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TCA9801: Inquiry about cascading two TCA9801 devices and noise immunity on B-side

Ryu Yamashita
Genius 4895 points

Part Number: TCA9801
Other Parts Discussed in Thread: OPT4048, , TCA9511A

Hi team,
 
Background:
We are considering an I²C topology where two TCA9801 devices are connected in series between an FPGA and an OPT4048 sensor. The purpose of using two stages is to improve noise immunity over relatively long cabling (approx. 510 mm + 650 mm). The B-side current‑source behavior is expected to help signal integrity, but no explicit information is found in the datasheet.
Questions:
  1. Is cascading two TCA9801 devices (A→B→A→B structure) officially supported or guaranteed by TI?
  2. Are there known limitations related to offset accumulation, rise‑time control, or threshold shifting when multiple TCA9801 stages are connected in series?
  3. Does the B‑side current‑source driver provide measurable improvement in noise or crosstalk immunity compared with a standard open‑drain pull‑up? If available, please provide any SI/EMI‑related data.
Requests:
  • Official statement regarding supportability of multi‑stage TCA9801 configuration
  • Any application notes, guidelines, or errata relevant to long‑distance cabling with TCA9801
  • Recommended alternative devices if cascading is not recommended

Best Regards,
Ryu.

  • 0
    TI__Guru 62928 points

    Hi Ryu,

    Ryu Yamashita said:
    Is cascading two TCA9801 devices (A→B→A→B structure) officially supported or guaranteed by TI?

    This is a valid connection. TCA980x has internal current source on B-side which cannot be connected to another B-side of another TCA980x. A-A, A-B, B-A connections are allowed. B-B is never allowed. 

    Two series buffers is okay. 

    Ryu Yamashita said:
    Are there known limitations related to offset accumulation, rise‑time control, or threshold shifting when multiple TCA9801 stages are connected in series?

    TCA980x is a true buffer and does not accrue offset voltage like TCA9511A (Vos spec). 

    I am not sure what you mean by rise-time control, but the internal current source on B-side helps to steadily pull-up the voltage on B-side through its current source. 

    The input threshold shift does not change when using multiple buffers in series. The VILA / VILB / VOLA / VOLB are as spec'ed in the datasheet regardless of series or parallel implementations. 

    Ryu Yamashita said:
    Does the B‑side current‑source driver provide measurable improvement in noise or crosstalk immunity compared with a standard open‑drain pull‑up? If available, please provide any SI/EMI‑related data.

    I think the "signal integrity" that we may be discussing here is about the rise-time control of the internal pull-up current source. There is not much external documentation about this, but it acts like a basic pull-up resistor and does not have major benefits over using a pull-up resistor. 

    When it comes to noise or x-talk, a standard open-drain pull-up would likely behave the same. 

    Regards,

    Tyler