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ADS8339: Conversion time and reference input current questions

Mark Pieper
Expert 4660 points

Part Number: ADS8339

Hi,

The reference input current spec is 75uA +/- 20%. Is this current the average over a single conversion time? If so, it is the average of the CDAC current spikes, correct?

The conversion time spec is very wide, 500ns - 3.3us. What drives this wide range?

Over what conversion time is the ref input current averaged? The wide conversion time range would imply that there should be an equally wide range to the reference input current, correct?

I'm trying to calculate the dV on the 10uF ref capacitor during a single conversion. For instance, picking some numbers, assume the 75uA is for a 1us conversion. The dV = 1us*75uA/10uF = 7.5uV. What would worst case dV be on 10uF?

Thanks in advance,

Mark

  • 0
    TI__Guru 69076 points

    Hi Mark,

    The reference current is averaged across the CDAC current spikes created with each bit decision in the ADC. This current will scale with the analog input voltage.

    The conversion time is the remainder of t_cycle minus t_acquisition. At full-speed, the ADS8339 samples at 250 kSPS, so the cycle time is 4 us. The minimum acquisition time of 700 ns can then be used to determine the maximum conversion time = 3.3 us. The minimum conversion time of 500 ns is usually set by an internal clock. The average current consumption in the data sheet assumes the max sampling rate of 250 kSPS and would decrease with longer conversion times.

    Regards,

    Ryan