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ADS1292ECG-FE: Measuring ECG with 2 electrodes

Andrea Castagnetti4
Prodigy 40 points
Part Number: ADS1292ECG-FE
Other Parts Discussed in Thread: ADS1292, ADS1292R

Hello,

I'm making a portable two electrodes ECG measurement system. We use standard dry AgCl electrodes. I would like to test our electrodes using the ADS1292ECG-FE demonstration kit. From the documentation I saw that the devkit is built for 4 electrodes ECG (ERA, ELA, ELL and ERL). Is it possible to configure the devkit for measuring an ECG signal using only two electrodes ? 

For example, let's say I just want to use ELA and ERA electrodes, can I leave the ELL and ERL disconnected and read the signal from channel 1 (IN1P and IN1N) ? Or do I have to also connect the two others electrodes (ERL and ELL) ?

Could you please give me some informations on how to configure the ADS1292 registers for this type of setup (2 electrodes).

Thanks in advance for your reply

  • 0
    TI__Guru 63156 points

    Hello Andrea,

    I'm glad to see you've purchased an EVM!

    You can certainly use only one channel for your evaluation if you'd like. The electrode labels ERA, ELA, and ELL are just to give an example, but really you can configure the signal inputs however you'd like. The ERL electrode is different, of course, since it is connected to the RLD output.

    If you are using ADS1292 (not the 'R' version), then the EVM should be configured such that both Channel 1 and Channel 2 are expecting ECG inputs. Channel 1 has different components installed if you are trying to measure respiration impedance. Alternatively, you can connect your LA electrode to pin 3 of the DB9 connector ('ELL') and measure LA-RA on Channel 2 instead. That way, you have a proper low-pass filter in the signal path. Whichever channel you decide not to use, the datasheet recommends powering down the channel and configuring the mux as Input-Short (CHxSET = 0x81).

    Without RLD, how do you plan to set the common-mode voltage of the ECG inputs? A 2-electrode system which does not use RLD needs to AC couple the inputs and bias the signals with a resistor divider or a pull-up to RLDOUT. There are component footprints in place in the Channel 1 path for you to try this if you'd like.

    To understand the requirements for the signal path, check out this training series we put together. In the "Signal Chain and Features" section, we mention the need to bias the inputs following the DC-blocking capacitor for systems without RLD:
    training.ti.com/how-measure-ecg-guide-signals-system-blocks-and-solutions

    Lastly - TI cannot suggest that you can connect the EVM to yourself, so I recommend using a patient simulator or a waveform generator to test the EVM. Be sure to share the ground connections between the source and the EVM.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Hello Ryan,

    Thanks for your response, very clear and a lot of useful information.

    I tried the configuration you suggested and I can measure an ECG signal. However, I have a lot of artifacts and sometimes it is impossible to get the ECG signal.

    I suspect as you suggested that these problems are caused by the loss of a proper common-mode setting. I will try to measure the ECG on CH1 by first making the HW modifications that you suggested and I would like to check with you if I correctly understood the modifications:

    AC coupling

    • Replace the R 0 ohm with 0.1nF at C48 and C46.
    • Replace the R 0 ohm with 2.2 nF at C50 and C49

    Biasing

    • Mount 10 M ohm resistances on R66, R72, R64, R62

    Also I wondering if I also have to mount a 47 pF capacitance at C89 and C90 to low-pass filter the signal.

    Are these the correct modifications needed to properly set the signal path for measuring the ECG on CH1 using two electrodes ?

    Finally, I tried to check the training series but I cannot play the videos. I tried on different web browser, different PC, and different OS but nothing seems to work. Could you please provide the lessons on another format (for example the slides). 

    Thanks a lot for your help.

    Best regards

    Andrea

  • 0 in reply to Andrea Castagnetti4
    TI__Guru 63156 points

    Hello Andrea,

    I believe it would be easier to simply use Channel 2 for your evaluation. You can expect the same performance on both channels. This way, you do not need to modify the PCB hardware. Simply connect your electrode inputs to Pin 1 and Pin 3 of the DB9 connector and read from Channel 2.

