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Completely Integrated 3-Wire RTD Measurement System Using a Low Power, Precision, 24-Bit Sigma-Delta ADC (CN0383)
The circuit shown in Figure 1 is an integrated 3-wire resistance temperature detector (RTD) system based on the AD7124-4/AD7124-8 low power, low noise, 24-bit Σ-Δ analog-to-digital converter (ADC) optimized for high precision measurement applications. With a two-point calibration and linearization, the overall 3-wire system accuracy is better than ±1°C over a temperature range of −50°C to +200°C. Typical noise free code resolution of the system is 17.9 bits for full power mode, sinc4 filter selected, at an output data rate of 50 SPS, and 16.8 bits for low power mode, post filter selected, at an output data rate of 25 SPS.
Figure 1. 3-Wire RTD Measurement Configuration
The AD7124-4 can be configured for 4 differential or 7 pseudo differential input channels, while the AD7124-8 can be configured for 8 differential or 15 pseudo differential channels. The on-chip programmable gain array (PGA) ensures that signals of small amplitude can be interfaced directly to the ADC.
The AD7124-4/AD7124-8 establishes the highest degree of signal chain integration, which includes programmable low drift excitation current sources. Therefore, the design of an RTD system is greatly simplified because most of the required RTD measurement system building blocks are included on-chip.
The AD7124-4/AD7124-8 gives the user the flexibility to employ one of three integrated power modes, where the current consumption, range of output data rates, and rms noise are tailored with the power mode selected. The current consumed by the AD7124-4/AD7124-8 is only 255 μA in low power mode and 930 μA in full power mode. The power options make the device suitable for non-power critical applications, such as input/output modules, and also for low power applications, such as loop-powered smart transmitters where the complete transmitter must consume less than 4 mA.
The device also has a power down option. In power-down mode, the complete ADC along with its auxiliary functions are powered down so that the device consumes 1 μA typical. The AD7124-4/ AD7124-8 also has extensive diagnostic functionality integrated as part of its comprehensive feature set.
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