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

3.5.1 DC-Coupled HCSL

General Guidelines

Traditional HCSL uses a current-mode driver to source 15mA from an open-emitter output. HCSL outputs must drive into resistive loads to provide a DC return path and generate a switching signal. Placing a 50Ω resistor to ground on each P and N leg sets the output swing to 750mV and the common-mode voltage to 350mV.

  • Terminate the HCSL driver with a 50Ω resistor to ground on each of the P and N outputs.
  • Series resistors (RS) can be placed close to the the output pins to correct overshoot and reduce the output slew rate. The value of RS is determined by the transmission line impedance and the internal output impedance of the driver. Fulfill the equation to maintain impedance-matched outputs:
    • ZO = RO + RS
  • When RO is unknown, set RS = 0Ω in the initial design. RS can be adjusted for impedance matching after testing the signal integrity of the PCB. The typically used RS values are listed:
Table 3-2 Typical Series Termination for HCSL
ZO (SINGLE-ENDED) [Ω] ZO (DIFFERENTIAL) [Ω] RO [Ω] RS [Ω]
50 100 17 33
42.5 85 15.5 27

Example Termination

DC-Coupled HCSL to HCSL

DC-Coupled HCSL to HCSL Figure 3-22 DC-Coupled HCSL to HCSL

When interfacing with an LP-HCSL receiver, follow the same guidelines as DC-coupled HCSL receivers.

DC-Coupled HCSL to LP-HCSL

DC-Coupled HCSL to LP-HCSL Figure 3-23 DC-Coupled HCSL to LP-HCSL