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

3.2.1 DC-Coupled LVPECL

For DC-coupled LVPECL signals, proper output termination is required to fulfill three requirements: to provide a DC return path, to keep the impedance matched, and to maintain the common-mode voltage.

General Guidelines:

DC Return Path

LVPECL outputs are based on open-emitters as described in Section 3.1.1 LVPECL Output Stage of the SLLA120 application note. A current return path to ground (such as pull-down resistors) is required to complete the circuit from the output stage and get a signal at the outputs.

Common-Mode Voltage

The traditional LVPECL output stage has a VCM of VCC – 1.3V, as listed in Table 3-3.

Table 3-1 Output VCM for Traditional LVPECL
VCC [V] OUTPUT VCM [V]
3.3 2
2.5 1.2\

Termination resistors (which are equal to the trace impedance) are pulled up to Vcc – 2V to produce an LVPECL output current of about 15mA. Keep the termination voltage less than the VCM of the LVPECL output driver to maintain the output driver stage in proper operation. More details on the LVPECL input and output stage design are found in the SLLA120 application note.

Impedance Matching

The transmission line impedance is impacted by the resistor network. If care is not taken to match the impedance and only the DC path is provided, then the outputs can experience significant overshoot, undershoot, and/or reflections. Use IBIS models to simulate the signal integrity through software such as HyperLynx. The following figures show IBIS simulations between an LVPECL driver and LVPECL receiver.

Figure 3-3 provides the oscilliscope simulated result when the output driver is better terminated. The DC return path is provided and impedance is matched.

Figure 3-2 provides the oscilloscope simulated result when the output driver is poorly terminated. The DC return path is provided but the impedance is mismatched.

HyperLynx LVPECL Simulation with Poor Termination Figure 3-2 HyperLynx LVPECL Simulation with Poor Termination
HyperLynx LVPECL Simulation with Better Termination Figure 3-3 HyperLynx LVPECL Simulation with Better Termination

DC-Coupled LVPECL to LVPECL Using a Traditional Termination

Traditional LVPECL Termination Figure 3-4 Traditional LVPECL Termination

DC-Coupled LVPECL to LVPECL Using a Thevenin Termination

When an external supply voltage of Vcc – 2V is not readily available, use a Thevenin termination instead. Use other resistor networks methods, such as π (PI) and Y-bias, to terminate without a supply voltage connection.

Example Termination

Configure the resistor network to have a bias voltage of VCC – 2V (to allow margin) and provide 50Ω impedance matching.

Equation 3. Z O   =     R T O P   ×   R B O T T O M R T O P   +   R B O T T O M =   130 Ω   ×   82 Ω 130 Ω   +   82 Ω ≈   50 Ω

If RTOP = 130Ω and RBOTTOM = 82Ω, then:

Equation 4. V T E R M   =   V CC   ×   R B O T T O M R T O P   + R B O T T O M   =   3.3 V   ×   82 Ω 130 Ω   +   82 Ω   ≈   1.3 V    
DC-Coupled LVPECL to LVPECL Using a Traditional Termination Figure 3-5 DC-Coupled LVPECL to LVPECL Using a Traditional Termination
DC-Coupled LVPECL to LVPECL Using a Thevenin Termination Figure 3-6 DC-Coupled LVPECL to LVPECL Using a Thevenin Termination