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Converter for 5V TTL signal to 1.8V signal

Other Parts Discussed in Thread: SN74LVC125A, TXB0104, SN7400, SN74GTL2003

H,

I'm looking for a 5V to 1.8V digital converter.

I have 4 TTL levels signals that are to be interfaced to my 1.5V/1.8V FPGA IO pins.

Can anyone let me know any part compatible with this application?

Thanks in advance.

Regards,

Nanjunda M

  • Hi Nanjunda,
    If you just need to down-translate incoming signals to the FPGA, that's easy. There are many devices that can support this, one of which is the SN74LVC125A.

    Note that the datasheet lists 6.5V as the absolute maximum input voltage. That means that inputs can be any voltage up to 6.5V regardless of the supply. Just supply the device with 1.8V, and all inputs will be down-converted to 1.8V logic.

    If you need to also send some signal back from 1.8V to 5V, things get a little more complicated. If this is the case, please give me details of how these signals behave (data rate, direction of signal, etc) and I can help find a suitable device.
  • Hi Emrys,

    Thanks for the part.

    This will help me to convert my 5V TTL signal inputs to 1.8V CMOS I believe as per the datasheet.

    I would also need the vice-versa too.
    The input singals are of 1.5V CMOS and 1.8V LVCMOS levels Signals are from the FPGA (with 1.8V and 1.5V) with maximum data rate up to 1Mbps.

    The output signal requirement would be 5V TTL standard logic level signals with same data rate.

    Please help me with this requirement too.


    Thanks & Regards,
    Nanjunda M
  • I would recommend looking at the TXB0104. There are a few requirements to use this translator - low line capacitance (<70pF) and significant drive strength (>3mA) are the two big ones.
  • Thanks for the recommendation.

    Does this support the TTL logic?
  • The TXB device will operate with 5V inputs, however it depends on what you mean by TTL (some people use the term to just mean 5V, while others mean the actual SN7400 TTL logic families).

    The old 5V logic families have output voltages that can be as low as 1.5V below the desired output voltage (so 3.5V at 5V). The TXB device probably would work with this, but it isn't desirable. If your driver outputs 5V signals, it will work fine.

    If you can tell me more about your system, I can be of more help.
  • Hi Emrys,

    On the 1.5V & 1.8V signals, we will be sending some data patterns (or pulses) to a check the data in the FPGA is properly set or not.
    The 5V signals we are planning to connect to the CRO (Oscilloscope) and check the outputs.

    Can you please tell me whether it will be fine with this application?


    Thanks & Regards,
    Nanjunda M
  • Is it really a cathode-ray oscilloscope? I've never seen one with digital inputs.

    If it's just a normal digital o-scope with 5-V inputs, it will be fine.
  • Yes. It's Digital O-Scope.

    I'm not aware of the load capacitance and current limitations on the Oscilloscope side.

    In the meantime, I could find a part SN74GTL2003PWR for the above requirement. Can you please tell me how well suitable is this device for my application?


    Thanks & Regards,
    Nanjunda M
  • There are likely 100 ways for you to achieve the same solution - SN74GTL2003 is one of those options. I would still recommend going with the TXB0104 since it has active drive where the GTL does not (ie your signal integrity will be better).
  • Hi Emrys,

    I selected the SN74GTL2003 looking at the logic specified in the datasheet i.e. supporting the TTL logic.
    Which I could not find in the TXB1004.


    Thanks & Regards,
    Nanjunda M