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ADS1299: AC lead off function using single-shot mode

Tomohiro Shirane
Prodigy 30 points
Part Number: ADS1299


Hello TI,

I would like to know whether AC lead-off function using single-shot mode is supported or not.

I want to trigger the single-shot mode on each 256Hz and want to get the AD value.

I've set the register like below.

========

CONFIG1 : 0x90      <- DR = fmod/64
CONFIG2 : 0xC0
CONFIG3 : 0xFC
LOFF : 0x0A          <-  6uA, AC lead-off at 31.2Hz
CH1SET : 0x00     <- Only ch1 is measured.
CH2SET : 0xE1
CH3SET : 0xE1
CH4SET : 0xE1
CH5SET : 0xE1
CH6SET : 0xE1
CH7SET : 0xE1
CH8SET : 0xE1
BIASSENSP : 0x02
BIASSENSN : 0x03
LOFFSENSP : 0x01
LOFFSENSN : 0x01
LOFFFLIP : 0x00
GPIO : 0x0F
MISC1 : 0x00
MISC2 : 0x00
CONFIG4 : 0x08

========

At first, I've prepared the 5K impedance environment according to the section7.2 in user's guide of EEG FE PDK(slau443b.pdf), then measured using above register settings. BUT the result was strange. (Pls see below image)

[Left side : voltage measurement (mV) ]                                          [Right side : FFT result]

       

The measured voltage was about -65mV, but I could not see the top peak at 31.25Hz. Instead of this I could see the top peak on 50Hz.

It seems the lead off current on 31.25Hz did not work during the single-shot mode.

Next, I measured using continuous mode, and the result was almost good. (Pls see below image).

[Left side : voltage measurement (mV) ]                                          [Right side : FFT result]

             

The measured voltage was +/- 60mV and the lead off current was 6uA. So the both of Zinp and Zinm are 5K. Also, I could see the top peak at 31.25Hz. That was good.

So, my questions are that

  • How should I modify the register settings in order for lead-off function to work correctly during single-shot mode triggered each 256Hz ?
  • Does the lead off current of 31.2Hz work during single-shot mode ?

I'm very appreciated if you give answer and comment.

Thanks.

  • 0
    TI__Genius 11550 points

    Hi Tomohiro-san,

    Thanks for the post. Please let me check with the team and get back to you. 

    Thanks

    -TC

  • 0 in reply to TCT
    Prodigy 30 points

    Hello TI,

    Thank you for your reply. I'll wait for your answer.

    Thanks.

  • 0 in reply to Tomohiro Shirane
    TI__Genius 11550 points

    Hi Tomohiro-san,

    Thanks for the patience.

    I checked with the team and confirmed that the AC leadoff would work with the Single-Shot mode. This is because the function of the AC leadoff circuit does not depend on how the ADC samples the channel input (either in continuous or single-shot mode). To verify the operation of the AC leadoff in single-shot mode, please set the PGA=1, ILEAD_OFF = 24uA, input floating, and probe the INxP and INxN pins. You should see the square wave with the ac leadoff frequency and magnitude close to VREF. Please see the attached picture below for the channel input with the test conditions listed above showing the AC leadoff at 31.2Hz.

     

    Thanks

    -TC

  • 0 in reply to TCT
    Prodigy 30 points

    Hello TI,

    Thank you for your reply.

    I checked your solution, however it didn't work.

    And I found possible cause that the lead-off current stop when the START-signal changes. Please see the below figure.

    Yellow line shows the lead-off current that works 31.2Hz.

    Blue line shows the START signal that changes each 256Hz. (The duration of rising edges is about 3.9msec (=256Hz) )

    At first, lead-off current works correctly.

    However, after the START signal start toggling, the lead-off current stop. Then the lead-off never work again.

    So my question is that

    • Why the lead-off current stop after the START signal change?  And Why the lead-off current never work again?
    • How should I modify in order for the lead-off function to work correctly under the START signal changes each 256Hz?

    Thanks.

  • 0 in reply to Tomohiro Shirane
    TI__Genius 11550 points

    Hi Tomohiro-san,

    Thanks for the test information. I will have to confirm this with the digital designer. Thanks for the patience.

    Thanks

    -TC

  • 0 in reply to TCT
    Prodigy 30 points

    Hello TI,

    Any update? I'd like to know the answer to my questions.

    Thanks.

  • 0 in reply to Tomohiro Shirane
    TI__Genius 11550 points

    Hi Tomohiro-san,

    Sorry for the delay. We are still waiting for feedback from the digital designer on this. It is taken longer than expected as the device was released a long time ago. Thanks for the patience.

    Thanks

    -TC

  • 0 in reply to TCT
    Prodigy 30 points

    Hello TI,

    OK, I understood. I'll wait for your update.

    Thanks.