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Frequently Asked Question

Why should I use differential rather than single-ended?

Apart from the maximum frequency of each output type, there are general advantages to using differential connections between inputs and outputs. The advantages are due to increased signal swing, higher slew rate, and the use of differential transmission lines. Differential transmission lines offer reduced EMI production, reduced sensitivity to induced or coupled noise, and better impedance matching (less distortion of edges due to signal reflections).

Whenever differential connections are used between devices which are separated by more than a very minimal distance, it is important that controlled impedance coupled differential transmission lines be used for the interconnect. This type of interconnect is easy to implement using microstrip techniques on PCB. It is important to terminate the far-end (receiver side) of a differential transmission line with its characteristic impedance in order to eliminate reverse reflections. In general, the transmission line is NOT matched at the output (transmitter) side.

Whenever a differential clock signal is not available to drive a differential input on one of the AD951x parts, it is possible to drive the input with a single-ended signal. This is done by AC coupling the single-ended signal into one of the differential inputs. The other differential input is connected via a capacitor to a quiet ground. The inputs are self-biased, so no common mode voltage must be set. The performance of the input is not reduced by this technique.

LVPECL and LVDS outputs are differential. CMOS outputs are single-ended (although an inverted output is also available; it is not a common differential driver, which means that the two outputs are not guaranteed to switch simultaneously). The inputs (REFIN, CLK, CLK1, CLK2) are also differential.