating conditions. Fortunately, more and more (IC and passive) components have emerged in recent years, with manufacturers specifying operating temperatures as high as 175 ° C and above. In addition, recent reference designs have also focused on performance, combining some of these components in the signal chain subsystem to enable sophisticated data acquisition, enabling system designers to adopt relevant technologies faster (such as CN-0365) and help them Reduce design risk and shorten time to market. However, before this, in the high-temperature precision data acquisition, there are still some gaps in the full-featured platform with good distance characteristics and wide availability.
In this article, we will introduce a new high-temperature precision data acquisition and processing platform with an operating temperature of up to 200 °C. The platform includes a high temperature circuit component, as well as a data acquisition front end and microcontroller, optimized firmware, data acquisition and analysis software, source code, design files, bill of materials, and test reports. The platform is suitable for reference design, rapid prototyping and high temperature instrumentation system laboratory testing. The size and structure of the circuit components are specifically designed to be compatible with the size requirements of oil and gas instrumentation, but can also be used as a basis for other high temperature applications.
Hardware Architecture OverviewInstrumentation (also known as downhole tools) used in oil and gas exploration is similar to many sophisticated data acquisition and control platforms, but has specific requirements for performance and reliability and can be used as a case study for this reference platform. In this application, the system samples signals from various sensors to collect information related to the surrounding geological formations. These sensors may be electrodes, coils, piezoelectric sensors or other sensors. Accelerometers, magnetometers, and gyroscopes provide information about the inclination and speed of the drill string. Some of these sensors have extremely low bandwidth requirements, while others provide information within or above the audio frequency range. Multiple acquisition channels are required, and high accuracy must be maintained at high temperatures (typically 175 ° C and above). In addition, a large part of these instruments are battery-powered or have limited available power, so they must have low power consumption and multiple operating modes to optimize power consumption.
In addition to the requirements of electronic systems, there are mechanical limitations in downhole applications that may determine the size of the electronic components and may affect the packaging and selection of the components. For the latter question, we will discuss in detail in the following sections. It is important to note that the circuit components of this segment generally have limitations on the board width. The electronic components must be placed in a tubular pressure vessel used in drilling operations, so their aspect ratio is narrow and long. This shape characteristic limits the size and density of the available components, and may also limit the component layout and signal routing. The result may affect the performance of high-precision electronics. Therefore, pay special attention to layout and other package design details. . Figure 2 shows a typical size, circuit assembly (transparent, top) mounted in a tubular pressure vessel, and a cross-section (bottom) of the tubular pressure vessel after the board is mounted.
The reliable reference design platform discussed in this paper is based on the CN-0365 analog front-end reference design, which is designed to provide a foundation for sophisticated data acquisition and control solutions based on high-temperature, low-power microcontrollers to meet many downhole instrumentation and other high-temperature electronics. Device requirements. Based on the AD7981 analog-to-digital SAR converter, this reference design demonstrates a full-featured system with two high-speed simultaneous sampling channels and eight additional multiplexed channels to meet the data acquisition needs of a wide range of downhole tools. 10 channels). The analog front end accesses the VA10800 ARM® Cortex®-M0 from alliance partners Vorago Technologies and Petromar Technologies via the SPI port. This design is the newest addition to the growing ADI ecosystem of high temperature applications and solutions.
Figure 1. High temperature reference platform.
After acquisition, the data can be processed locally or via the UART or the optional rs-485/'target='_blank'>RS-485 communication interface. Other companion components on the board (including memory, clock, power supply, and passive components) are vendor-specified devices that support high operating temperatures and are proven to be reliable at temperatures of 200°C or higher. Work. Figure 1 and Figure 2 show the actual board diagram and high-level functional block diagram of the high-temperature reference platform. The board shown in Figure 2 shows the layout and dimensions of the downhole board, which is about 11.4 inches long and 1.1 inches wide.
Figure 2. Dimensions of downhole electronic components.
The design of the precision data acquisition channel of the platform is comprehensively introduced in the CN-0365 application note. This design is the basis for the three ADC inputs on this platform, but to meet board size requirements, the platform can operate reliably at temperatures up to 200 ° C, with some adjustments in passive component selection and optimization. The reference acquisition channel circuit is shown in Figure 4. There are two digital multiplexed channels that operate at high sample rates, each with a complete data acquisition channel (similar to CN-0365). There is also an analog multiplex channel that adds an ADG798 multiplexer before the input and is optimized for low throughput inputs. R1 and R3 provide a 1.25 V bias voltage for the non-inverting input of U1, preventing it from floating to the supply rail of the analog input when it is disconnected or when the multiplexer is deselected. R8 and R9 can be changed to increase the gain of U1. R4, R7, and C1 are anti-aliasing filters, but they can also be reconfigured as either attenuators or alternate filter configurations. R5, R6, and C4 form the RC filter between the ADC driver and the ADC input. The purpose of this filter is to limit the amount of out-of-band noise reaching the ADC input and attenuate the kickback voltage from the ADC's input switching capacitor.
