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Differential Reference And Feedback Input Capability

Last Updated: December 29, 2011

How should the differential reference and feedback input be connected?


RoboClockII has two muxable reference and feedback inputs (REFA and REFB; FBKA and FBKB). Each reference or feedback input is capable of receiving a differential mode signal or a single-ended mode signal. The required input is selected by (REFSEL) FBSEL. When LOW, (REFA) FBKA is selected. These inputs have an internal pull-down resistor, therefore, when it is left open, the (REFA) FBKA input is automatically selected.

Each input consists of two input pins; a "+" and a "-" . When connecting to the input in single-ended mode, the output signal can be connected to either the "+" pin or the "-" pin; the other input pin must be left open. When the signal is connected to the "+" pin, it is the same as connecting to any single-ended input and the polarity of the signal is preserved. When the signal is connected to the "-" pin, then the polarity of the input signal is inverted.

When the inputs are differential, the negative input becomes the reference for the positive input, which enables RoboClockII to accept PECL inputs. The minimum differential voltage swing and the common mode range specifications in the data sheet provide enough flexibility to allow RoboClockII to accept HSTL, LVDS, and SSTL inputs.

Any RoboClockII output can be connected to the feedback input. When directly feeding back a RoboClockII output to the selected feedback input, the single-ended mode configuration is used. Generally, the output used for feedback is connected to the "+" input pin.

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