The circuit shown in Figure 1 provides a dual-channel, channel-to-channel isolated, thermocouple or RTD input suitable for
programmable logic controllers (PLC) and distributed control
systems (DCS). The highly integrated design utilizes a low
power, 24-bit, Σ-Δ analog-to-digital converter (ADC) with a
rich analog and digital feature set that requires no additional signal conditioning ICs.
Each channel can accept either a thermocouple or a RTD input.
The entire circuit is powered from a standard 24 V bus supply. Each channel measures only 27 mm × 50 mm.
Figure 1. PLC/DCS Channel-to-Channel Isolated Temperature Input
(Simplified Schematic: All Connections and Decoupling Not Shown)
The circuit shown in Figure 1 is an integrated 2-wire, 3-wire, or 4-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.
Figure 1. 2-Wire, 3-Wire, or 4-Wire RTD Measurement Configuration
This circuit note uses a Class B Pt100 RTD sensor with an accuracy of ±0.3°C at 0°C but it can support other classes such as Class A, Class AA, 1/3 DIN, or 1/10 DIN that are higher accuracy RTDs. This circuit also has provision for Pt1000 RTDs that are useful in low power applications.
The AD7124-4/AD7124-8 can achieve high resolution, low nonlinearity, and low noise performance as well as high 50 Hz and 60 Hz rejection, suitable for industrial RTD systems. The typical peak to peak resolution of the system is 0.0043°C (17.9 bits) for full power mode, sinc4 filter selected, at an output data rate of 50 SPS, and 0.0092°C (16.8 bits) for low power mode, post filter selected, at an output data rate of 25 SPS. These settings show that the system accuracy is significantly better than the sensor accuracy.
The AD7124-4/AD7124-8 integrate several important system building blocks required to support RTD measurements. Functions, including programmable excitation current sources and a programmable gain amplifier (PGA), excite and gain the RTD, respectively, which allows direct interfacing with the sensor and simplifies the design while reducing cost and power consumption.
Several options of the on-chip digital filtering and three integrated power modes, where the current consumption, range of output data rates, settling time, and rms noise are optimized, provide application flexibility. The current consumed in low power mode is only 255 μA and in full power mode is 930 μA. In power-down mode, the complete ADC along with its auxiliary functions are powered down so that the AD7124-4/AD7124-8 consume 1 μA typical. The power options make the AD7124-4/AD7124-8 suitable for nonpower critical applications, such as input modules, and also for low power applications, such as loop-powered smart transmitters where the complete transmitter must consume less than 4 mA.
The AD7124-4/AD7124-8 also have extensive diagnostic functionality integrated as part of its comprehensive feature set. This functionality can be used to check that the voltage level on the analog pins are within the specified operating range. These devices also include a cyclic redundancy check (CRC) on the serial peripheral interface (SPI) bus and signal chain checks, which leads to a more robust solution. These diagnostics reduce the need for external components to implement diagnostics, resulting in a smaller solution size, reduced design cycle times, and cost savings.
The circuit shown in Figure 1 is an integrated thermocouple
measurement 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.
Thermocouple measurements using this system show an overall system accuracy of ±1°C over a measurement temperature range
of −50°C to +200°C . Typical noise free code resolution of the
system is approximately 15 bits.
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 low noise 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, bias voltage generator, and
internal reference. Therefore, the design of a thermocouple
system is simplified when the AD7124-4/AD7124-8 is used because most of the required 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.
