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ADS1292: Input Interface Review

Tomisin Sanni
Prodigy 30 points
Part Number: ADS1292

Tool/software:

Hello, 

I was wondering if I could get the input interface of my ECG circuit (see attached) reviewed. The goal is to use both channels of the ADS1292 for ECG (no respiration), where one channel is used when three electrodes (including RLD) are connected and the other for 2 electrodes use (no RLD). Hence I have replicated the anti aliasing filtering on both but added a DC block with a pullup to the RLDOUT signal for the 2 electrode configuration. 

I also have a couple follow up questions on this: 

1. Do I need a BAT54S on the RLDOUT signal as well?

2. Is it okay to reuse the same electrode connections for either configuration? Whereby I can swap what channel I am reading from in software depending on if RLD lead off is detected on power up. Am I right in thinking that?

3. How would I approach DC lead off detection in the second channel in this case given that there is a DC block present. Would I still be able to do this using the channel without the DC block (channel 1) since they are still connected to the same electrode as it would still saturate the comparators on that channel? 

  • 0
    TI__Guru 66186 points

    Hello Tomisin,

    Thank you for your post.

    1. You can place a BAT54 or similar protection diode on the RLDOUT as well. This node should be driven from the ADS1292 RLD amplifier itself and thus limited by the supply rails, but you can take additional care to protect this pin from possible overvoltage or ESD events as it comes in contact with the external source.
    2. That sounds possible if you are able to determine RLD connection status.
    3. The challenge with monitoring DC lead status of the two electrodes without RLD is that there is no longer a defined common-mode voltage on the body. Therefore, the inputs may be connected, but the voltage on the IN1P/N pins would still be allowed to drift above/below the comparator thresholds. It would be better to utilize the AC lead-off detection method on CH2 itself and analyze the magnitude of the received peak-to-peak signal swing at the lead-off frequency to determine ON/OFF status. 

    For best SNR and CMRR performance, we recommend using low tolerance passives in the signal path. Also, where possible, use thin film resistors and C0G/NP0 type capacitors for signal chain components as well.

    Regards,

    Ryan