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ADS8329 offset in 0.3ms from its conversion start

Other Parts Discussed in Thread: ADS8329

Hi, Folks

I got question from customer. They said that the digital value are offset in 0.3ms from its conversion start.

Conditions are below,

1. Using SCLK for conversion clock.

2. SCLK is 20MHz, CONVST is 840MHz.

3. They are doing intermittent conversion by SCLK pause and resume as below.

  

In this condition, the conversion value during the first 0.3ms of each group will be lower.

The red dot represent the comparison of 4th period. After 0.3ms of 1st, 2nd and 3rd period shows same trend, but

the first 0.3ms shows different. (After 0.3ms from resuming SCLK)

Could you let me know why it occurs?

  • Hello Keiichi,

    Can you please provide an schematic showing the source circuit (amplifier and RC filter) driving the ADS8329 (IN+) (IN-) inputs, the external reference circuit, reference bypass capacitor and supply connections including bypass capacitors?

    The ADS8329 is a SAR converter therefore the inputs of the ADC must be driven with a low impedance source.  The driver circuit must be able to charge the internal sampling capacitor (40pF) to a 16-bit settling level within the acquisition time. The acquisition time is 3 CCLKs minimum. 

    Thank you and Best Regards,

    Luis

  • Hello Luis,

    Thanks for your reply.

    I will confirm to our customer but unfortunately Japan is now big holiday weeks, reply may delay.

    BTW, the internal 40pF sampling capacitor should be settling... means that
    If external sync mode,
    Does the SCLKs should apply at least 9SCLKs(3SCLK*3=9SCLK) while posedge of SCLK to negedge xCONVST?
  • Luis,

    I got schematic from customer, but this is private information, could you please allow me to send it directory via E-mail?
    Could you send me your e-mail adress to me? My mail address is keiichi_kajino@ktl-corp.co.jp

    Thank you in advance.
  • Hi Keiichi-san,
    I received the schematic via e-mail and will follow up with this case off-line
    Best Regards,Luis