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ADS1299: Questions about design with uni-polar power supply, referential montage, and AC lead-off impedance check

Part Number: ADS1299

Hello,

I am designing a schematic around the ADS1299 for EEG acquisition. I plan to use a referential configuration with the 8 positive pins, SRB1 and Bias out.

I want to keep the PCB as small as possible, so was planning to go with a uni-polar supply (avoid extra inverter, -2.5V LDO and 2.5V LDO). 

I am using a 3.7V Lipo battery with a charge IC, 5V boost IC and 3.3V LDO.

1. I read here that there is no disadvantage in performance by using uni-polar supply versus bi-polar. I understand that a common-mode offset must be applied to bring the positive pins to mid-supply, but don't fully understand the rest of the configuration. Is there any additional hardware I need to consider for this? Does it change anything on the ADS1299 pin connections on the schematic? Or is it purely just register configuration?

2. I don't fully understand AGND and DGND connections. I've seen some designs that just use a single ground. Is the grounds I've labelled correct? All grounds in my power circuitry are AGnd except for the ground on the 3.3V LDO, which is my DGnd? Do I still need separate analog and digital grounds when using a uni-polar supply? I understand if using bi-polar supply as AGnd will be -2.5V and DGnd will be 0V. But for uni-polar, both are 0V? I saw that in Fig 62 of the EEGFE-PDK datasheet that AGnd and DGnd are tied together using a 0Ohm resistor. Is this recommended? If so at what stage should I tie them together? 

3. Related to the previous question, are decoupling capacitors only needed between AVdd-AGnd and DVdd-DGnd? I've seen designs with caps between AVdd-DGnd too, is this necessary? How many decoupling caps do I need between AVdd-AGnd? I've seen some designs with just one pair of parallel 1uF and 0.1uF caps, and other designs with 3 of these parallel circuits.

4. I have connected AVdd1 with AVdd and AVss1 with AVss. Is this ok? The datasheet says AVdd1 is for the charge pump block and that it should be star-connected with AVdd.

5. The 5V boost converter can supply 5V with 500mA output current and an output voltage ripple of +/-25mV. The 3.3V LDO can supply 250mA output current. Firstly, I assume this is more than enough current to drive the ADS1299, an STM32F4 uC, and an AN1325A BT chip? Secondly, will 25mV ripple affect the ADS1299's performance at all?

6. I want to measure all positive electrode inputs with reference to SRB1. Is it best to tie all negative inputs together with SRB1 like I have done? Or should I use the internal input multiplexer via MISC1 register instead and leave the negative pins floating? I need to use AC lead-off too. So I want to output the AC lead-off signal on the SRB1 electrode and record it on the 8 positive electrodes to get individual impedance measurements for each electrode. Is this possible? Does this affect how I should connect the negative inputs to SRB1 (either by wire or using the input multiplexer).

7. As I am using referential montage, I can't use a differential capacitor between positive and negative pins (as all negative pins are tied together with SRB1). Instead I am using a shunt capacitor to ground. Will this have a large effect on the CMRR? Is there any way of using a differential capacitor with SRB1 for all channels?

8. Protection! I have a series 5k resistor between each electrode and pin. Is this enough? I have seen designs with ESD chips. Is it necessary? Also, on the power management side, should I have a protection IC for over-voltage/discharge/current and short-circuit to protect the ADS? Or is the fact that the switch disconnects the 5V boost and supply to the ADS when the charger is turned on?

9. Finally, I would appreciate it if any poor elements of my design could be pointed out, or if there are any suggestions on how to improve the design to achieve higher quality EEG.

Thanks in advance for any support with this.

 

Mark

  • Hi Mark!

    Thank you for the great ADS1299 questions! We're on Easter break till Monday the 22nd. One of our Bio-Potential experts will be in touch with you after the holiday weekend. In the meantime, please do have a look at some of the frequently asked ADS129x questions here:
    e2e.ti.com/.../772058
  • Hi Mark,

    Let me start by saying that we really appreciate your interest in our ADS1299. We know you've been using our devices for quite some time now.

    I think it would me more comprehensible if you could please split your questions into separate posts. There are just too many different topics in this one thread to address at once.

    Lastly, I think you'll find that some of your questions have been asked and answered on this very forum in the past. Please try doing a quick search and include the search term "ADS129*" to see if you can find the answers there.

    Best regards,
  • Thanks for your time Ryan.

    Sorry for the long post! It wasn't originally intended, but I added more and more as I went.

    I have split it into 2 separate posts now (one regarding power management and one regarding SRB1).

    I spent some time searching the forum and found some interesting posts, which refined my questions, but I would still like some clarification on the topics if possible.

    Feel free to delete this thread.

  • Thanks, Mark!