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ADS8568: Acquisition time and charging internal capacitor

David Park
Expert 6120 points
Part Number: ADS8568

Hi,

There are a couple of questions and issue.

1. Does t(ACQ) mean charging time of internal capacitor or acquisition time of conversion data from ADC to MCU?

2. If it's charging time, is internal capacitor charged in t(CONV)?

3. If it's acquisition time of conversion data, why is there only minimum value in datasheet?

4. Can we know when charging of internal capacitor is completed?

Below is analog input signal during conversion. With CONVST high, input voltage is dropped and increased to original voltage.

It seems to be due to charging internal capacitor so now testing with adding capacitor at analog input or changing dividing resistor to smaller.

C26=0, C27=1, blue = analog input, pink = convst

analog input has voltage drop 2 times. first one is at convst high, second is with busy/int high.

Thanks.

  • 0
    TI__Mastermind 24970 points

    Hello,

    1.  t(ACQ) means the charging time of internal capacitor 

    2. No, the internal capacitor is only charge during the t acq time.

    3. there is no max for acquisition time, because the acquisition time can be set by the user to be as long as they need to,  there is no max limit. For example, the time between two sample can be set to be 1 second, or 10 seconds, based on the user's needs, this would proportionately elongate the acquisition time, thus there is no max time. 

    4. the internal capacitor will be charged until a rising edge of the CONVST. This is set by the user. The internal capacitor needs to be charged within half an LSB within the acquisition time, this is a matter of circuit design. Does the customer need support designing their input circuit 

    It is common to see a response on the input signal when a conversion is initiated, but it should recover and not affect the conversion result. 

    Does the customer see incorrected conversion results?

    Can you share schematic and sampling rate?

    There is also good educational material on the web in TI Precision labs, that covers ADC input drive that would be helpful to provide background on this topic: Introduction to SAR ADC front-end component selection ~click here~

     

    Regards

    Cynthia