In industrial applications, the equipment must operate reliably in harsh environments. In an analog IO module, input power and analog signal protections are needed to operate in harsh environments, fulfilling various standards. Field transmitters set the current loop based on the value of physical quantities such as temperature and pressure. The set current is then converted into voltage using a burden resistor inside the analog input module. The goal of the design is to ensure that the precision burden resistor and the analog-to-digital converter (ADC) front end are well protected.

Focusing on analog signal protections, it is important to be compliant with mandatory standards to protect the module from surges (IEC61000-4-5) and electrical fast transients (EFT). It is also necessary to consider and manage accidental events such as mis-wiring during installation and wire-short, the unwanted connection between two cables which causes a short circuit during operations that can occur in a factory.

Figure 2-1 shows a typical discrete solution based on a positive temperature coefficient device (PTC) used to protect the current input of an analog input module.

Figure 2-1 Discrete Solution With PTC

The downsides of a typical solution are:

• System size due to the BOM in terms of size and count of the components (see Figure 2-1)
• Constraint of high-power dissipation of the burden resistor in overcurrent condition reduces the choice of selection
• Temperature dependency of the front end (mainly from PTC)
• Higher voltage drop at the analog input limits the length of the current loop
• Lack of status (overcurrent versus normal operation)