4 Devices With Termination Resistors

• The following scenario is used when the device that is being used for the receiver already contains a termination resistor. The process and the equations are roughly the same with just a few adjustments due to the added resistance. Ensure that the receiver being used contains a termination resistor and obtains the value of the resistance from the data sheet.
• If your device does contains internal termination, then the equations shown in Figure 4-1 will be used.

Figure 4-1 Equations for a Receiver With 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
  • R_T = 100Ω, value of termination resistor inside device
• Using the formulas a, b, and c from Section 2 creates 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 = 222Ω
  • R₂ = 32Ω
  • R₃ = 95Ω

Proceed to simulate the circuit in order to verify the correct behavior.

Figure 4-2 Schematic for Receivers With Internal Termination

• The simulation shown in Figure 4-3 demonstrates a voltage swing of approximately:
  • V_ID ≈ 160 mV
  • V_OD ≈ 325 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 and V_A then we obtain the following results:
  • V_ID = 163 mV
  • V_B = 0.9 V
  • R_E = 50Ω

Figure 4-3 Simulation Results for Devices With Internal Termination