Designers of electronic systems that are specified for operation up to 200˚C face challenges as state-of-the-art components that operate at such extreme temperatures are not often readily available. Typically, designers will select automotive / industrial grade components and up-screen them to select outliers that they are confident will operate at high temperature. This is of course not without risk, but is often the only feasible option.
Such high temperature (HT) requirements are common in oil and gas drilling, industrial and avionics systems. The benefit of HT electronics is that the control system can be located closer to the action – this often facilitates a reduction in size, weight and power as well as enhanced precision by eliminating signal degradation and electrical noise issues. Active cooling systems can be excluded if components can operate reliably at higher temperatures. There are many benefits that can result, but the challenges still remain in finding HT-rated components and designing a robust circuit that works at 200˚C.
A typical HT data acquisition and control system has a basic set of requirements that include the ability to interface to different types of sensors (temperature, pressure, position, orientation), a processor and serial communications. The circuit must be able to operate reliably up to 200˚C, deal with an electrically noisy environment and consume as little power as possible. We have seen the same basic requirements come up over and over again from engineers working on different systems across the world.
To simplify the solution to this ubiquitous problem, a reference design was created to address these requirements. The reference design was developed using only components that are specified to operate at up to 200˚C, along with a board design using appropriate HT-rated materials. The board was tested thoroughly, including operating at maximum temperature for 100 hours in an oven.
A block diagram of the high temperature data acquisition and control system is shown in Figure 1. Board mounted right-angle D-Sub connectors are used to interface board signals with external systems. The connector on the left side of the diagram is used for digital signals and the connector on the right side of the diagram is used for analog signals. The analog signals will typically be sensor inputs and the digital signals are serial communications ports that interface with other subsystems or a host control system. There are two separate TTL-level UART ports and an RS-485 port. The RS-485 option uses differential signals so data can be transmitted more reliably over longer distances. The housing of the connectors is grounded on the PCB assembly to reduce signal crosstalk.
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Figure 1 – High temperature data acquisition and control system (Source: VORAGO Technologies)
The analog and digital signal paths are segregated on the board layout to minimize noise coupling in the circuit. Three analog-to-digital conversion subsystems are implemented on the reference design. There are two separate high speed (400 KHz) 16-bit channels and another channel that multiplexes eight analog inputs through a third analog-to-digital converter (ADC). Low noise precision amplifiers are used as unity gain buffers to couple the analog sensor signals to the ADCs with a low source impedance. The input sensor signal range is 0 to 2.5V. The AD7981 ADC that is used on the reference design is capable of > 85dB typical SINAD (Signal to Noise and Distortion ratio) and ±0.6 LSB typical INL (Integral Nonlinearity) with no missing codes when used with the precision 2.5V reference voltage device (also implemented in the design). One of the inputs to the multiplexer is connected to an RTD mounted on the board so that board temperature is monitored. Another is connected to the 3.3V supply rail for the system so that any fluctuations in supply voltage are apparent.
When the analog input signals are sampled and converted, they are sent to the VA10800 microcontroller over Serial Peripheral Interface (SPI) channels. The microcontroller then processes the data and communicate status to the host or any other subsystem using either of two UARTs or RS-485.
The VA10800 is an ARM© Cortex© -M0 based microcontroller that operates at 50MHz. A block diagram of the device is shown in Figure 2. A high temperature 200˚C rated flash memory is used to store program memory. When the system is powered-up, the internal SRAM on the microcontroller is loaded over a third SPI port and the microcontroller executes from internal memory. This configuration reduces power consumption in the flash memory. There is a JTAG interface that allows access to the VA10800 for programming and debug. This can also be used for flash reprogramming through the MCU.
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Figure 2 – VA10800 Microcontroller Block Diagram (Source: VORAGO Technologies)
The VA10800 includes twenty-four 32-bit timers that can be used for different types of timer-based operations such as input captures, output compares, pulse width modulation and pulse counting. GPIO lines from the MCU are connected to the D-Sub connectors and can be used for system control operations such as triggering different signal sampling modes, controlled by the MCU.
