I need help with Hi-Res ECG with good SNR

Discussion in 'General Electronics Chat' started by MGray, Feb 19, 2008.

  1. MGray

    Thread Starter New Member

    Feb 19, 2008
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    Hi,
    I need to build an ECG system with a resolution of 3000Hz, and good signal to noise ratio. It needs to be battery powered. Also, I need to record three differential inputs (total of 6 leads). I really need some help here... can anyone suggest a circuit diagram I can use???
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. SgtWookie

    Expert

    Jul 17, 2007
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    Well, were the EKG signal a sine wave, 2Hz would probably be sufficient - but they're fairly narrow spikes.

    I agree, the hardware looks OK. However, the output has a low-pass filter with a corner frequency of about 20Hz; far too low for stated requirements.

    MGray, you also stated that you needed to record this data. Were you needing to capture ALL of the data, or simply statistics, like maximum/minimum/average rate, amplitude, etc?

    Recording a minimum of 8 bits of data at a 3kHz rate for three single-ended channels would require 4.2 megabytes of storage per minute.
     
  4. MGray

    Thread Starter New Member

    Feb 19, 2008
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    Thanks for replies beenthere & SgtWookie.

    Yeah, 3000Hz seems quick, but I’m researching alternan activity, which these days requires at least 1000Hz resolution. In addition, I want to use this in an MRI scanner. To remove RF interference (tuned to excite hydrogen proton resonance in a 1.5 tesla field = 60mHz) I need a little extra resolution.

    I will need to record the entire waveform... I am currently working with a dataset in which we recorded blood pressure waveforms, transforming analogue waveforms into fibre optic frequency signals at a resolution of 40,000Hz. We recorded this and other signals using Spike2 (Cambridge Electronic Design) for approximately 30mins. The files were rather big.

    I am trying to give our electronics boys a few suggestions about the circuit. I was thinking of this basic circuit:
    http://plus1plus1plus.org/Resources/EEG_EKG/eeg_ekg04.11x17.pdf

    ..replacing the first section with something like Linear Technology's LTC1057, an INA128 or an OP177.

    Id also need to ensure the current limiting resistors included and up to scratch.

    Any adive greatly appreciated
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    The INA128 for the 1st section would be good. You really need to use an instrumentation amp, as they're internally trimmed - otherwise, you'll be fiddling around with precision pots forever trying to get the CMRR down. :rolleyes: With the INA128, you just select the gain you want with a single resistor, and you're done.

    I do like the LTC1057 for the subsequent stages. You could possibly get away with eliminating the amplification stage, and use a single LTC1057 or OPA200 for the high/low pass filter & amplification section. Haven't fiddled around in SPICE with it yet though.
     
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