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SN65LBC179: Differential Input Resistance Values Check

David Higginson
Prodigy 50 points
Part Number: SN65LBC179

When checking the resistance values to Ground from Pin 8 (Differential Input A) and Pin 7 (Differential Input B) I am getting values higher than I expect. Based on the image below from the datasheet and some quick calculations I am getting that I should get around 30k ohms for input A, and about 23k ohms for input B. Now what I am actually getting when measuring is 35k ohms for input A and 26.5k ohms for input B for a recent lot I am using. However, these seem pretty high. Looking back at the datasheet I noticed that the 18k ohm and 100k ohm are specified as nominal. So I assumed a 10% tolerance on those and also the 12k ohm and still got values below what I should be measuring (~32.3k ohm & ~24.18k ohm). 

What I would like to know is what is the tolerance of the Nominal Resistors? +/- 20%? 

I am guessing that these nominal resistances would vary from Lot to Lot of parts so I should but should be able to measure the resistance to ground within some sort of definable range. 

Thanks, David

  • 0
    TI__Guru 62920 points

    Hi David,

    Since only a nominal resistance value is given in the figures, if you want to make sure the input impedance of a given unit is within spec you may want to measure the input current instead.  This is an item in the electrical characteristics table for the receiver portion of the device (see "II" spec).  You could measure the current directly or you could translate this specification to a resistance by dividing the applied voltage (given as a test condition in the spec) by the measured input current.

    Out of curiosity, how are you measuring the resistance now?  Is the device powered or unpowered?  If powered I would be concerned that the bias voltage present on the A/B lines (created by the resistance divider circuitry shown in the figures copied above) might be confusing your Ohmmeter.

    Regards,
    Max

  • 0 in reply to Miguel Robertson
    Prodigy 50 points

    We test the resistance with the device unpowered. I will try taking the current spec with the applied voltage to see if we can define a range. However, I would think that Texas Instruments has designed this with some sort of tolerance for these resistors.

  • 0 in reply to David Higginson
    TI__Guru 62920 points

    David,

    Sorry for the delay while I was out the last several days.

    That's a safe assumption, but the reasons I recommend referencing the current spec are: (1) it is equivalent to what you are looking for, (2) it is listed in the electrical characteristics table so there are firmer min/max limits for you to reference (which are warranted per our terms of sale), and (3) since this product was originally developed 27 years ago I'm concerned we may not be able to locate accurate info on the design targets.

    Max