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AFE4300 question about 10M resistors between current channels

Gaetano Anania
Prodigy 50 points
Other Parts Discussed in Thread: AFE4300

Hi,

I am going to design a new board for impedence measurement using the AFE4300 and following the guidelines of the AFE4300EVM board.

I found this discussion about the use or not of the 10M resistors between the two current channel but I don't understand the right suggestion .

e2e.ti.com/support/applications/medical/f/30/p/425106/1517888?tisearch=e2e-sitesearch&keymatch=AFE4300%2010M#1517888

I think there are four cases:

use 10M resistors between the two current channels+ use the isolation capacitors (1 uF) on each current channel
use 10M resistors between the two current channels+ use the isolation capacitors (1 uF) with a 10K resistor in parallel on each current channel
not use 10M resistors between the two current channels+ use the isolation capacitors (1 uF) on each current channel
not use 10M resistors between the two current channels+ use the isolation capacitors (1 uF) with a 10K resistor in parallel on each current channel
Could you tell me the final solution and why ?

Could you let me know if an application note or technical document exist?

I should have another question:

I would like to mix the six current channels togheter in order to have six indipendents points for injection of the current ( e.g : I0-I1,I2-I0, I1-I3,I3,I4 and so on for all combinations possible). If the previous response is 1 or 2 I should put one 10M resistor between each current channel combination ( 10M between I0-I1,10M between I0-I2, 10M between I0-I3,10M between I0,I4 etc.). If so I think I can't use the AFE4300 for my pourpose. Is that right?

I am sorry for the amount of questions.

Thank you in advance.

  • 0
    TI__Mastermind 49956 points
    Hello Gae,

    We have received your inquiry about the device AFE4300 and will get back to you with a response next week.
  • 0 in reply to Praveen Aroul
    TI__Genius 12575 points
    Hi Gae,

    The excitation current is generated by the amp, which needs the DC feedback for the stability.
    Now if you have purely capacitive network as the impedance to be measured, the amp will not get the DC feedback and will not be stable.
    This is the reason we suggest adding a resistor between the IOUT pins. The value of this resistor totally depends on the impedance being measured (i.e. body impedance), it should be high enough to make sure almost all of the AC current flows through the body impedance.
    The isolation cap is purely for the patient saftey, whereas 10KOhm across it along with the resistive component of the body impedance gives the DC feedback to the amp. In this case you don't need the additional 10MOhm.

    In summary, if your body impedance have a resistive component, you can connect a network (i.e. R || C , 10KOhm and 1uF ) in series with all IOUT pins. Using this configuration you can use any combination of IOUTs.


    Regards,
    Prabin
  • 0 in reply to Prabin Yadav
    Prodigy 50 points

    Hi Prabin,

    thank you for your reply.

    However I have an other question about the patient isolation. If I use the 10K resistor in parallel to the capacitor I won't have the right isolation between the patient and the device. I'm  making a medical device (CLASS 2) so I cannnot risk to induce DC current on the patient.

    Could it be a real (possible) problem during the impedence measurement or for  the certification phase (risk analisys)?

    Have you any advices about that ?

    Regards

    Gae

  • 0 in reply to Gaetano Anania
    TI__Genius 12575 points
    Hi Gae,

    Most of the DC current is blocked by the series cap between the DAC_FILT_IN and DAC_OUT pin.
    However if you still think there is a risk, you can increase the resistance across the isolation cap to higher value. The main idea is to provide the DC feedback to amp as well as to provide high impedance to the DC current. Because of the parallel cap the AC current will not see this big resistor.

    I have no idea about the certification phase.

    Regards,
    Prabin