Circuit functions and advantages

The circuit shown in Figure 1 is a fully programmable general-purpose analog front end (AFE) suitable for process control applications. It supports the following inputs: 2/3/4-wire RTD configurations, thermocouple inputs with cold junction compensation, unipolar and bipolar input voltages, 4 mA to 20 mA inputs.

Today, many analog input modules use wire links (jumpers) to configure customer input requirements, and configuring and reconfiguring inputs requires time, knowledge, and manual intervention. This circuit provides a software-controllable switch to configure the operating mode and a constant current source to excite the RTD. This circuit can also be reconfigured to set the common-mode voltage for the thermocouple configuration. A differential amplifier is used to condition the analog input range of the Σ-Δ ADC. This circuit provides industry-leading performance at the lowest cost.

Because the AD8676 and AD8275 provide voltage gain, this design is ideal for small signal inputs, all types of RTDs, or thermocouples.

The AD7193 is a 24-bit Σ-Δ ADC that can be configured as four differential inputs or eight pseudo-differential inputs. The ADuM1400  and ADuM1401 provide all the signal isolation needed between the microcontroller and the ADC. This circuit also contains standard external protection functions and complies with IEC 61000 standards. 

Circuit description

This circuit provides a fully programmable universal analog front end (AFE) for process control applications, supporting 2/3/4-wire RTD configurations, thermocouple inputs with cold junction compensation, unipolar and bipolar input voltages, 4 mA to 20 mA input, as shown in the configuration diagram in Figure 2.

A serially controlled 8-channel SPST switch   ADG1414 is used to configure the selected measurement mode.

Voltage measurement

This circuit supports measurements of unipolar and bipolar signals up to ±10 V. The input signal passes through a signal conditioning stage and is converted by the AD7193 ADC. The AD8676 amplifier buffers the input signal, which is then amplified by the gain stage. AD8275 is used to level-convert the input signal and provide gain to make it comply with the input range of AD7193. The common-mode voltage connected to the REF pin is used to bias the output of the AD8275. This voltage is generated by the 4.5 V precision reference REF194 .

RTD measurement

As shown in the connection table, this circuit supports 2/3/4-wire RTD configurations. In this case, the sensor is a 1000 Ω platinum (Pt) RTD (Resistance Temperature Detector). The most accurate configuration is the 4-pin RTD configuration. In the application shown, an external 200µA current source provides the excitation current required by the RTD, and the AD7193 operates at a gain of 16, maximizing the dynamic range of the circuit. When RTD measurement mode is selected, the AD8617 amplifier is configured as a current source. When thermocouple measurement is selected, it is reconfigured in closed loop to set the common mode voltage. The AD8617 is a dual channel low noise amplifier, so it can drive both input channels on the board simultaneously. The resistor configuring the current source must have a low temperature coefficient to avoid introducing temperature drift errors in the measurement circuit.

Thermocouple measurement

In thermocouple applications, the voltage generated by the thermocouple is measured based on an absolute reference voltage external to the ADC. Cold junction compensation is implemented using the 16-bit temperature sensor ADT7310 . Because the signal of the thermocouple is small and to maximize the dynamic range of the circuit, the AD7193 operates at the highest gain of 128 times. The input channels are buffered so that large decoupling capacitors can be placed on the front end if necessary to eliminate the effects of noise that may appear on the thermocouple pins. The common-mode voltage used for thermocouple measurements is provided by the amplifier AD8617.

4 mA to 20 mA current measurement

This circuit also supports 4 mA to 20 mA current measurements. An on-chip sense resistor is used to convert the current to voltage. To use the full dynamic range of the ADC in current measurement mode, use a 200 Ω resistor. The sense resistor must have a low temperature coefficient to avoid introducing temperature drift errors in the measurement circuit.

Regulator and Reference Selection

This circuit chooses ADP1720 as the 5 V regulator.

The ADP1720 is a high voltage micropower linear regulator especially suitable for industrial applications. This circuit selects 4.5 V REF194 as the reference voltage source. The E-class device has an initial accuracy of ±2 mV at 25°C and a maximum temperature drift of 5 ppm/°C. It is a low dropout device that consumes less than 45 μA and is specified over the −40°C to +125°C temperature range.

isolation

The ADuM1400 and ADuM1401 are four-channel digital isolators based on Analog Devices' iCoupler® technology. They are used to achieve isolation between the field side and the system microcontroller. The isolation rating is 2.5 kV rms. The ADuM1400 uses 4 lines, all used to send data (SCLK, DIN, ADG1414, ADT7310). The ADuM1401 also uses 4 wires, 1 wire for transmitting data (AD7193) and 3 wires for receiving data (INT1, INT2, DOUT). The DIN, DOUT and SCLK lines are connected to the SPORT interface.

The design also includes external protection features such as standard protection diodes and transient voltage suppressors (TVS devices) to enhance the circuit's robustness. For more information, see the schematics and other resources in the CN0209 Design Support Package: http://www.analog.com/CN0209-DesignSupport .

Table 1 shows the performance in other modes based on 1000 ADC data sample points.

Figure 3 shows a histogram of the AD7193 output performance when configured in bipolar input mode and the input is tied to ground. This histogram shows the effect of equivalent input noise. The effective resolution achieved in this mode is 19.2 bits.