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Grounding scheme for Right Leg Drive ECG circuit

Mir
Intellectual 485 points
Other Parts Discussed in Thread: ADS1298

Hi,

What is the appropriate grounding scheme for a Right-leg drive? I was refering to the link below:

http://www.ti.com/lit/an/sbaa188/sbaa188.pdf

This assumes that the person is not connected to earth.

If the system is operated on 0-5V single supply, then if the person inadvertently touches earth, like operating an equipment or simply walking on the floor, then would not the RLD be a problem? Considering a proper electrode connection to body, body impedance can be ~500 ohms. So, if the DC feedback by RLD is 2.5V w.r.t. electrical ground, and I assume the electrical ground ~= earth potential, then 2.5V/500 ohm  = 5mA flows through the body, which is close to the macro-shock limit!

Please correct my above understanding.

Or is there any particular note on the grounding scheme? or some other electronics should be added to account for the above safety?

Also, is there any application note from TI regarding the safety limits/procedures for biomedical electronics?

Thanks in advance,

Warm regards.

  • 0
    TI__Guru 57306 points

    Hi Mir,

    Thanks for your question. Before we get too far, can you tell us which device you are using? Is it the ADS1298?

    I'd like to bring your attention to Figure 3 from the app note you referenced. In the event that the patient does come into contact with the system's ground, Rp provides a path for the current from Vref. To limit this current, with Vs = 5V, it is recommended to use an Rp value of 100k ohms. This should limit the current to 50uA in accordance with IEC standards.

    Thanks and regards,

  • 0 in reply to Ryan Andrews
    Intellectual 300 points
    Hi. I saw in this app note, that you are using a 100k safety resistor (Rp) parallel with a Cp in the Figure7, when you test with the ADS1298.
    1. Can you tell me which value is the most used for this Cp if i use the 100k resistor?
    2. Putting this Cp wont ruin any safety regulations? As it is not in serial with the Rp. Which type of capacitor should we choose for this Cp?
    Thanks
  • 0 in reply to ECGlou
    TI__Mastermind 38745 points
    Hey ECGlou,

    1) The article describes placing a pole at 60 Hz. To do that with a 100k Rp would require a 25.5 nF capacitor.
    2) Figure 7 doesn't give a great illustration of what is really going on in the circuit since the cable also has resistance (remember the reason for the cap was to compensate for a pole created by the cable). Figure 5 shows a better illustration of what is really going on.

    Brian Pisani
  • 0 in reply to Brian Pisani
    Intellectual 300 points
    Hi Mr. Brian!

    Thank you for the information!

    Best regards