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Right leg drive: stability

Kasper Pedersen
Prodigy 20 points
Other Parts Discussed in Thread: INA128

Hi all.
First of all i apologize if my question isn't entirely clear - English isn't my native language.

I'm currently designing a EMG frontend with a "driven right leg" electrode.
Looking at different datasheets and ECG/EMG application notes I have found a alot of designs using right leg drive: The following circuit is from the INA128 datasheet

To ensure stability, most of the circuits found in apllication notes is frquency compensated by a capacitor across the feedback resistor as seen on page 28 in
http://focus.ti.com/lit/an/sprab36b/sprab36b.pdf

- How is it posible to determine the right cut off frequency without a electrical model of the patient electrodes?

I would like to be able to optimize my design for the highest possible cmmr while ensuring a gain-phase marging of eg. 45%, either by simulation of the circuit in spice or by approximating the loop transfer function.

Thanks in advance
Kind regards Kasper

  • 0
    Prodigy 130 points

    Kasper-

    As a start you could try this.  Each patient electrode can be modeled as 51K ohms in parallel with 47nF best case with fresh electrodes going up to about 620K ohms in parallel with 47nF worst case with electrodes that are starting to dry out.

         +----- 47nF -----+

         |                |

    -----+-- 51K ~ 620K --+-----

    Then you put a few hundred ohms in series with each of these electrodes and connect them together in a Y arrangement.  Essentially, the electrode and skin impedance looks like on average about 51K to as much as 620K at frequencies below a few KHz and it starts to look like a short at frequencies above about 20 KHz or so.  Obviously it's somewhere in between, in between those frequencies.  The patient's body itself contributes the several hundred ohms (100 to about 2K).  These are the models that are used in the ANSI/AAMI EC13 standard for cardiac monitors.  Tina will nicely model all this.

    -Randy