Design Steps

In the following description the component reference designators referred to are those used in the circuit diagram on the first page of this document.

• Choose R1 to match the characteristic impedance of the cable used. In this case, with 24 AWG twisted pair cable, R1 = 100 Ω.
• Choose R2 and R3 so that when the output is low the differential input voltage is ≤ –50 mV. When the output is low the voltage on the positive input is 0 V and the voltage on the negative input is given by the following equation:
  
  
  

  This yields the following equation for the differential input voltage:

  
  
  
• Choose R4 and R5 so that when the output voltage is high, the differential input voltage is ≥ 50 mV. When the output is high the voltage on the positive input is given with the following equation:
  
  
  

  Therefore, the following equation shows the differential input voltage:

  
  
  
• C1 and C2 set the time constant of the AC-coupling network. If this time constant is long enough to cause the receiver to change state, the absolute value of the AC-coupling time constant is unimportant. The positive feedback network formed by R5 ensures that the receiver stays in the last state it was actively driven to, and thus the minimum operating frequency is independent of the values of C1 and C2. The time constant of C2, R2, and R3 is given in the following equation:
  
  
  

  The time constant of C1, R4, and R5 is given in the following equation:

  
  
  
• Choose capacitors C1 and C2 with a voltage rating that exceeds the isolation voltage required. For fail-safe circuits, two capacitors in series can be used; however, each individual capacitor must be rated for the full isolation voltage.