Figure 3. Functional block diagram of the high temperature reference platform.
Figure 4. ADC driver configuration.
The platform was designed to take advantage of several key features of the AD7981 ADC. This 16-bit, 600 kSPS converter improves the typical SINAD over 85 dB and the typical INL of ±0.6 LSB with a 2.5V reference and no missing codes. SINAD above 90 dB can be achieved with a 5 V reference, but in this platform we have not chosen this specification in order to maintain compatibility with lower voltage systems. Since the ADC core automatically enters a power-saving state during the conversion cycle, the power consumption of the ADC automatically varies linearly with throughput. This saves energy when using a low sample rate converter.
Software overviewfirmware
The platform's firmware is based on the FreeRTOS operating system and can easily integrate tasks such as data processing and other communications. We optimized the code so that non-multiplexed channels 0 and 1 can efficiently perform fast ADC conversions, with multiplexed channels 2 through 9 taking as little as 10 μs. The conversion result can be processed locally or transmitted from the UART channel at a rate of 2 Mbps. The conversion result buffer is 16 kB (8k samples) and can be shared between multiple channels or dedicated to one channel. The firmware is available in an open source format that end users can customize and use as a basis for the final application.
Data acquisition and analysis softwareFigure 5 shows the data acquisition and analysis software, based on the .NET interface design, with power components through a USBUART-TTL level shifter. Communication with hardware (including control and data streams) is possible with well-defined protocols. Data can be collected in burst mode or continuously. Data analysis capabilities are also included to analyze and validate SNR, THD, and SINAD (such as FFT) in both time and frequency domains. Data can also be logged to a file (such as exported to Excel) for storage or for processing in other applications. Just like the firmware, we provide the source code of the data acquisition software for free, and the end user can customize it.
Figure 5. Data acquisition and analysis software.
High temperature structureThis reference platform is made of components and other materials suitable for operation at 200 °C. All components used on the platform are high temperature working components specified by their respective manufacturers (except as otherwise stated), and the global dealer network has begun to supply large quantities. All BOM, PCB layout drawings and assembly drawings are provided free of charge with the reference design package.
capacitanceSmall-capacitance filters and decoupling with C0G or NP0 dielectric capacitors. The temperature coefficient of these dielectric capacitors is extremely flat and, in general, is more resistant to buckling stress. In order to make the RC filter have a high Q, a low temperature coefficient, and stable electrical characteristics at varying voltages, it is recommended to use a C0G or NP0 type capacitor. We use a small size 0805 or higher ceramic device to reduce the CTE mismatch between the component and the PCB. For a large amount of storage needs, we chose a high temperature tantalum capacitor and balanced it between size and ESR. In fact, Figure 8 finally shows that stability is really affected when using 100 kΩ and 1 MΩ resistors. Since the output voltage is subject to strict filtering, the gate voltage becomes a ringing detector. Ringing indicates that the phase margin is poor or negative, and the ringing frequency shows the crossover frequency.
resistanceThe main part of the design uses thin film SMT resistors (automobile grade PATT series), and the supply on the market is sufficient. In addition, some thick film SMT resistors were selected for specific values ​​and sizes, depending on the design needs.
ConnectorThe board is connected to a Micro-D rated at 200 ° C, which is commonly used in high reliability industries. To reduce signal crosstalk, we have specially treated the connector housing to ground it to the PCB in the assembly. For applications requiring the highest signal integrity and lowest crosstalk, use high temperature professional connectors (or no connectors) and coaxial or shield balanced inputs to reduce crosstalk.
PCB design and layoutLong and narrow PCBs should be selected for downhole applications because the boards in these applications must meet the drilling and pressure shell limitations. The board material chosen is a high temperature, halogen-free polyimide. Specify a board thickness of 0.093 inches instead of 0.062 inches as this will increase stiffness and flatness.
Nickel-gold surface treatment, in which nickel provides a barrier to prevent metal-to-metal growth, gold provides a good surface for joint welding.
For a selected 0.093 inch board thickness, a typical four layer stack has a copper barrier of approximately 13 mils and a large core of 60 mils. In the case of a six-layer structure, the barrier layer is typically 9.5 mils and 28 mils thick. To this end, we have adopted a six-layer design so that a ground plane can be placed on each signal layer to improve noise performance.
