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Recommended amp for measuring ICG

Evert Dek
Prodigy 50 points
Other Parts Discussed in Thread: OPA314, OPA316, LMV851, TLV2460

Hi Folks,

I need some help to choose the correct amplifier for measuring ICG signal on the human body. ICG is the thorax impedance.
To explain simple how you measure ICG , you apply an 100kHz 1mA constant current to the back of the body, then you measure on the front of the body the impedance. The base of the impedance is 20-35 ohm en changes every time the hart pumps blood, this change in impedance is 100-400 mOhm. In this last value we are interested, this will be a signal of 0.1-0.4mV 50kHz to be measured.
More detailed explanation


We need to redesign the existing concept to make it more wearable.
So we need an amp with low noise, low power consumption and a small footprint, power supply will be 3.3V


  • 0
    TI__Guru* 93105 points

    Hi Evert,

    I assume you intend to use the amplifier in the measurement function? What sort of power level do you mean in regards to low power consumption? How low does the noise need to be in the 50 to 100 kHz range?

    Thanks, Thomas

    PA - Linear Applications Engineering 

  • 0 in reply to Thomas Kuehl
    Prodigy 50 points

    Hi Thomas,

    Thanks for the quick reply.
    Yes, it's indeed the amplifier for the measuring. The application that we have now is measuring with 50kHz, the 100kHz was taken from the pdf to explain how it's working.

    We are now using the MC33174D in our application. It uses 200uA and has an en of 29nV/HZ.
    The goal is the get the noise down factor 10 to 3nV/Hz so we can measure change in the impedance from 0.5mOhm. This is hard without using more power, 500~800uA will be the limit.

    Regards,

    Evert

  • 0 in reply to Evert Dek
    TI__Guru* 93105 points

    Hello Evert,

    Amplifier voltage noise tends to be inversely proportional to operating current. It may difficult to find an amplifier having 3 nV/rtHz with operating currents in the sub-milliampere range.

    When I review TI's operational amplifiers having operating current below 800 uA, the voltage noise is in the range of 11 to 14 nV/rtHz. Half, or less than half, the noise of the MC33174D's 29 nV/rtHz, but certainly not 1/10th. Some amplifiers you may want to take a look at:

    Model       Current     en (nV/rtHz)

    OPA314     210 uA           14

    OPA316     500 uA           11

    LMV851     520 uA           11

    TLV2460    575 uA           11 

    Most of these are offered in very small SC70 and SOT23 packages.

    Regards, Thomas

    PA - Linear Applications Engineering