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ADS52J90: Low Frequency Modulation

Joel Gaudreault
Prodigy 41 points

Part Number: ADS52J90

I use the ADS52J90ZZE with mutliple ADA4932-1 to drive the input channels. When I sample the data, I see an undesired low frequency modulation (AM) on top of my carrier frequency of 2.5 MHz. The low frequency modulation is in the orded of hundreds of Hz.

The low frequency modulation is only present when each of the following conditions are met.

  1. Measure between the ADC driver and the ADC input.
    1. Either with a scope or with the ADC itself.
    2. When I disconnect the ADC diver from the ADC + measure on the scope, the modulation is gone.
  2. Enable the ADC core Clock (60 MHz)
    1. This means the ADC has to be sampling.
  3. Use Carrier frequencies that are multiples of 1.25MHz, 1.5MHz or 2 MHz.
    1. Other AC signals as well as DC do not show any modulation. See Graph below.

  • 0
    TI__Intellectual 1410 points

    Hi,

    The problem statement is not very clear to us. Can you please share the full FFT spectrum data with carrier frequency of 1.25MHz 

  • 0 in reply to Sheetal Gupta
    Prodigy 41 points

    Hi  ,

    The original carrier frequency is 2.5 MHz. Look at the top and bottom of the first picture. You will see the undesired AM modulation. This low frequency modulation is only present at certain carrier frequencies.

    Look at the second picture. I can only see this undesired AM modulation when using a carrier frequency of 1.25 MHz, 1.5 MHz, 2 MHz or their harmonics.

    This happens when the ADC core clock is enabled at 60 MHz producing a 30 MHz sampling rate since I am using the ADC in 32 input mode. If I change ADC core clock to lets say 62 MHz, then the same happens but with carrier freqencies slighlty higher (1.29167 MHz, 1.55 MHz, 2.0667 MHz). This leads me to think that the ADC's sampling is influencing the measurement itself.

  • 0 in reply to Joel Gaudreault
    TI__Intellectual 1410 points

    Hi Joel,

    This kind of time domain data can be observed if the input signal and the clock are not locked properly. If there is a drift in the frequencies over time, you might observe this. But if this happens, the FFT of this data will also show spectral leakage around the fundamental tone. The solution to this is locking the input signal and clock. Also using coherent frequency at the input.