BAV199LT1G diodes used to fix transient voltage
About the Circuit
Transients and overvoltages may occur during production or in use. To ensure effective protection and complement the internal protection mechanisms of CIs, additional external protection measures are necessary. External protection leads to additional capacities, resistances and leakage currents. These effects must be carefully considered to achieve high accuracy. Figure 5 shows the additional protection circuit.
Leakage currents can significantly affect RTD measurements and must, therefore, be taken into account. Leakage currents can also cause faults in thermocouple measurements if significant line resistances are present.
In this circuit, the TVS PTVS30VP1UP quickly sets the voltage transients at 30 V with a typical leakage current of only 1 nA at 25°C. A 30 V TVS has been selected to cope with 30 V DC overvoltages. A 1.69-kΩ resistor followed by Schottky BAV199LT1G low leakage diodes is used to fix the transient voltage and DC overvoltage to the 5V supply line. The resistor 1.69-kΩ limits the current through the external diodes to about 15 mA at an overvoltage of 30 V DC.
To ensure that the supply line can absorb this current, a Zener diode is used to clamp the supply line. This ensures that the absolute maximum values of all integrated circuits connected to the power supply are not exceeded. The 5.6 V NZH5V6V6B Zener diode was chosen for this purpose. A 300Ω resistor limits any additional current that may enter the AD7193 or ADG738 components.
The ADuM5401 and ADuM1280 components use Analog Devices iCoupler technology. They provide an isolation voltage of 2500 Vrms between the measurement side and the controller side of the circuit. The ADuM5401 also provides isolated power for the measurement side of the circuit. The isoPower technology included in the ADuM5401 uses RF switching elements to transmit power via a transformer. Particular attention must be paid to the disposal of PCBs in order to comply with emission standards. Recommendations for PCB layout are contained in application note AN-0971.
Thermocouple configuration test results
The performance of the circuit depends heavily on the sensor and configuration of the AD7193. The power of the thermocouple type K varies from -10 to 60 mV in the temperature range from -200 to 1350°C. The PGA of AD7193 is configured for G = 32. The voltage variation at the PGA output is -320 mV at 1.92 V or 2.24 Vpp. With "Chop" enabled, 50Hz/60Hz noise reduction and FS filter word[9:0] = 96, the noise distribution histogram for 1024 samples is shown in Figure 6.
The resolution of AD7193 is 24 bits or 224 = 16 777 216 codes. The total dynamic range of AD7193 is 2 × UREF = 2 × 4.096 V = 8.192 V. The output voltage of the thermocouple after the PGA is only 2.24 Vss and does not occupy the entire dynamic range of the AD7193, so the system range is reduced by a factor of 2.24V / 8.192V.