For a design using the ADS8167 we are looking at using the output of the REFby2 pin into a single supply op-amp, somewhat like Configuration2 of Figure 38 on page 27 of the datasheet. However, we are implementing a band-pass filter with gain of about 50 on the op-amp. When we analyze the noise gain for our REFby2 input we see peak gain of about 62 just above our filter center frequency and our noise equivalent bandwidth is about 1.2 kHz. We would really like to understand the expected noise on the REFby2 pin when using the internal reference. The datasheet doesn't provide much past Figure 22 and Figure 24. The graphs under typical characteristics state "REFIO configured as output pin, … (unless otherwise noted)" which imply the internal reference is used. However, clearly Figure 24 is not using the internal reference, leaving me concerned that neither is Figure 22.
Specific questions (feel free to add more information, I am sure I am overlooking something):
- What is the noise density of the internal reference? (at REFIO and at REFP, we are interested mostly between 10 Hz and 10 kHz.)
- What is the noise density of the REFby2 output?
- The REFby2 has a minimum capacitance of 1 µF, how much higher can this go and how does a higher capacitance effect the noise.
If the REFby2 has more noise than we can handle (we are targeting about 14 ENOB at a 3000 Hz sampling rate) then we have various design options. It would be good to have this information while we are still working through some architectural details instead of discovering an issue during testing. (We could shift some gain to an earlier stage, add a separate unity gain stage for the single supply offset, and/or look at external references. All of these options have trade-offs in our system. The relative benefits of each option depend on the noise levels.)
Thanks, Chip Weller
