Post Go back to editing

AD623 gain ecg

Hi

I need help in the design of ECG, and more precisely with the selection of amplifier gain ad623.

I applied scheme as in the Annex but used the amplifier ad623.

I think the value of the resistor Rg is wrong. How to choose the correct value resistors R2, R3 and Rg ?

Sincerely

Parents
  • Hi Waldas,

    Unless you reject the offset at the input, you can't really apply much gain. Because of the 300mV half-cell potential of Ag/AgCl electrodes, and your limited supply and reference voltages, you cannot apply more than a gain of 2 or 3. This is obviously not enough for ECG signals that are only a couple mV. Your options after that is to do ac-coupling (by adding an RC at the output of using a DC servo on the reference pin) and then place an additional gain stage or follow the output of AD623 with a high resolution ADC.

    Here are other options:

    1. Have you considered using the AD8232 instead? This device was designed for hear-rate monitoring, cardiac monitors and fitness applications. If this is the type of system you are designing, I think it will work very well for you.

    2. For more demanding diagnostic ECG systems, perhaps you could consider AD8420 or AD8237. Just like AD8232, these two inamps use an Indirect Current Feedback architecture which would allow you to eliminate the offset before it get amplified. The way you do this is shown in page 24 of the AD8420's data sheet. This will allow you to increase the gain (g=101 as shown in the schematic) even in the presence of large offsets.

    The overall gain is mainly a function of the sensitivity and resolution you expect and the ADC selection (and of course, the power supplies and reference voltage which sets the ADC input range). It also depends on how large is the signal you expect at the input. If the electrodes are applied at the chest is one thing, and if they are applied at the hands is another one.

    Regards,

    Gustavo

  • This is an old thread, and perhaps nobody is looking at it any more. But Gustavo's answer doesn't quite make sense to me -- in all ECG applications that I know of, you measure the voltage *difference* between two electrodes. In that case, why wouldn't the half-cell voltages cancel out? I.e., the differential amplifier inside of the in-amp should remove any half-cell voltage, since it will be common to both inputs.

    Am I missing something here? Is the issue that the two electrodes may be mismatched for some reason?

    Thanks,

    /Joel

Reply
  • This is an old thread, and perhaps nobody is looking at it any more. But Gustavo's answer doesn't quite make sense to me -- in all ECG applications that I know of, you measure the voltage *difference* between two electrodes. In that case, why wouldn't the half-cell voltages cancel out? I.e., the differential amplifier inside of the in-amp should remove any half-cell voltage, since it will be common to both inputs.

    Am I missing something here? Is the issue that the two electrodes may be mismatched for some reason?

    Thanks,

    /Joel

Children
No Data