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

2.2.2 AC-Coupled Signal

Outputs are typically AC-coupled in the following cases:

  • The receiver data sheet specifies an AC-coupled input signal.
    • In this case, the receiver has an internal DC biasing network that must be used. The external AC-coupling capacitor placed in series with the input signal "clears" or separates the DC bias from the driver to avoid conflict with the internal bias from the receiver.
  • The common-mode voltage of the driver does not match the receiver input specifications.
    • AC-coupling capacitors must be used to reconfigure the DC bias. Use an external or internal biasing network.

AC-coupled outputs can require termination on both the driver and receiver side depending on the signal type. The termination structure is broken down into three steps as illustrated and explained in Figure 2-3.

Example of AC-Coupled Termination Using Three StepsFigure 2-3 Example of AC-Coupled Termination Using Three Steps
  1. Terminate on the driver side if the driver requires a DC return path to ground. This step is optional if the receiver is internally terminated and biased.
  2. Add AC-coupling capacitors (typically 0.1uF) after the driver termination and before the receiver termination to shift (or "clear") the DC bias from one level to another. The AC-coupling capacitor isolates the DC offset in the signal allowing only the AC signal to pass through.
  3. Terminate on the receiver side to reconfigure the common-mode voltage to the receiver specification. This step is optional if the receiver is internally terminated and biased.