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Input Signal Switch for ECG Applications

Martin Daricek
Prodigy 70 points
Other Parts Discussed in Thread: ADS1298R, TS5A12301E

Hello,
I would like to switch/mux ADS1298R input channels to different signal sources, e.g. to switch between V6 electrode and respiration circuitry to channel 1. Switch should be dual SPDT digitally controlled IC, with similar (or better :) specification like mux inside the ADS1298R. I viewed at several IC, like TS3A24159, TS3A24157 or TS5A23157. Are these circuits suitable for this purpose? I can't compare switches spec with internal mux in ADS1298R, because there isn't its spec in datasheet (offset, noise, signal distortion, charge injection, etc.). Which is the best one? Isn't s better choice to use two separated SPDT IC`s (due to crosstalk)?

 Thank you for answer, Martin

  • 0
    TI__Guru 66186 points

    Hello Team, 

    Martin is one of my customers using the ADS1298R. We are looking to include an external switch in his design to alternatively select two different electrode inputs.

    These biopotential signals are of low amplitude (~1 or 2 mV) and generally low frequency (up to 64kHz modulation signals for Respiration measurements, otherwise < 150Hz). 

    Low offset, noise, signal distortion, charge injection, etc. would be important, as well as minimal cross-talk.

    Could you kindly guide Martin towards a few solutions that would be appropriate for these types of applications?

     

    Thanks and regards,

  • 0 in reply to Ryan Andrews
    TI__Genius 9860 points

    Hi Martin,

    Does your signal have a DC bias? If so, what is the voltage?

     

    -Ryan

  • 0 in reply to C. Ryan Land
    Prodigy 70 points

    hi Ryan,

    there is two options:

    1. ADS1298R input connected to respiration circuitry, so there is AC coupling signal through series capacitors, but DC biased by pull up/down resistors to middle of supply (e.g. Vcc = 3V, DC bias is 1.5V).

    2. ADS input connected directly through low pass filter to human body, which is biased by RLD, so there is usually 1.5V DC bias, as a result of negative feedback of RLD referenced to (Avdd+Avss)/2

    in both options, there is DC bias, in most cases placed in the middle of supply - in this case = supply is 3V (one Li-Poly cell reduced by LDO), DC bias = 1.5V

    regards, martin

     

  • 0 in reply to Martin Daricek
    TI__Genius 9860 points

    Hi Martin,

    It would probably be better to have one switch than two separate switches. Turn on/turn off timing is more predictable with a dual switch than with two separate single-pole single-throw switches.

    The ICs you mentioned are good choices - the ones with the lowest on-resistance will be the best for your application. Some of the switches have more paths than you will need, so you can float the unused pins. However, the TS5A12301E is a SPDT switch that would be a good choice if board space is a concern.

    -Ryan

  • 0 in reply to C. Ryan Land
    Prodigy 70 points

    Hi Ryan,

    thanks for advice, I will try several switches to choose the best one, we will see.

    Regards, Martin