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AFE4300 DC feedback resistors selection between current channels

Daniyal Noor
Prodigy 145 points
Other Parts Discussed in Thread: AFE4300

Hi,

I came across a thread about the DC feedback resistor across the current channels (10M used on the development board as well as the hand BCM reference design), Amy mentioned that you guys recommended to use 10k with 1uf parallel earlier instead of the 10M used on all the reference design, is this documented anywhere like an application notes ?

I will be using AFE4300 on a custom designed instrument on patients to do a clinical trial, I wouldn't want constant DC current induced on the patient, can you please elaborate on this matter. can we use a small value resistor like 1M for the dc feedback instead?


Thanks :)

  • 0
    TI__Mastermind 18285 points
    Daniyal,

    The reason for changing the recommendation is as follows:
    There is an op amp inside the AFE4300 that drives the excitation current into the body. The external electrical network between the active IOUT pins (IOU0 and IOUT1 for example) serves as the feedback network for the op amp. While the excitation signal is AC, for the op amp to work in a stable manner, it needs a mechanism that sets the DC feedback. Our earlier recommendation was to use a 10MOhm resistor between IOUT0 and IOUT1. However this was found to be unsatisfactory from a DC feedback perspective. That is why we changed our recommendation to add a 10 KOhm resistor in parallel with the 1 uF caps. The two series 10 KOhm resistors in series with the resistive component of the body impedance will now be the mechanism that sets the DC feedback for the excitation op amp. The 10 MOhm (or 1 MOhm) can be retained if possible as it adds additional redundancy to the DC feedback mechanism.
  • 0 in reply to Amy Schnoor
    Prodigy 145 points
    Hey Amy,

    Thanks for the reply, as I mentioned that we are designing a device which will be connected to patients, from safety point of view the design of the equipment needs to follow the IEC 60601 standard.
    1. The allowable DC leakage current is very small so by having two 10KOhm resistors will it fulfill the standards ?
    2. If two 10KOhm resistors were used what amount of current will be going through the patient (assuming 200 ohm for patient body impedance)?
    3. If this newer method is not used in the design what will be the effect to the system, unstable op-amps? and what will that do to the measurements?

    I know these questions are very specific and it might be hard to find out the answers so please give me as much info as you can and we can figure this out together.
    Thanks.
    :)