    If you still want to modify the board:

    • C48 and C46 should be 0.1uF. This forms a high-pass filter with the 10-M resistors.
    • C47 and C45 should be uninstalled
    • C50 and C49 should be 0 ohm
    • Mount R62, R64, R66, and R72.
    • R71, R63, C89, and C90 can be installed to add a low-pass filter. The values are really up to you, 51k and 47pF are used on channel 2 as an example.

    I don't know of a reason why the videos should not work. Let me see if I can find out.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 20 points

    Hello Ryan,

    I work with Andrea on the development of our ECG.

    We followed your recommendations (exept the 51k resistors that we replaced with 2k to change the cutoff freq) and I thank you for that.

    On one side, I have this hardware : nordic NRF52 dev kit + ADS1292R ECG/RESPIRATION BREAKOUT KIT

    On the other side: Our custom board with a nrf52 and the ADS1292

    On the dev kits, the values returned by the ADS are fluctuating when I touch the electrodes. That's what I expect.

    However, on our custom board nothing happens: The values returned by the ADS are stuck to  0x7FFFFF (8 388 607). No fluctuation at all.

    Of course I have the same code running on both hardware.

    Here are the registers config :

    #define ADS1292_REG_ID                  0x00

    #define ADS1292_REG_CONFIG1             0x01

    #define ADS1292_REG_CONFIG2             0x02

    #define ADS1292_REG_LOFF                0x03

    #define ADS1292_REG_CH1SET              0x04

    #define ADS1292_REG_CH2SET              0x05

    #define ADS1292_REG_RLD_SENS            0x06

    #define ADS1292_REG_LOFF_SENS           0x07

    #define ADS1292_REG_LOFF_STAT           0x08

    #define ADS1292_REG_RESP1               0x09

    #define ADS1292_REG_RESP2               0x0A

    #define ADS1292_REG_GPIO                0x0B

    I tried to reduce the gain to 1, but the input ADS is still saturating.

    It is supplied with 3,1V and input voltage on IN1N and IN1P is 1,04v, touching or not the electrodes.

    We tried to connect electrodes directly on R5 and R6, without better result.

    I attached the schem to this message.

    Would you have any recommendation to help us ?

    Regards,

    Pierre

  • 0 in reply to Pierre LE COZ
    TI__Guru 63156 points
    Hello Pierre,

    Thank you for the update.

    Can you please share the register values that you are using? You only provided the register addresses.

    I would first double-check that all supply voltages and grounds are correct. Then check VREFP and VREFN. If the ADC is saturating, it could mean the reference voltage is not enabled properly.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 20 points
    Yes, sorry,

    Here are the values :

    #define ADS1292_INIT_VALUE_CONFIG1 0x02
    #define ADS1292_INIT_VALUE_CONFIG2 0x80
    #define ADS1292_INIT_VALUE_LOFF 0x10
    #define ADS1292_INIT_VALUE_CH1SET 0x00
    #define ADS1292_INIT_VALUE_CH2SET 0x00
    #define ADS1292_INIT_VALUE_RLD_SENS 0x00
    #define ADS1292_INIT_VALUE_LOFF_SENS 0x00
    #define ADS1292_INIT_VALUE_LOFF_STAT 0x00
    #define ADS1292_INIT_VALUE_RESP1 0x00
    #define ADS1292_INIT_VALUE_RESP2 0x02
    #define ADS1292_INIT_VALUE_GPIO 0x0C

    Pierre
  • 0 in reply to Pierre LE COZ
    TI__Guru 63156 points
    Thanks, Pierre.

    CONFIG2[5] must be set to '1'. Currently, you do not have the internal reference buffer enabled. According to your schematic, you do not have the option for an external reference voltage source, so the internal reference must be used.

    Additionally, RESP1[1] must be set to '1' and RESP2[0] must be set to '1'.

    Best Regards,