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ADS9120: ADS9120 input signal freq. limitation

jay chen
Prodigy 100 points
Part Number: ADS9120
Other Parts Discussed in Thread: ADS9218

Hi Gent.,I have one quesiton.. on ADS9120, could you please help check it? thanks.

In the d/s, this can support 2.5MSPS sample rate, I want to know what the maximum input signal bandwidth is? Based on Nyqusit law, the maximum input signal freq. is 1.25MHz, but in d/s, it has no relative description on the input signal bandwdith limitation. Also the electrial char. spec., when Fin is 500KHz, the SNR only is 84dB. As we can see as well, when the input signal is 100KHz, it can get much better ADC SNR perforamnce. I'm curious of that when the input signal is 1MHz, if ADS9120 can support it? how about the SNR spec. at this condition? In my system, the Fin is 1000KHz, I only can get ~70dB SNR spec., I want to confirm this is chip limitation  or my system design limitation? thanks.

Have a good day!

Jay

  • 0
    TI__Mastermind 36190 points

    Hello Jay,

    Yes, this is a limitation of the ADC.  As you have noted, the SNR at 500kHz is 84dB, having reduced by about 12dB from 250kHz, per Figure 23.  You can expect the SNR to further reduce at 1MHz input by about another 12dB, or around 72dB.

    So yes, the practical maximum input frequency is around 250kHz.  Although the ADC will still function above this frequency, the SNR will degrade quite a bit as you have noted.

    TI does have other ADC options that are specified at 1MHz with much better performance.

    ADS9224: SNR 89.5dB at f-IN=1MHz

    ADS9218: SNR 94.1dB at f-IN=1MHz

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 100 points

    Thank Keith, it is confirmed.

    btw, could you please help give me some ideas why ADS9120 has worse input signal bandwidth limitation than ADS9224's? As I know, all these are SAR ADC, the ADC architeture is same. thanks.

    best regards,

    Jay

  • +1 in reply to jay chen
    TI__Mastermind 36190 points

    Hi Jay,

    Both ADS9120 and ADS9224 are SAR architecture, but there are differences.  The ADS9120 uses a variation of SAR that is commonly used for higher resolution ADC's with resolution greater than 16b.  This architecture has a limited input frequency range that is typically less than the maximum sample rate frequency of the device.  The ADS9224 uses a more standard SAR architecture that can support much higher input frequency.

    Regards,
    Keith