Circuit functions and advantages
The circuit shown in Figure 1 is a complete loop powered thermocouple temperature measurement system, which uses the PWM function of precision analog microcontroller to control the output current of 4 mA to 20 mA.
Figure 1. Aducm360 controls 4 mA to 20 mA loop based temperature monitoring circuit (schematic diagram: not all connections and decoupling are shown)
This circuit integrates most of the circuit functions on the precision analog microcontroller aducm360, including dual channel 24 bit Σ-Δ Type ADC, arm cortex ™- The M3 processor core and PWM / DAC features for 4 mA to 20 mA loops with control loop voltages up to 28 V provide a low-cost temperature monitoring solution.
Among them, aducm360 is connected to a T-type thermocouple and a 100 Ω platinum resistance temperature detector (RTD). RTD is used for cold junction compensation. The low-power Cortex-M3 core converts ADC readings to temperature values. The supported temperature range of T-type thermocouple is − 200 ° C to + 350 ° C, and the output current range corresponding to this temperature range is 4mA to 20mA.
This circuit is similar to the circuit described in circuit note cn-0300, but this circuit has the advantage of driving 4mA to 20mA loop with higher resolution PWM. PWM based output provides 14 bit resolution. For details about the interface between temperature sensor and ADC and the linearization techniques of RTD measurement, please refer to circuit note cn-0300 and application note an-0970.
The circuit is powered by linear regulator adp1720, which can adjust the loop plus power supply to 3.3 V to provide power for aducm360, operational amplifier op193 and optional reference voltage source adr3412.
This part of the circuit is similar to the temperature monitor circuit described in cn-0300, using the following characteristics of aducm360:
24 bit Σ-Δ PGA is built in the model ADC, and 32 gain is set for thermocouple and RTD in the software. ADC1 switches continuously between thermocouple and RTD voltage sampling.
The programmable excitation current source drives the controlled current through the RTD. Dual channel current source can be at 0 μ A to 2mA are configured in certain steps. This example uses 200 μ A setting to minimize errors caused by RTD self heating effects.
The ADC in aducm360 has a built-in 1.2 V reference voltage source. The internal reference voltage source has high precision and is suitable for measuring thermocouple voltage.
The external reference voltage source of ADC in aducm360. When measuring RTD resistance, we use ratio setting to connect an external reference resistance (rref) to the external VREF + and VREF − pins. Since the reference voltage source in the circuit is high impedance, it is necessary to enable the on-chip reference voltage input buffer. The on-chip reference voltage buffer means that the impact of input leakage can be minimized without an external buffer.
Bias voltage generator (Vbias). The Vbias function is used to set the thermocouple common mode voltage to avdd_ REG/2 （900 mV）。 Similarly, the common mode voltage of the thermocouple can be set without external resistance.
Arm Cortex-M3 core. The powerful 32-bit ARM core integrates 126 KB flash memory and 8 KB SRAM memory to run user code, configure and control ADC, and use ADC to convert thermocouple and RTD input into final temperature value. It also controls the PWM output to drive a 4 mA to 20 mA loop. For additional debugging purposes, it can also control communication on the UART / USB interface.