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ADC requirements for RTD temp measurement systems

14 Aug 2015  | Mary McCarthy, Aine McCarthy

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Other ADC requirements
Power
The current consumed in a system is dependent on the end application. In some industrial applications, such as temperature monitoring in factories, the complete temperature system containing the sensor, ADC and microcontroller are contained on a stand-alone board which is powered from the 4 – 20 mA loop. Therefore, the stand-alone board has a current budget of 4 mA maximum. In portable equipment such as gas analysers, used to analyse the gases present in mines, the temperature must be measured along with the gas analysis. These systems are operated from a battery, the aim being to maximise the lifetime of the battery.

In these applications, low power is essential but high performance is still required. In process control applications, more current can be allowed for the system. For this type of application, the requirement may be to sequence through a higher channel count in a certain period of time while still achieving a certain level of performance. The AD7124-4/AD7124-8 contains 3 power modes, user selectable via 2 bits in one of its registers. The power mode chosen determines the range of output data rates along with the current consumed by the analogue blocks on-chip. Therefore, the part can be operated in mid or low power mode for loop powered or battery powered systems. In process control systems, the part can be operated in full power mode, where higher current consumption leads to improved performance.


Diagnostics
Diagnostics are becoming increasingly important in industrial applications. Typical diagnostic requirements are:

 • Power supply / Reference voltage / Analogue input monitoring
 • Open wire detection
 • Conversion / calibration checks
 • Signal chain functionality check
 • Read/Write monitoring
 • Register content monitoring

For systems which are being designed for fail safe applications, on-chip diagnostics save the customer in design time, external components, board space and cost. A part such as the AD7124-4/AD7124-8 includes the above diagnostics. The failure modes effects and diagnostic analysis (FMEDA) of a typical temperature application using this device has shown a safe failure fraction (SFF) greater than 90% according to IEC 61508. Two traditional ADCs are normally required to provide this level of coverage.


Conclusion
The ADC and system requirements for a temperature measurement system are quite stringent. The analogue signals generated by these sensors are small and must be amplified by a gain stage whose noise is low to ensure that the gain stage's noise does not swamp the signal from the sensor. Following the amplifier, a high resolution ADC is required so that the low level signal from the sensor can be converted into digital information. ADCs which use a S-D architecture are suitable for such applications since high resolution, high precision ADCs can be developed using these architectures. Along with the ADC and gain stage, a temperature system requires other components such as excitation currents and reference buffers.

Finally, the end application dictates the current budget allowed for the system. Portable or loop powered systems must use low power components and, with redundancy being included for fail safe systems, this reduces the current consumption allowance per component further. For systems such as input modules, there is a desire for a certain level of performance at higher throughputs, leading to increased channel density. Using a device with multiple power modes eases the burden on the user in that one ADC can be designed into multiple end systems, reducing design times.


About the authors
Mary McCarthy and Aine McCarthy are with Analog Devices.


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