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ADS5400 input analog signal

Adam Gropp
Prodigy 130 points
Other Parts Discussed in Thread: ADS5400, ADS5400EVM

Hello,

I am working with the ADS5400 EVM and I am wondering whether is analog input can to be bipolar, unipolar, or both. In the slau293 document, it specifies to use a frequency generator set to a 0 V offset -1dBFS sine wave. With a zero volt offset it leads me to believe that a bipolar signal is acceptable but the fact the terminology frequency generator vs. function generator (as used to specify the clock source) confuses me. Is the documentation talking about the same thing (frequency generator/function generator)? I know that this may be a silly question I was want to make sure I do not mess up the board. 

Also with full scale set  2 V (default), a -1 dBFS input analog signal would be around 1.78 Vpp (2*10^-1/20). Am I thinking about this correctly? This is the first time I have seen dBFS but my thinking is that it would be 1 dB below 2V so using 20log(x/2)=-1 I get 1.78 V.

Is there a minimum input analog frequency? 

Thank you for your time in advance.

Adam

  • 0
    TI__Mastermind 43910 points

    Hi Adam

    This ADC has a differential analog input, and the applied signal and converted value will be bipolar around a 0V differential level. Please refer to the ADS5400 device datasheet, and the ADS5400EVM User Guide (http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slau293&fileType=pdf)

    Figure 5.1 of the EVM User Guide shows the input circuit path, from the single ended 50 ohm input, through the 1:1 transformers T4 and T5, to the differential inputs of the ADC. The generator setting of 0V offset is included to ensure users do not apply a DC voltage to the transformer inputs.

    The terminology of frequency generator or function generator is basically referring to the same thing. Function generators may also generate waveforms other than sine-waves, while frequency generators are sine-wave only. The transformer coupled input path on this EVM is only intended for sine-wave type signals.

    The transformers are rated for a minimum frequency of around 5 MHz.

    I believe your approximate amplitude calculation is correct. It is always best to start at a lower signal amplitude and get everything working, and then increase the signal level until the fundamental power reported by High Speed Data  Converter Pro Single Tone analysis is at -1dBFS.

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Prodigy 130 points
    Jim-

    Thank you for your reply. I was able to get the board working.

    Adam