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SN74LVC244A: Delay Variation Issue

Part Number: SN74LVC244A

Tool/software:

Hello,

I am experiencing an unexpected delay variation issue with the SN74LVC244A IC and would appreciate your support in diagnosing the potential causes.

Issue Description:

I am using the SN74LVC244A in two identical setups, referred to as Sample #A and Sample #B. The delay measurements of the outputs passing through the SN74LVC244A are as follows:

  • Sample #A: 2.4 ns
  • Sample #B: 3.4 ns

However, after swapping the SN74LVC244A ICs between Sample #A and #B, the delay measurements changed as follows:

  • Sample #A: 2.8 ns
  • Sample #B: 2.3 ns

Based on the swap, I initially expected that Sample #A would show a delay similar to the previous 3.4 ns value, and Sample #B would show a delay close to the previous 2.4 ns value. However, the results are significantly different, and I need to understand the potential reasons behind this unexpected behavior.

Question:

What factors could cause such variations in delay when swapping the SN74LVC244A between two identical setups? Are there specific parameters, IC characteristics, or environmental conditions that could lead to this kind of behavior?

Your insights would be greatly appreciated.

Thank you for your assistance.

  • Strange. Might there be large temperature variations? I assume the (capacitive) loads are identical?

    Does it go back after you switch the ICs back?

  • Thank you for your response.

    Regarding the temperature, there are no components nearby that could generate high heat, and the tests were conducted in an environment exposed directly to ambient air, so I don't expect significant temperature variations.

    As for the loads, I used identical components on the same PCB design, so I also don't expect any differences in the capacitive loads.

    Additionally, when the ICs are switched back to their original positions, the delay values show only very slight changes, approximately 0.1 ns, but no significant variations are observed.

    I would appreciate any further insights you might have on this matter.

  • This looks as if the differences were caused by the soldering. But for the drive strength to be noticeably reduced, the solder connection would have to be very bad.

  • Could you please explain the reasons behind your suggestion? I would like to understand how a poor solder connection could cause the observed delay differences. Specifically, how does soldering quality affect the delay in such a noticeable way?

  • If the solder connection has a high resistance, then the output would taker longer to (dis)charge the load. (This would require the load to have a noticeable capacitance. Is this the case?)

  • Namsik,

    Would you be able to provide a schematic?

    Best,

    Malcolm

  • Thank you for the suggestion.

    I have checked the soldering quality, and it does not seem to be the cause of the issue. Even after reworking the solder joints, the symptoms remain the same, and the same results occur when someone else performs the work. Additionally, since the initial assembly was outsourced to an SMT vendor, it is unlikely that a soldering issue would only affect this particular component.

    Could you please advise on any other factors we should consider or check?

    Thank you for your continued support.

    Best regards,

  • Here is the schematic you requested. I apologize if the image quality is low; I hope it is still clear enough to review. Thank you for your assistance.

    Best regards,

  • Namsik,

    Have you tried performing an ABA swap (measure delay, swap devices and measure delay, swap devices back and measure one final time) to see if the results remain consistent between swaps?

    From my perspective the cause here is likely either measurement error, something happening to the parts while they are being swapped, or some difference that's inherent to the two boards and is distinct from the parts entirely. I don't see any external resistors/capacitors from your schematic connected to the device, so I assume that there aren't any?

    Best,

    Malcolm

  • Hello Malcolm,

    I have performed the ABA swap test as you suggested, and the differences between the boards remained consistent without any significant changes.

    There are no external resistors or capacitors connected to the data lines.

    As you mentioned, measurement errors or device swapping characteristics seem unlikely, as similar results were observed in repeated tests. Currently, we are preparing to investigate the PCB characteristics further. If there are any other factors or tests you would recommend for us to consider, please let me know.

    Thank you for your continued support.

    Best regards,
    Namsik You.

  • Namsik,

    I don't have any other ideas here. Let me know what the PCB characteristics look like.

    Best,

    Malcolm