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TXB0104: Unable to drive outputs to zero volts

Dayne Kemp59
Prodigy 30 points
Part Number: TXB0104

Hi

I am using the TXB0104 to interface SPI between an ISLA214P50 ADC and an FPGA, at less than 1MHz.

Port A is powered with 1.8V, and Port A I/Os are directly connected to the ADC. The ADC does not have any pull-ups or pull-downs internal to its digital inputs.

Port B is powered with 2.5V, and Port B I/Os are directly connected to the FPGA. The FPGA has a drive strength of 16mA.

The OE pin is directly connected to the 1.8V rail, with a 100k pull-down.

Each power rail has a 100nF capacitor located at the relevant pins on the TXB0104 IC.

When sending SPI commands from the FPGA to the ADC, the port B side looks perfect, swinging from 0V to 2.5V. However, the Port A side only swings from 1.1V to 1.8V.

At 1.8V, the TXB0104 should be able to source 450uA which should be sufficient to drive to zero volts, particularly on the CSB line.

Any idea what I'm doing wrong here?

Thanks.

  • 0 in reply to Karan Kotadia
    Prodigy 30 points

    Page 25 of the ADC datasheet (ISLA214P50) shows the internal structure of the digital inputs and no pull-ups or pull-downs are present. This may not be a true representation of the internal structure though.

    There are no pull-ups/downs external to the ADC.

    Where does the 20uA limit on the TXB come from? From a 1.8V rail, through a 4k resistor, does that not equate to 450uA if driving to zero volts?

  • 0
    TI__Genius 16725 points
    Hello,

    Can you confirm that there are not pull ups either internal to the ADC or on the line? The TXB can only sink 20uA of current so anything above that will cause the line to drive to the 1.1V. Can you check how much current is on that line?

    Thanks!
    -Karan
  • 0 in reply to Dayne Kemp59
    TI__Genius 16725 points

    Hi,

    Please confirm that there are not internal pull ups. Also please check how much current is there on the line, there must a place this current is coming from.

    The TXB is driven low not only through the 4k but also through the NMOS which lower the output impedance of the device. I am basing the 20uA based on the IOL test provided in the datasheet. Here is an FAQ on the auto bidirection translation families:

    e2e.ti.com/.../719537

    Thanks!

    -Karan