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[FAQ] LMH0303: How to Tune LMH0303 LMH0307 Cable Fault detection

Part Number: LMH0303
Other Parts Discussed in Thread: LMH0307,


The LMH0307/3 termination fault detection was never designed to be precise; its purpose is to provide an indication when no cable is connected to the output (near end). The termination fault detection works by detecting reflections on the output.  The device measures the peak-to-peak output voltage.  The output amplitude is normally 800 mVp-p.  No termination results in 2x the output voltage (1600 mVp-p) due to the 100% reflection.

At the lowest threshold settings, the LMH0307/3 detects fault even with the amplitude down near 800 mV (perfect termination).  At the highest threshold settings, the LMH0307 will detect no fault even as the amplitude approaches 1600 mV (completely unterminated).

With the default register settings (HDTFThresh = 9) and a 3G input, the cable driver will detect the “unloaded condition” or a fault if it sees an amplitude greater than about 450 mV below the maximum (that is, the output amplitude, including the reflection, is about 1.15V).  This is not characterized but each step of the HD threshold register changes this decision voltage by approximately 100 mV, but this can range from about 50 to 200 mV. Please note: This feature was not intended to be precise, and these results are dependent on the board layout/passive components used for ESD and etc.


Since the termination fault detection threshold is dependent on the PCB layout, the threshold may need to be fine-tuned for each design.  To help in setting the termination fault threshold, we provide the termination fault counters (registers 0Ah and 0Bh) to help gauge how the cable driver is interpreting the output termination.

The termination fault counter counts the termination faults seen at the LMH0303 output.  Internal 10MHz state machine counts up or down: up one tick when a termination fault is detected, and down one tick when a proper termination is detected.  The counter ranges from 0 to 31 (decimal).  When there is no termination fault, the value will be near 0.  When the count hits 31, the termination fault indicator is asserted.

These registers can be used to fine tune the termination fault threshold setting.  If there are many termination fault counts when the output is properly terminated, then the termination fault threshold should be 

increased.  If the register is not showing consistent counts of 31 when the output is unterminated, then the termination fault threshold should be decreased.