Other Parts Discussed in Thread: OPA333, REF3333, ADS8166
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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
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?
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.
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