There are several types of temperature sensors that can be used in a temperature system. The temperature sensor to use depends on the temperature range being measured and the accuracy required. Along with the sensor, the accuracy of the temperature system depends on the performance of the analog-to-digital converter (ADC) to which the sensor is interfaced. A high resolution ADC is required in many cases as the magnitude of the signal from the sensor is quite small. Sigma-delta (Σ-Δ) ADCs are suitable for these systems as they are high resolution devices. They also have additional circuitry embedded on-chip, which is required in a temperature system such as excitation currents and reference buffers. This article describes 3-wire and 4-wire resistance temperature detectors (RTDs) that are commonly used. It describes the circuitry needed to interface a sensor to an ADC along with explaining the performance requirements needed from the ADC.
RTDs are useful for measuring temperatures in the range of –200°C to +800°C and these devices have a near linear response over this temperature range. Typical elements used for RTDs are nickel, copper, and platinum, with 100 Ω and 1000 Ω platinum RTDs being the most common. An RTD is made up of either 2-wire, 3-wire, or 4-wire versions, with 3-wire and 4-wire versions being the most commonly used. These are passive sensors that require an excitation current to produce an output voltage. The output voltage levels of such RTDs vary from tens of millivolts to hundreds of millivolts depending on the RTD chosen.
3-Wire RTD Interface and Building Blocks
Figure 1 shows a 3-wire RTD system. The AD7124-4/AD7124-8 includes all the building blocks needed for the system. To fully optimize this system, two identically matched current sources are needed. These two current sources are used to cancel the lead resistance errors produced by RL1 and RL2 of the RTD. One excitation current flows through both the precision reference resistor, RREF, and the RTD. The second current flows through lead resistance RL2 and develops a voltage that cancels the voltage drop across RL1. The voltage generated across the precision reference resistor is used as the reference voltage REFIN1(±) to the ADC. Since one excitation current is used to generate both the reference voltage and the voltage across the RTD, the current source accuracy, mismatch, and mismatch drift has a minimal effect on the overall ADC transfer function. The AD7124-4/AD7124-8 offers a choice of excitation current values, which allows the user to tune the system so that most of the ADC input range is used, resulting in increased performance.
The low level output voltage from the RTD needs to be amplified so that most of the ADC’s input range is used. The AD7124-4/AD7124-8’s PGA is programmable from a gain of 1 to 128, allowing the customer to trade off excitation current value vs. gain and performance. Filtering is required between the sensor and the ADC for antialiasing and EMC purposes. Reference buffers allow unlimited values for the R and C components of the filter; for example, these components do not impact the accuracy of the measurement.
Calibration is also required in the system to eliminate gain and offset errors. Figure 2 shows the temperature error measured for this 3-wire Class B RTD following an internal zero-scale and full-scale calibration, the overall error being much less than ±1°C.
Having the precision reference resistor on the high side of the RTD works well for systems using a single RTD. When multiple RTDs are needed, the precision resistor should be placed on the low side so the reference resistor is shared among all the RTD sensors. For this implementation, better excitation current matching and matching drift is required. To minimize the errors due to the mismatches in the excitation current sources, two different techniques can be used:
- Measure the two individual currents using the cross multiplexer functionality of the AD7124-4/AD7124-8, the precision reference resistor and the ADC’s internal low drift reference.
- Perform system chopping where the currents are swapped to the different sides of the RTD and the average of the two results is used in the overall calculation of the temperature.
4-Wire RTD Interface and Building Blocks
4-wire RTD measurements require one excitation current source only. Figure 3 shows a 4-wire RTD system. Like the 3-wire RTD system, the reference input used is REFIN1(±) and the reference buffers are enabled to allow unlimited antialiasing or EMC filtering. The current through the RTD also flows through the precision reference resistor, RREF, which is used to generate the reference voltage for the ADC. This configuration is resulting in a ratiometric measurement between the reference voltage and the voltage generated across the RTD. The ratiometric configuration ensures that variation in the excitation current value has no influence on the overall system accuracy. Figure 4 shows the RTD temperature error measured for a 4-wire Class B RTD after an internal zero-scale and full-scale calibration. Similar to the 3-wire configuration, the overall error recorded is much less than ±1°C.
For temperature systems, the measurements are mainly low speed (up to 100 samples per seconds typically). Therefore, a low bandwidth ADC is required, but the ADC must have high resolution. Σ-Δ ADCs are suitable for these applications since low bandwidth, high resolution ADCs can be developed using the Σ-Δ architecture.
With Σ-Δ converters, the analog input is continuously sampled, and the sampling frequency is considerably higher than the band of interest. They use noise shaping also, which pushes noise out of the band of interest into a region not being used by the conversion process, further reducing the noise in the band of interest. The digital filter attenuates any signal outside the band of interest.
The digital filter has images at the sampling frequency and multiples of the sampling frequency. Hence, some external antialiasing filters are required. However, due to the oversampling, a simple first-order RC filter is sufficient for most applications. The Σ-Δ architecture allows 24-bit ADCs with a peak-to-peak resolution of up to 21.7 bits to be developed (21.7 stable or flicker free bits).
Filtering (50 Hz/60 Hz Rejection)
Along with rejecting the noise as discussed previously, the digital filter is also useful to provide 50 Hz/60 Hz rejection. Interference occurs at 50 Hz or 60 Hz when systems are operated from the mains power supply. There are mains-generated frequencies at 50 Hz and its multiples in Europe, and 60 Hz and its multiples in the U.S. The low bandwidth ADCs mainly use sinc filters that can be programmed to set notches at 50 Hz and/or 60 Hz along with multiples of 50 Hz and 60 Hz, thereby providing rejection at 50 Hz/60 Hz and its multiples. There is an increasing requirement to provide 50 Hz/60 Hz rejection using filtering methods that have low settling time. In a multichannel system, the ADC sequences through all enabled channels, generating a conversion on each. When a channel is selected, it requires the filter settling time to generate a valid conversion. The number of channels converted in a given period of time is increased if the settling time is reduced. The AD7124-4/AD7124-8 includes post filters or FIR filters, which provide simultaneous 50 Hz/60 Hz rejection at lower settling times compared to a sinc3 or sinc4 filters. Figure 5 shows one digital filter option, this post filter has a settling time of 41.53 ms and provides simultaneous 50 Hz/60 Hz rejection of 62 dB.
Other ADC Requirements
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 standalone board, which is powered from the 4 mA to 20 mA loop. Therefore, the standalone board has a current budget of 4 mA maximum. In portable equipment such as gas analyzers used to analyze 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 maximize 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 three 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 analog blocks on-chip. Therefore, the part can be operated in mid power 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 are becoming increasingly important in industrial applications. Typical diagnostic requirements are
- Power supply/reference voltage/analog input monitoring
- Open wire detection
- Conversion/calibration checks
- Signal chain functionality check
- Read/write monitoring
- Register content monitoring
For systems that 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.
The ADC and system requirements for a temperature measurement system are quite stringent. The analog 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 Σ-Δ 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, which can be designed into multiple end systems, reducing design times.