Frequently Asked Question
What is the proper termination (value and location) for outputs?
The "proper" termination for an output depends on the type of output (LVPECL/LVDS/CMOS) and the environment in which it is being used. The AD951x data sheets show recommended, or example terminations, for each of the types of outputs.
CMOS outputs are generally not terminated - that is, they drive a high impedance input. CMOS is usually single-ended, and so differential transmission lines are not used. Even if it is differential, controlled impedance transmission lines are not usually used.
LVDS outputs are differential by nature. LVDS normally drives a controlled-impedance differential transmission line, terminated at the pins (or on-chip) of the receiver in the characteristic impedance of the transmission line (usually 100 ohms). No termination is needed at the output of the source; only at the input of the receiver. AC coupling may or may not be used.
LVPECL outputs are differential, but can be used as single-ended or differential. The LVPECL output driver is an emitter-follower, and must have current flowing at all times in order to keep the output impedance low. If current cannot flow, the output is high impedance. This means that an LVPECL output must always be terminated in such a way that current flows. This is done in one of three ways:
The load resistors (typically 200 Ω) set the current in the emitter followers. The AC coupling capacitors separate the common mode of the outputs from the common mode of the receiver. The differential coupled transmission line is typically 100 Ω. The far-end termination must match the characteristic impedance of the transmission line (100 Ω). If the receiver inputs are self-biased, no further steps need to be taken. However, if the common mode voltage of the receiver inputs must be set, this can be done by splitting the far-end termination resistor and feeding the Vcm at the center point.