SNAA377 December   2025 LMK05318 , LMK05318B , LMK5B12204 , LMK5B12212 , LMK5B33216 , LMK5B33414 , LMK5C22212A , LMK5C23208A , LMK5C33216 , LMK5C33216A , LMK5C33414A

 

  1.   1
  2.   Trademarks
  3.   Abstract
  4. 1Introduction
  5. 2General Termination Guidelines
    1. 2.1 Identify the Driver and Receiver Requirements
    2. 2.2 Determine the Coupling Type
      1. 2.2.1 DC-Coupled Signal
      2. 2.2.2 AC-Coupled Signal
  6. 3Differential
    1. 3.1 Setting the Common-Mode Voltage (Thevenin Termination)
    2. 3.2 LVPECL
      1. 3.2.1 DC-Coupled LVPECL
      2. 3.2.2 AC-Coupled LVPECL
    3. 3.3 LVDS
      1. 3.3.1 DC-Coupled LVDS
      2. 3.3.2 AC-Coupled LVDS
    4. 3.4 HSDS
      1. 3.4.1 DC-Coupled HSDS
      2. 3.4.2 AC-Coupled HSDS
    5. 3.5 HCSL
      1. 3.5.1 DC-Coupled HCSL
      2. 3.5.2 AC-Coupled HCSL
    6. 3.6 LP-HCSL
      1. 3.6.1 DC-Coupled LP-HCSL
      2. 3.6.2 AC-Coupled LP-HCSL
  7. 4Single-Ended
    1. 4.1 LVCMOS
      1. 4.1.1 DC-Coupled LVCMOS (Series Termination)
      2. 4.1.2 AC-Coupled LVCMOS (Series Termination)
    2. 4.2 Differential P or N
      1. 4.2.1 DC-Coupled Differential P or N
  8. 5Summary
  9. 6References

3.1 Setting the Common-Mode Voltage (Thevenin Termination)

When the driver VCM and the receiver VCM do not match, translation is required and the VCM must be reconfigured to meet the receiver specification. Use the Thevenin termination to adjust the common-mode voltage of differential outputs.

The Thevenin equivalent circuit consists of a resistor network, as shown in Figure 3-1. Impedance matching occurs when the two resistors are configured using Equation 1 and Equation 2. A signal with Thevenin termnation is less susceptible to signal degradation because the circuit can provide additional current and reduce the driver load. However, a Thevenin termination can increase the power consumption as a result. For power-sensitive designs, use a Y-bias termination instead.

Equation 1. ZO =  RTOP × RBOTTOMRTOP + RBOTTOM≈ 50Ω
Equation 2. VTERM = VCC × RBOTTOMRTOP + RBOTTOM

Where,

  • ZO is the characteristic impedance (such as 50Ω single-ended).
  • RTOP is the resistor pulled-up to the supply.
  • RBOTTOM is the resistor pulled-down to ground.
  • VTERM is the termination voltage (typically the common-mode voltage).
  • VCC is the supply voltage (such as 3.3V).
Thevenin Termination for AC-Coupled OutputsFigure 3-1 Thevenin Termination for AC-Coupled Outputs