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Optimizing the power consumption of the system was one of the design objectives. The VA10800 offers different power saving modes and the ARM instruction set includes a WFI (Wait For Interrupt) instruction that idles the CPU in a low power state. The AD7981 ADC also powers down automatically in between conversions. Both the MCU and the ADC power consumption scale linearly with operating frequency, so it is possible to optimize system power consumption by scaling processing speed and sampling rates to match system requirements. In the event that higher resolution sampling is required, ADC oversampling can be used. Oversampling a signal by 4X will gain an additional bit of resolution.
As well as all of the ICs being specified to operate at 200˚C, the same is true of the capacitors and resistors in the design. There are 62 capacitors and 81 resistors on the board. C0G (NP0) ceramic capacitors with a very stable dielectric and thin film SMT resistors are used. Thin film resistors are accurate, have a good temperature coefficient and are stable.
The board design was laid-out with a high aspect ratio resulting in a long narrow PCB (11.5” by 1” dimensions). This form factor was driven by the requirement in downhole drilling systems to house the PCB in a cylindrical vessel mounted vertically in a borehole. The PCB dimensions also simplified the organization of signal paths to keep physical separation between digital signals and analog signals. This is important as high speed digital signals can introduce noise into the analog signal path. A diagram of the PCB layout is shown in Figure 3.
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Figure 3 – Reference design layout (Source: PetroMar Technologies)
Commercial grade PCB material (FR-4) cannot be used for high temperature capable designs due to delamination after prolonged exposure to temperatures exceeding 130°C. For this reference design, a polyimide board is used that has appropriate thermal characteristics and can handle 200°C. The PCB itself is 0.093” thick and has six layers with a ground plane isolating each of the signal layers. High temperature solder was used for manufacturing the boards and components were pre-tinned with Sn100C solder.
Developing and testing of the reference design was not trivial. Even commercial temperature range systems that combine a precision analog front end with high speed digital processing can be problematic. Elevating the operating temperature to 200°C exaggerates the problem. This design will shortcut the development issues that arise for a designer of extreme temperature data acquisition / processing platforms. It will also accelerate software development time.
The reference design includes supporting firmware for the VA10800 microcontroller and a data capture software package. The VA10800 firmware uses the FreeRTOS operating system that provides simplified incorporation of tasks such as data processing and communications. The ADC data that is piped into the MCU from the ADC SPI channels can be either processed on the MCU itself or passed through the MCU via a UART to another subsystem or host controller. This source code is available to designers to evaluate the reference design or modify it as they see fit.
A data capture and analysis software package is also available for the reference design. The software runs on a PC that interfaces to the high temperature board by means of an interface board (that is also included in the reference design) with a USB-to-UART converter that allows a UART on the VA10800 to communicate with the PC. Continuous or burst mode data is streamed to the PC and it is displayed on the screen. Figure 4 shows a data capture and analysis software window that is displayed on a PC.
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Figure 4 – Data capture and analysis software screenshot (Source: PetroMar Technologies)
The tool itself provides the ability to analyze and verify SNR (Signal-to-Noise Ratio), THD (Total Harmonic Distortion) and SINAD (Signal-to-Noise and Distortion ratio) in both the time and frequency domains. This data can also be logged to files that can be stored or exported to Excel. The data capture software source code is also available for customization by the designer.
The reference design was developed in a collaboration between VORAGO Technologies, Analog Devices and PetroMar Technologies. All of the design files and software are available for no cost. For more details, see www.voragotech.com/products/htdab1 or email firstname.lastname@example.org.
Ross Bannatyne attended the University of Edinburgh and The University of Texas at Austin and has spent 25 years in the semiconductor industry in Silicon Glen, Silicon Valley and Silicon Hills in various engineering, marketing and management roles. Ross has published two books, “Using microprocessors and microcomputers” (Prentice Hall) and “Electronic Control Systems” (Society of Automotive Engineers) and holds patents related to failsafe electronics and microcontroller development tools. He currently serves as director of marketing for VORAGO Technologies in Austin, Texas.