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TXB0104: I2S generator affected by level translator

Eric Wiebols33
Prodigy 30 points
Part Number: TXB0104
Other Parts Discussed in Thread: SN74AVC4T245, SN74AVC4T774

Hi !

I want to listen to an I2S bus with my analyzer. However, the bus runs at 1.5V but my analyzer require, at least, 2V levels. I tried using the TXB0104 as a level translator but the audio gets severely distorted.

To debug I tried a loop-back configuration from the combined I2S generator/analyzer:

1. All the I2S generator signals, i.e. SCK,FSYNK & DATA, are connected to Port A of the TXB0104. VCCA is powered from the generator. OE is pulled to GND by 1k.

2. I connect the analyzer input signals to Port A, i.e. the same pins to which the generator signals are connected. Audio is, as expected, good.

3. I connect OE to VCCA. Audio is still good.

4. When I power VCCB with a 3.8V battery, i.e. VCCB > VCCA,  the audio gets distorted. Nothing is connected to the PortB data pins (yet).

Any idea of what is happening ?

\Eric

  • 0
    TI__Guru* 97046 points
    Hi Eric,
    I have asked our local expert on translation to take a look at this post. He should be back with you soon.
  • 0
    Guru 316860 points

    The datasheet says:

    For proper operation, the device driving the data I/Os of the TXB‍0104 must have drive strength of at least ±2 mA.

    Apparently, your generator is too weak, or the capacitance of the lines is too high.

    Does the same happen without the analyzer connected?

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Clemens,

    Thanks for your input. You might be right that the generator is too weak. I have not found any official data (R&S UPV Option B41) but I will try to look it up. Maybe I should try with a buffer chip in between ?

    In any case; here are some screen capures of SCK (the other signal show the same behaviour):

    Generator SCK connected to Port A. Power on port A, No power on port B.

    Generator SCK, and anayzer SCK, connected to Port A. Power on port A, No power on port B -> More capacitance seems to reduce ringing.

    Generator SCK, and anayzer SCK, connected to Port A. Power on port A, Power on port B.-> Audio starts to sound like ***.

    Generator SCK connected to Port A.. Anayzer SCK connected to PortB.  Power on port A, No power on port B. -> Audio (obviously)l sound like ***.

    To me it seems as if the chip is confused about port directions ...

    \Eric

  • 0 in reply to Eric Wiebols33
    Guru 316860 points
    The UPV-B41 has a specified output impedance of 50 Ω; this should not be a problem.

    The TXB0104's outputs are extremly weak (to allow auto-sensing of the direction). They are designed to drive capacitive loads of up to 70 pF; larger loads prevent fast switching.
    This is possibly what is shown in the third waveform. Apparently, all your adapters and cables exceed that 70 pF limit.

    You do not need an auto-sensing bidirectional translator for I²S. Try a unidirectional or direction-controlled translator, such as the SN74AVC4T245.
  • 0 in reply to Clemens Ladisch
    Prodigy 30 points
    Hi,

    Makes sense. I thought about a uni-directional variant but could not find a readily available "break out board" so ...
    Thanks for your assistance !
    \Eric
  • 0 in reply to Eric Wiebols33
    TI__Guru 62120 points
    Clemens ,
    Thanks for the contribution. Always a great support to have on e2e.

    To drive larger cap load , buffered devices like the SN74AVC4T245 or SN74AVC4T774 might work.
    Unfortunately , there is no EVM currently for this , however , I am working on a generic board which should be released soon online by end of August.