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AFE4300 amplifying current

David_
Prodigy 120 points
Other Parts Discussed in Thread: AFE4300, AFE4300EVM-PDK

Hello,

I'm looking into measuring very small impedances (<5ohm) and want to inject a bigger current than the normal 375uA.

I was wondering if it is possible to achieve this by amplifying the voltage between pin 19 and 20 (DAC filter)? Are there any safety measures not mentioned in the datasheets, which could interfere with that?

Are there any other good ways to get a higher current, that you know of?

Thanks,

David

  • 0
    TI__Mastermind 49176 points

    Hello David,

    The injected current to the human body is limited and It is under the demands of the IEC- 60601 spec.

    The device follows the IEC restrictions for the injected current.

    There may not be other options to increase the injected current.

    Could you provide more details about your application? Why do you require higher current?

    Where will you be measuring such small impedance?

    Best Regards.

  • 0 in reply to Praveen Aroul
    Prodigy 120 points

    I am aware of IEC- 60601 and most definetly will not test any equipment on a patient without the necessary safety measures. 

    However, my tests showed a rising inaccuracy for impedances smaller 10 ohms. I would like to measure them for testing an EIT-phantom (small impedance network). I hope increasing the current might enhance SNR. So I'm not injecting that current into the patient, but want to test out the limits of the AFE4300. Are there any built-in current-limiters? Especially in  higher frequency range, the current is way below the limits and I would like to improve that to get better test results.

  • 0 in reply to David_
    TI__Mastermind 49176 points

    David,

    The current is limited to 375uA RMS.

    With calibration, you should be able to measure 10 ohm kind of impedance with high accuracy.

    If you have done calibration, what are the reference resistor(s) that you chose for calibration?

    Can you provide details about the AFE configuration and your test results?

  • 0 in reply to Praveen Aroul
    Prodigy 120 points

    I used 4.7ohm and 22ohm resistors for calibration when measuring a 3.3ohm, 10ohm and 33ohm resistor. FWR-mode was used and frequencies were 8,16,32 and 64kHz. For all frequencies the error for 33ohms was good below 1%. For the other two resistor the error rose to 1%-2% (10ohms) and 3%-4% (3.3ohms). Also the error got worse with rising frequencies and the standard deviation (measurements were done 25 times) increased almost ten-fold, compared to 33ohms.

    Further measurement with hiher loads (and other calibration) had better results. The layout for my measurement board was almost identical to the AFE4300EVM-PDK. 

    I guess with these low voltages the noise increases, so I'm looking for a way to amplify the signal.

  • 0 in reply to David_
    TI__Mastermind 18275 points

    David,

    I will need to try to replicate this in our lab. I will let you know my results when I have taken the necessary measurements.

  • 0 in reply to Amy Schnoor
    Prodigy 120 points

    As mentioned in the other thread, a defective DAC signal caused alot of damage. Replacing the AFE4300 increased accuracy and the error is now down to a max of 1.5% for 3.3ohms @64kHz. That seems more reasonable, than my first results.

    The standard deviation, however, still differs greatly depending on the load. Could this be due to increased noise effects for small impedances?