SDAA162 July   2026 ADS125H18 , ISO7721 , ISO7730 , ISO7731 , SN6505B , SN74LVC1G17 , TUSB320 , TVS3301

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Design Overview and Measurement Performance (Normal Operation)
    1. 1.1 Design Overview
    2. 1.2 EMC Test Board Voltage Measurement Performance During Normal Operation
    3. 1.3 EMC Test Board Current Measurement Performance During Normal Operation
  5. 2EMC Test Board Circuit and PCB Layout Considerations
    1. 2.1 Circuit Design Considerations for EMC Compliance
      1. 2.1.1 High-Voltage Capacitors and Resistors on Every Input Connector Pin
      2. 2.1.2 TVS Diodes
      3. 2.1.3 Protecting the Current Shunt: PTC and Zener Diodes
      4. 2.1.4 Series Resistors on Digital Signals
      5. 2.1.5 Digital Isolation
      6. 2.1.6 Power Supply and Protection
      7. 2.1.7 High-Voltage Capacitors and Resistors for Discharging Path
    2. 2.2 PCB Layout Considerations for EMC Compliance
      1. 2.2.1 PCB Layer Stack-up and Ground Plane
      2. 2.2.2 Avoiding a Long Return Path
      3. 2.2.3 Avoiding 90-Degree Bends in PCB Traces
      4. 2.2.4 Using a Guard Ring to Isolate Interference Signals
      5. 2.2.5 Decoupling Capacitors
      6. 2.2.6 Differential Signal Routing
      7. 2.2.7 Stitching Vias
      8. 2.2.8 Layout for Isolation Barrier
      9. 2.2.9 Component Placement
  6. 3EMC Test System, Standards, and Results
    1. 3.1 EMC Test System
    2. 3.2 EMC Test Standards
    3. 3.3 EMC Test Results
      1. 3.3.1 Electrostatic Discharge (ESD)
      2. 3.3.2 Radiated Immunity (RI)
      3. 3.3.3 Electrical Fast Transients (EFT)
      4. 3.3.4 Surge Immunity (SI)
      5. 3.3.5 Conducted Immunity (CI)
  7. 4Schematic, PCB Layout and Bill of Materials
    1. 4.1 Schematic
    2. 4.2 PCB Layout
    3. 4.3 Bill of Materials (BOM)
  8. 5Summary
  9. 6References

2.1.3 Protecting the Current Shunt: PTC and Zener Diodes

Figure 2-3 shows that the EMC test board includes 249Ω shunts (R3) on certain analog inputs. These shunts convert the current output from a field transmitter to a voltage that the ADC measures during normal operation. However, fault conditions can occur that apply a sustained overvoltage to the shunt and potentially cause its destruction. For example, the most common overvoltage event occurs when the nominally 24V power supply gets accidentally shorted to the inputs. Equation 9 and Equation 10 calculate the current through and the power dissipated by a 249Ω shunt that has 24V applied, respectively:

Equation 9. IShunt=VSupplyRShunt=24V249Ω=96.4mA
Equation 10. PShunt=IShunt2×RShunt=96.4mA2×249Ω=2.31W

Equation 10 shows that a typical 0.1W or 0.25W shunt would be destroyed under these fault conditions. Therefore, the ADS125H18 EMC test board uses back-to-back Zener diodes and a PTC fuse to help protect the shunt as follows:

  • The back-to-back Zener diodes — D1 and D2 in Figure 2-3 — protect the shunt by clamping to a specific voltage during both positive and negative overvoltages. This clamping voltage limits the current through the shunt such that any remaining fault current flows through the diodes
  • The PTC fuse — F1 in Figure 2-3 — protects the Zener diodes by increasing its resistance due to increasing temperature. The temperature increase results from excessive power dissipation because of the large fault current flowing through the diodes. The elevated PTC fuse resistance then reduces the total fault current sourced from the supply and stabilizes the system

Figure 2-4 shows a current input channel on the ADS125H18 EMC test board with the back-to-back Zener diodes and the PTC fuse highlighted in yellow:

Shunt Protection Circuitry on the ADS125H18 EMC Test BoardFigure 2-4 Shunt Protection Circuitry on the ADS125H18 EMC Test Board

Table 2-1 describes important Zener diode parameters, their definitions, and example values:

Table 2-1 Important Zener Diode Parameters, Definitions, and Values
ParameterDefinitionValue (@25°C)
Zener voltage (VZ)Amount of current that can pass through the PTC without tripping11V
Surge power (PS)Amount of time the diode can sustain a given input current90W for 1.5ms
Forward voltage (VF)Diode voltage drop when it is forward biased0.7V
Leakage current (IR)Residual device current in the off state that flows through the shunt and creates a measurement error1µA
DC Zener current (IZ)Current at which the Zener diode clamps136mA

