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AFE4300 multiplexed bio-impedance measurement questions

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

Hi, I'm currently working on a system which needs to obtain bio-impedance measurements reasonably fast, system will have up to 35 electrodes I'm planning to use external analog multiplexers to do all the switching, periodically system needs to do impedance measurements across different electrodes with different frequencies. 

I would like to know:

1. What is the minimum time required to obtain a valid impedance reading. (settling time, conversion time ? I can only find 860sps sampling frequency)

2. Will this time change if I change excitation frequency ? by how much ?

3. If possible how can I reduce this time?

4. Can you please give some ideas how should I utilise the AFE4300 dev board to achieve this, is there an API so I can control the process with scripting language and so on if so can you please tell me where I can find it.

Thanks

Kind regards,

  • 0
    TI__Mastermind 18275 points

    Daniyal,


    We have received your post and hope to respond soon.

  • 0 in reply to Amy Schnoor
    Prodigy 145 points
    Hey Amy,

    Is there any answer on this?
  • 0 in reply to Daniyal Noor
    TI__Mastermind 18275 points
    Daniyal,

    You will need to use FWR mode since IQ mode requires calibration between each measurement. FWR mode only needs calibration when you switch frequencies and at power up of course.

    You will also want to be in continuous conversion mode. In Continuous conversion mode the AFE4300 continuously performs conversions. Once one conversion is complete, the AFE places the result in the conversion register and immediately begins another conversion. Single shot mode will need to wait for the ADC_PD bit to be set high.

    You will have a power-up time of about 1ms.

    The best way to reduce time of measurement is to have the maximum sampling frequency, since a conversion at 860 SPS only requires ~1.2ms.

    We have not done extensive testing to reduce measurement time, however you can adjust our EVM to test your setup and we do have some sample code that you can find on our product folder under tools and software.

    www.ti.com/.../toolssoftware
  • 0 in reply to Amy Schnoor
    Prodigy 145 points

    Hey Amy,

    Thanks for the information, just to clarify if I use FWR mode I will not be able to obtain phase information, and from my understanding when using IQ mode the calibration only needs to happen for the purpose of obtaining relative phase. 

    The reason I need to get lots of measurement done is that the experiment I'm running gives me limited time with each patient thus the idea is to just log all the I and Q data within the time limit and do all the calculation and calibration after the time frame. Can you please tell me if the following order is a good idea for such situation.

    1. Device power on

    2.Set device to IQ and continues mode with 860sps rate

    3. Connect the unknown impedance with the external multiplexer

    4. Set stimulating frequency to 8kHz

    5. Do IQ measurements  for a x period of time on the unknown impedance and log the I and Q data

    6. Repeat step 4 to 5 with changing frequency at step 4 to 16,32 and 64kHz

    Above steps will be done when I'm with patient the following steps I will do after the time frame

    7. Connect the one of the resistors from step 3 set the stimulating frequency to 8kHz and log the I and Q 

    8. Repeat step 7 with stimulating frequency set to 16, 32 and 64kHz

    9. Use the I and Q data obtained from step 7 to 8 to calculate the parameter needed for magnitude equation, also calculate the phase at all four frequencies.

    10. Use the magnitude equation and the phase data from step 9 to post calculate all the magnitude (resistance) and the phase of the measurements obtained from step 1 to 6

    Also can you please suggest me what kind of stimulating period as marked bold in step 5 is suitable, and to get a stable reading how many samples should I take to do the averaging during each measurements.

     Thanks in advance Amy.

     

  • 0 in reply to Daniyal Noor
    Prodigy 145 points
    Hey Amy any answer on the above question?

    I also asked on another thread about the DC feedback resistor, you mention that you guys recommended to use 10k with 1uf parallel, but I will be using this device on patient 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?

    Thanks :)
  • 0 in reply to Daniyal Noor
    TI__Mastermind 18275 points
    Daniyal,

    We have not verified this testing, but it looks ok to me as long as you are ok with not obtaining phase information. For step 5, I would suggest calibrating this to see what a good number of samples is to average. Then, you can get the least amount required as you said you want to take a lot of measurements quickly.

    Also, responded to your question on the DC feedback in this post. e2e.ti.com/.../448695
  • 0 in reply to Amy Schnoor
    Prodigy 145 points

    Hey Amy,

    Thanks for the reply, we do need the phase information and my suggestion was to do post calculation as mentioned from step 7 to 10.

    Is it possible to obtain phase information by doing post calculation ?

    Thanks. 

  • 0 in reply to Daniyal Noor
    TI__Mastermind 18275 points
    Daniyal,

    We have not tried to obtain phase information by doing a post calculation so I cannot verify it will be accurate. This procedure seems okay though. If you verify it is accurate, it will be good to followup on your post to any other user who might find that useful.