JAJSO87 may 2023 LMK04368-EP
PRODUCTION DATA
AC coupling allows for shifting the DC bias level (common mode voltage) when driving different receiver standards. Since AC coupling prevents the driver from providing a DC bias voltage at the receiver it is important to ensure the receiver is biased to its ideal DC level.
When driving non-biased LVDS receivers with an LVDS driver, the signal may be AC coupled by adding DC blocking capacitors, however the proper DC bias point needs to be established at the receiver. One way to do this is with the termination circuitry in Figure 9-9.
Some LVDS receivers may have internal biasing on the inputs. In this case, the circuit shown in Figure 9-9 is modified by replacing the 50 Ω terminations to Vbias with a single 100 Ω resistor across the input pins of the receiver, as shown in Figure 9-10. When using AC coupling with LVDS outputs, there may be a startup delay observed in the clock output due to capacitor charging. The previous figures employ a 0.1 µF capacitor. This value may need to be adjusted to meet the startup requirements for a particular application.
LVPECL drivers require a DC path to ground. When AC coupling an LVPECL signal use 120 Ω emitter resistors close to the LVPECL driver to provide a DC path to ground as shown in Figure 9-11. For proper receiver operation, the signal should be biased to the DC bias level (common mode voltage) specified by the receiver. The typical DC bias voltage for LVPECL receivers is 2 V. A Thevenin equivalent circuit (82 Ω resistor connected to VCC and a 120 Ω resistor connected to ground with the driver connected to the junction of the 82 Ω and 120 Ω resistors) is a valid termination as shown in Figure 9-11 for VCC = 3.3 V. Note this Thevenin circuit is different from the DC coupled example in Figure 9-8.