Circuit explanation

Discussion in 'General Electronics Chat' started by kavin6, Feb 3, 2009.

  1. kavin6

    Thread Starter Member

    Jan 2, 2009
    I want to know the working/operation of the following circuit. Please could anybody explain it.
    Thanks in advance!:)
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    What is your interest and background?

    IMHO persons should not be promoting circuits connecting across the human heart without knowing what they are doing.
  3. kavin6

    Thread Starter Member

    Jan 2, 2009
    To design an ECG amplifier. I have done it and its working fine but I need to know the working of it.
  4. SgtWookie


    Jul 17, 2007
    The schematic was copied from page 11 (Figure 9) of the Burr-Brown (now Texas Instruments) datasheet for the INA118; attached.
  5. ifixit

    Distinguished Member

    Nov 20, 2008
    I have had some design experience with instrumentation amplifiers (INA) connected to patients many years ago. I doubt the design requirements have been relaxed much. The main requirement is to measure a bio signal from within the body, while at the same time reject all the extraneous electrical noise that is around us at all times. You are not allowed to harm the patient either.

    Signal common mode noise is always present and must be kept to a minimum to be within the operating range of the INA118. Taking a sample of the center of the gain resistor (Rg) gives you the common mode signal of LA and RA, which is noise and 60Hz hum from around the patient. This signal is buffered to get a low impedance output used to drive the shield and "guard" the input from stray leakage currents.

    The guard signal is then inverted, amplified, and converted to a current, which is "injected" into the patient’s leg. This has the effect of canceling the unwanted noise on both RA and LA equally. This connection also provides a safe virtual ground reference to the patient. I recently had an ECG myself and both legs had a connection, which I suspect distributes the feedback signal more evenly.

    Connecting electrical equipment to a human patient is a situation which scrutinized very carefully by the electrical safety authorities and a lot of safety requirements need to be met. Even just connecting a grounding electrode to a patient can be a problem if that ground is used by, or effected by, other equipment in the building (e.g. a hospital).

    Please be very careful with what you are doing.

  6. kavin6

    Thread Starter Member

    Jan 2, 2009
    Thanks a lot ifixit :D