I'd like to know more about the best practice in designing the hardware around the ADS1298 chip family. First off, I'm really satisfied with my results with these chips but there is always something which could be done better. My subquestions are:
1. I'm using the AVDD/AVSS +/- 2.5V supplies with the recommended TI parts (+ve,-ve LDO and charge pump inverter). Is it known how significant is the noise contribution of this supply design and if it has such which is the best practice to reduce it (extra caps etc.)?
2. I'm using the internal 2.4V reference. It is not clear from the datasheet that when using the +/-2.5V supplies can I select the 4V reference? Also, would this yield any significant benefits in noise reduction?
3. There is no output voltage noise data for the internal reference, only noise drift per temperature. However, if I use a - seemingly better - external reference (REF3225 or REF3240) will I experience better noise performance?4. In case of using external reference, it is not clear in the datasheet what to hook up on VREFN and VREFP pins. According to the datasheet, VREFN should be AVSS which is -2.5V in my case and VREFP would be AVSS+2.5V = 0V...where to connect the external reference actually?Thank you in advance!
If you've followed the same basic implementation of the supplies we used on the ADS1298ECG-FE board, you should have good results. We don't see any significant noise from the switchers in our board because of the delta-sigma architecture used in the ADS1298.
The reference (internal or external) is based on the lowest supply rail. If you were using a single supply from 0-3.3V for instance on the AVDD/AVSS nodes, you would be limited to the 2.4V reference. Since you are using +/-2.5, going with the 4V internal reference is not going to be an issue for you. If you were to measure the REFP pin relative to GND, you should see ~1.5V. I don't have noise numbers handy at the moment for the internal vs. an external reference, but you would have better (less) drift using an external source.
While we did not install the external reference on the ADS1298ECG-FE boards, the schematic in the Users Guide (page 64, sheet 3 of 5) does show the proper connection for an external reference. You would tie the external reference GND pin to your AVSS rail along with REFN. The reference output would then tie into the REFP terminal (preferably through a buffer amp).
thank you very much for the detailed answer. Much clearer now.Best regards,
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