Power and digital communication signals are fed into one connector and analog signals are fed into the reverse connector. This allows for good isolation and signal flow between the digital and analog domains. The division of the ground is placed in the middle of the circuit board, and the power supply filtering is located near the separation. Minimize the digital control lines that intersect the separation layer, using series termination to reduce digital noise coupling. The digital and analog ground planes are soldered at one point using a copper network connection to provide a low impedance loop for the drive source.
The multiplexer control signal is the same length as the analog portion, but its routing path is separated from the critical analog signal path. In practice, these multiplexed control lines are synchronized with the acquisition data measurements, minimizing crosstalk effects.
weldingThe Sn95/Sb05 was chosen to provide a sufficiently high melting point ("230 °C") at an operating temperature of 200 °C, while also considering good operability and existing processing capabilities of the assembly plant.
Board mountingWe provide mounting posts on this board for convenience, and are only suitable for benchmark or laboratory environments and are not suitable for high impact and strong vibration environments. If you want to use it in strong impact and strong vibration environments, you can first attach the component to the board with epoxy. For fragile components such as IDC connectors, they can be sealed or removed from the assembly. In downhole or other harsh environments, the typical installation method is to use a rail mounting system that secures the entire board with a flexible, impact-resistant mounting washer. It is also possible to completely seal the assembly and install it into the mounting hardware, then secure the mounting hardware to the chassis or housing.
Performance test resultWe have extensively tested multiple boards to evaluate their typical performance over the operating temperature range; they also immersed for 200 hours at 200 °C ambient temperature to determine assembly process and board reliability.
AC and DC signal chain performance is a key accuracy indicator for precision data acquisition systems based on SAR ADCs. When the ADC is operating at 600 kSPS and operating at 200 °C, the robust ratiometric platform has crosstalk performance of more than –100 dB and a maximum offset drift of ±60 mV. For AC testing, use a 1 kHz low distortion tone as the input signal and power the board with a +5 VDC/–2.5 VDC analog supply. Figure 6 shows the FFT and spectrum analysis results of the signal at 400 kSPS. At 200 ° C, the SNR is better than 84 dB and the THD is –96 dB. Figure 7 shows the SNR and SINAD, and Figure 8 shows the THD of the non-multiplexed channel over the operating temperature range when the same input tone is used.
Figure 6. FFT and spectrum analysis results at 200 °C.
Figure 7. SNR and SINAD over the operating temperature range.
Figure 8. THD over the operating temperature range.
We measured the power consumption of the analog and digital rails over the operating temperature range. The results are shown in Figure 9. The total power consumption at room temperature is 155 mW, which increases to 225 mW at 200 °C. The power dissipation on the 3.3 V rail is dominated by a microcontroller running at full clock rate and a precision oscillator. The burst sampling rate we set for the converter is 8192 samples per second.
Figure 9. Power Consumption of 2.5 V, 3 V, and 5 V Supply Rails
Please refer to the reference platform for the test results of the additional parameters. The rated parameters are in accordance with the 200 °C operating temperature requirements.
Application exampleMultiple applications in oil and gas exploration, aerospace and heavy industry enable orientation and vibration detection through accelerometers. The accelerometer with analog input has the highest accuracy and is very flexible, allowing the sensor output to be adjusted to suit the application.
The ADXL206 is a complete precision low power dual axis iMEMS® accelerometer for high temperature environments. It has a range of ±5 g and a bandwidth from 0.5 Hz to 2.5 kHz. The output of the ADXL206 is centered at 1â„2 VCC and is proportional to VCC. If the ADXL206 and EV-HT-200CDAQ1 share VCC (provided on the connector), the DC offset and power drift can be cleared by the VCC reference on the multiplexer S7 channel. Figure 10 is an example circuit. The signal range (0 V to 5 V) of the ADXL206 must be adjusted to fit the range of 0 V to 2.5 V of the precision data acquisition system in a 1â„2 scale factor. The specific method is to buffer the output first, and then use the attenuator inside the data acquisition system. C2 and C3 set the bandwidth of the ADXL206; the example in Figure 9 shows a bandwidth of 33 Hz. Low bandwidth applications can use multiplexer inputs; to achieve the highest bandwidth and accuracy, two non-multiplexed input channels can be used.
Figure 10. Interface of the high temperature accelerometer to the EV-HT-200CDAQ1.
summaryThis paper introduces a new, highly integrated and robust precision data acquisition reference platform, EV-HT-200CDAQ1. The platform has been measured and its parameters meet the 200 °C operating temperature requirements. With this platform, high-temperature electronics designers can use state-of-the-art components in prototyping and evaluation, reducing development time and time to market.
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