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ADS1299: Does applying DC offset causes 50Hz signal?

masahiko hayase
Prodigy 40 points
Part Number: ADS1299

Hello, 

We developed EEG device by using ADS1299.

When applying +/-150mV DC offset voltage to the channel in series ,however, we obseved undesired 50Hz signal.

Although our area's line frequency is 50Hz ,observed 50Hz signal differs from line frequency.(Graph 1)

This signal level depends on the DC offset voltage level. (Table 1 and Graph 2)

I attach the test circuit(test circuit.pdf) ,measured data(data.pdf), and register setting(register.txt).

We want to know if applying series DC offset voltage to ADS1299 causes this problem.

Best regards,

test circuit.pdf data.pdf 

Fullscreen register_setting.txt Download 
address  value
00h      3Eh
01h      96h
02h      C0h
03h      ECh
04h      0Ch
05h      68h
06h      68h
07h      68h
08h      68h
09h      68h
0Ah      68h
0Bh      68h
0Ch      68h
0Dh      FFh
0Eh      FFh
0Fh      00h
10h      00h
11h      00h
12h      00h
13h      00h
14h      0Fh
15h      00h
16h      00h
17h      00h

  • 0
    TI__Genius 11550 points

    Hi Masahiko,

    Welcome to the E2E forum !!!

    I consulted with the team on your observation. The difference in the channel input DC offset voltage will cause the design more susceptible to power line interference. This also explains the signal level dependency on the DC offset voltage level. It is crucial to minimize the power line noise coupling in the system design for this scenario.

    Thanks

    -TC

  • 0 in reply to TCT
    Prodigy 40 points

    Thank you for your response.

    We think that observed 50Hz signal differs from Line frequency.
    Formally attached data(Graph.1) was obtained by applying DC offset to the channel and applying power line signal to the other channel simultaneously.
    The attached data(Graph3,Graph4) is original data of Graph.1, which show the increasing phase difference as time progressed.
    (Sampling frequency is converted from 250SPS to 200SPS in this data.)

    Best regards,

    data2.pdf

  • +1 in reply to masahiko hayase
    TI__Genius 11550 points

    Hi Masahiko,

    It is difficult to tell the frequency difference between the output and the line frequency. I am not sure where else the interference signals can be coupled into the test system. The only internal frequency for the device is the internal oscillator which is operating at a frequency of 2.048 MHz.

    Thanks

    -TC

  • 0 in reply to TCT
    Prodigy 40 points
    Thank you for your support.
    We reexamine our system and find out the cause of 50Hz noise.
    ADS1299 is not the cause of this problem.
    Best regards,