Tackling high-temperature data acquisition and processing platform development

February 22, 2018

Bannatyne-February 22, 2018

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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