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SN65LBC176: Regarding some Oscillations Observed

Darren (FAE)
Expert 5483 points
Part Number: SN65LBC176

There are two devices being used in the same application.

Device "A" : SN65LBC176
Device "B" : SN65LBC176A

It seems there are quite a few little differences between these two parts; timings and ESD protection being two of them.

There has been some oscillations/ringing observed with Device "A" only.
Below waveforms (YELLOW = Pin#6 (A) // GREEN = Pin#7 (B)) are the differential outputs.

Left-Image is a "good example" with no ringing; and Right-image is a "bad example" with ringing (2nd image has trouble with B output))
Is there any idea what could be causing this behavior?

i.e. I know slow inputs on D(pin#4) could cause unstable behavior; does this seem possible given the waveforms?
If so, are there different input transition Tr/Tf requirements for the two devices (max/min)?
https://e2e.ti.com/support/logic-group/logic/f/logic-forum/737694/faq-how-does-a-slow-or-floating-input-affect-a-cmos-device 

  • 0
    TI__Mastermind 43295 points

    Hi Darren,

    So it is a possibility  the parts do have different input structures on "D" (highlighted in datasheets) - how fast are the input transition times being used? We don't have a direct specification for it because it generally isn't a problem in most systems - but it could become one if the input rise time is especially slow (between the VIL and VIH). When this spec is included on CMOS input devices - I generally have seen the max rec.  be 20ns/V or 10ns/V - but I haven't seen that spec on RS-485/422/232 devices so I am not sure how fast it is moving. Knowing the data rate that is trying to be targeted as well as the input signal transition - is there a reason why it would be slower such as an RC circuit? 

    Also the "A" pin on the "A" device doesn't seem to outputting much of a signal at all - have more than 1 "A" device been tested? If yes how many and are the results similar to above? 

    Please let me know!

    Best,

    Parker Dodson