3 Devices Without Termination Resistors

• Verify in your data sheet whether the receiver that you are using for your design contains an internal termination resistor. The formulas used are similar to obtain values with small differences taking the termination resistor into consideration.
• If your device does not contain internal termination then the equations shown in Figure 3-1 will be used.
  Figure 3-1 Equations for a Receiver Without Internal Termination

• For the simulation, the following values will be used:
  • V_CC = 3.3 V
  • V_A = 1.2 V, LVDS driver output fixed common mode voltage
  • V_B = 0.9 V, Sub-LVDS driver output fixed common mode voltage
  • R_E = 50Ω
• Using the formulas a, b, and c from Section 2, create a system of equations in order to solve for the resistance values that will best fit the desired parameters for Sub-LVDS. The following values were obtained from the system of equations:
  • R₁ = 110Ω
  • R₂ = 35Ω
  • R₃ = 50Ω

Now proceed to simulate the circuit in order to verify the behavior.

Figure 3-2 Schematic for Receivers Without Internal Termination

• The simulation shown in Figure 3-3 demonstrates a voltage swing of approximately:
  • V_ID ≈ 185 mV
  • V_OD ≈ 320 mV
• V_ID falls within the desired range. If the formulas are used to obtain values for V_ID, V_CMF, and R_E with the resistor values and the measured values for V_OD & V_A, then the following results can be obtained:
  • V_ID = 188 mV
  • V_B = 0.85 V
  • R_E = 48Ω

Figure 3-3 Simulation Results for Devices Without Internal Termination