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ADS8866: Analog Reference

Rlc Rlc
Prodigy 30 points
Part Number: ADS8866
Other Parts Discussed in Thread: OPA333, REF3333, ADS8166
Hi,
I am planning to use the ADS8866 in a daisy chain configuration. My main concern is the AREF. 
Although it is recommended to use 5V, I will need 3.3V. Is this going to affect the actual resolution bits (INL)?
Moreover, do I have to use a reference or can I just connect it to the Vdd? It should be more noisy, but is it going to affect it's lifetime?
Finally, the manual recommends REF60xx or OPA333 depending on the sampling speed. 
- Can i use only ONE reference and feed multiple ADS8866?
- If yes, can the reference and each ADS be in separate boards and be connected via a 10+cm wire?
Thanks
  • 0
    TI__Mastermind 36170 points

    Hello,

    Vref can be set to any voltage from 2.5V to 5V.  However, your effective resolution is the full scale range divided by the noise.  Since the noise will be the same, but your full scale input will now be reduced from 5V to 3.3V, you will see a reduction in effective resolution.  The INL will remain about the same.

    If you can tolerate the accuracy of your 3.3V supply, you can use it as the reference input.  Make sure you have a large amount of capacitance at the pin to handle the transients.  If the source impedance is too high, this configuration may limit your maximum sample rate to less than 100ksps.  As long as the ABS maximum specs are not exceeded on the reference pin, using this configuration will not reduce lifetime of the part.

    If you plan to use multiple ADS8866 on different boards, then I would suggest that you use a single reference (REF5033, REF3333, etc.) and then use a buffer amplifier for each ADC, similar to figure 54 configuration.

    One other option is to use the ADS8166, which includes an internal reference buffer, plus an internal reference.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 30 points

    Thanks for your prompt response. 

    Do you have a suggested value for the AREF? I guess it should be in the nF range?

    Can you please elaborate more on how the high source impedance can limitt the sampling rate?

    I guess if I use multiple I can still avoid the reference and just use a big capacitor on AREF for each board, right?

  • 0 in reply to Rlc Rlc
    TI__Mastermind 36170 points

    Hello,

    Typically, for a reference pin capacitor, at least 10uF is suggested, but this depends on the reference that is used.

    Take a look at the TI Precision Labs that discusses REF pin drive requirements for a SAR ADC.

    https://training.ti.com/voltage-reference-overview-adc?context=1139747-1140267-1128375-1139107-1136002

    The purpose of the REF pin cap is to supply the charge current  for the ADC internal sampling capacitor.  The external source must recharge the capacitor before the next conversion cycle, and this recharge time depends on frequency response, as well as any impedance between the source and the REF capacitor.

    Using a large capacitor on each REF input does not eliminate the need for a buffer when distributing a reference to multiple boards.  As discussed, the capacitor must be fully  recharged between conversions, and this must be done with a low impedance source (buffer) that can quickly recharge the cap before the next conversion cycle.

    Regards,
    Keith