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Termination fault detection issue in LMH0307 (SDI cable driver)

paul baby54
Prodigy 20 points
Other Parts Discussed in Thread: LMH0307, LMH0303

Termination Fault detection is not proper in default setting of the SDI cable driver[LMH0307].

Setup used and testing procedure as follows:

Hardware connections are as given in reference schematic in datasheet.

Input 700mv Pk-Pk.[HD SDI signal]

Output Connector is 75ohm with 75ohm BNC cable.

Testing in default condition :

always showing termination fault.

After changing HD threshold value of all channels to Maximum, Differential channels[SDO bar] were able to detect Termination fault properly.but other channels are not detecting termination fault.

After reducing the Amplitude to lowest value Normal channels [SDO] are detecting termination fault properly but differential channels are now continuously showing no termination fault.

Kindly suggest how we can resolve this issue.As we will be monitoring the fault conditions periodically. 

  

  • 0
    TI__Guru* 88895 points
    Hi Paul,

    Please review the description below. Below, we discuss the background on cable fault detection and then how we can fine tune the cable fault detection.

    Background:
    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 with a 3G input, the cable driver will detect the “unloaded condition” 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.

    Procedure:
    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. It 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. Please fine tune these register settings and let us know your comments/result.

    Regards,,,nasser
  • 0 in reply to Nasser Mohammadi
    Prodigy 20 points
    Dear Nasser,
    Thanks for your information, but my problem is if I keep the HDTFThresh to default values it shows termination fault detected always even if we reduce the amplitude values , if I increase the Threshold then some channel [SD0]Toggle with respect to termination properly. and if I make the Threshold to Max then the differential outputs toggle with respect of termination. As per data sheet it should detect termination properly in default values only.

    Regards Paul.
  • 0 in reply to paul baby54
    TI__Guru* 88895 points

    Hi Paul,

    HDTFTHreshLSB and HDTFTHresh define the threshold for both positive and negative 75 ohm output(SDO0+ and SDO0-). This is assuming both of these outputs have the same impedance characteristics. If you use these two registers and can fine tune to declare fault for just one output(for example SDO0+) but not the other(SDO0- in this case) this means for some reason the reflection levels are different between these two outputs.  

    Regards,,nasser