Table 2-2 describes important PTC fuse parameters, their definitions, and example values:

Table 2-2 Important PTC Fuse Parameters, Definitions, and Values
ParameterDefinitionValue (at 25°C)
Hold current (IH)Amount of current that can pass through the PTC without tripping at 25°C200mA
Trip current (IT)The current at which the PTC begins to trip at 25°C600mA
Max current (IMAX)Maximum current that can flow through the PTC without damaging the device30A
Power dissipation (PD)The power dissipated by the PTC in the tripped state0.9W
Time to trip (tTrip)Amount of time for the PTC to trip for a given input current1.5ms at 8A

It helps to walk through an example to understand how these components protect the shunt. Assume a 24V power supply with an 8A current limit accidentally shorts across the shunt. The Zener diodes instantaneously clamp the voltage across the shunt while the fault current passes through the Zener diodes instead of the shunt. Equation 11 calculates the total clamped voltage across the shunt using the values from Table 2-1:

Equation 11. VClamp=VZ+VF=11V+0.7V=11.7V

The Zener diodes clamp the voltage across the shunt at 11.7V. Equation 12 and Equation 13 calculate the current through the shunt as well as the power dissipated by the shunt under these conditions, respectively:

Equation 12. IShunt=VClampRShunt=11.7V249Ω=47mA
Equation 13. PShunt=VClamp2RShunt=11.7V2249Ω=0.55W

The remaining power supply current passes through the Zener diodes. Equation 14 and Equation 15 calculate the current through the diodes as well as the power dissipated by the diode under these conditions, respectively:

Equation 14. IZener=ISupply-IShunt=8A-47mA=7.953A
Equation 15. PZener=VZener×IZener=11V×7.953A=87.48W

The diode surge power (PS) specification indicates how long the diodes can survive during the overcurrent event. The diode datasheet often provides a plot showing pulse duration on the x-axis and peak power on the y-axis because the diode can support higher current as the pulse duration decreases. In this example, the Zener diode supports PS = 90W for 1.5ms. Therefore, the diode can support the overcurrent conditions for approximately 1.5ms given the result in Equation 15. Figure 2-5 shows the circuit behavior at the instant the power supply shorts to the input. Note that some components have been removed for simplicity.

Shunt Protection Circuit Immediately After the Power Supply ShortsFigure 2-5 Shunt Protection Circuit Immediately After the Power Supply Shorts

Select a PTC fuse such that the component trip time versus current is less than PS so the diode does not get damaged. The PTC fuse protects the diode in this example because tTrip = 1.5ms (at 8A) according to the specifications in Table 2-2. The PTC fuse resistance increases significantly in the tripped state, reducing the current sourced by the power supply and dropping the remaining supply voltage across the fuse as shown in Equation 16:

Equation 16. VPTC=VSupply-VShunt=24V-11.7V=12.3V

The PTC fuse also maintains the power dissipation specification (PD) in the tripped state. Equation 17 calculates the PTC fuse current given the PD specification from Table 2-2:

Equation 17. IPTC=PDVPTC=0.9W12.3V=73mA

The remaining current passes through the Zener diode as shown in Equation 18:

Equation 18. IZener=IPTC-IShunt=73mA-47mA=26mA

Confirm that IZener is less than the Zener maximum DC current specification (IZ) when the PTC fuse trips. Equation 18 shows that IZener = 26mA, which is well within the limit of IZ = 136mA specified in Table 2-1. Figure 2-6 shows the circuit behavior when the PTC fuse trips after 1.5ms.

Shunt Protection Circuit 1.5ms After the Power Supply ShortsFigure 2-6 Shunt Protection Circuit 1.5ms After the Power Supply Shorts.

The circuit remains in the state shown in Figure 2-6 until the fault condition is removed. Therefore, design the system to ensure all components can support the required voltages and currents indefinitely. Also consider the following factors to select shunt protection components in an industrial application:

  • The PTC fuse behavior changes with temperature. Take care at low temperatures because the trip current and power dissipation are much higher. This behavior requires the Zener diodes to support much larger DC currents after the PTC fuse trips. Comparatively, the PTC fuse trips at a much lower current at higher temperatures. This behavior might cause the PTC fuse to trip while the system is measuring a valid current
  • Two shunts in parallel can help reduce the total power dissipation across each device because the current reduces by a factor of two while the resistance increases by a factor of two. However, the power equation squares the current (P = I2R) such that the power dissipated by each shunt reduces by a factor of two compared to single shunt.
  • The Zener diode leakage current adds to the measured input current and creates an error. Choose a low leakage component to maintain a high-accuracy system