Internet Explorer is not a supported browser for TI.com. For the best experience, please use a different browser.
Video Player is loading.
Current Time 0:00
Duration 14:19
Loaded: 0.00%
Stream Type LIVE
Remaining Time 14:19
 
1x
  • Chapters
  • descriptions off, selected
  • en (Main), selected

    Hello, and welcome to the next video in the series. In this video, we will talk about electrode configuration and interface circuitry for an ECG in wearable devices.

    We will start with an overview of ECG signal acquisition on wearable devices. We will specifically look at the challenges posed by the high contact impedance resulting from small form factor electrodes. Next, we will look at the different electrode configurations and compare 2-electrode and 3-electrode configurations. We will specifically examine the role of the right leg drive electrode in suppressing common mode interferences.

    We will also look at the electrode interface circuitry. This is additional circuitry that might be needed between the ECG electrodes and the ECG input pins of the analog front end. We will examine the merits and demerits of DC and AC coupled configurations. We will also look at the considerations related to additional buffering and filtering that may be required between the ECG electrodes and the analog front end.

    This illustration depicts possible electrode locations for ECG signal acquisition on a wearable device. In this illustration, E1 and E2 are electrodes at the bottom surface of the wearable device that make contact with the left wrist of the user. Electrodes E3 and/or E4 are on the top side, and one or both of them can be contacted by the right hand of the user whenever the ECG signal is to be acquired. This illustration shows the user making contact using the fingers on the right hand to the electrodes on the top side of the watch whenever an ECG signal needs to be acquired. The biopotential associated with the electrical activity of the heart in the vector direction between the right and left arm is recorded by such a system.

    A sample ECG signal is also shown. The waveform in gray depicts the raw data from the device. Additional low-pass filtering may be used to reduce the noise, resulting in the cleaner waveform in blue.

    The illustration depicts the connection of the electrodes on the smartwatch to the input circuitry of a TI analog front end. In this illustration, three electrodes are shown. The three electrodes are labeled LA for left arm, RA for right arm, and RLD for right leg drive. These terminologies are carried over from naming conventions in a clinical ECG system.

    The illustration shows the LA and RLD electrodes in contact with the user's left wrist and the RA electrode on the side of the watch. The number and position of these electrodes can vary based on the mechanical design of the watch. The potential between the RA and LA electrodes is differentially gained up by the instrumentation amplifier of the analog front end.

    A feedback loop drives the RLD electrode through an RLD buffer. The intent of the RA feedback loop is to drive the body with a common mode potential so as to suppress any common mode interferences picked up by the body. As we will see in the latter part of the presentation, a common mode interferer picked up by the electrodes can translate into a differential signal and can affect the quality of the ECG signal, so the RLD electrode plays an important role in the ECG signal acquisition system.

    Here, we see the same electrode interference as previously shown, but with a contact impedance introduced between each electrode and the corresponding pin of the analog front end. Such a contact impedance results from the contact of the electrode to the skin and is depicted as a parallel resistor and capacitor. The contact impedance plays a significant role in the ECG signal quality, and it governs several aspects of the system design.

    A wearable device has dry electrodes of a small size. Factors like poor contact with the electrodes and dry skin can result in a scenario where the contact impedance is high. Getting good ECG signals in such a scenario of high contact impedance can be challenging.

    Some of these challenges are as follows. High contact impedance can cause ECG signal attenuation. High contact impedance also adds thermal noise, and this might warrant low-pass filtering before the signal is converted by the ADC. Additionally, a mismatch in the contact impedance between the electrodes can degrade the Common Mode Rejection Ratio, or CMRR, of the system. We will look at each of the above factors in more detail.

    First, let's look at the effect of the contact impedance on ECG signal attenuation. The network comprising the electrode contact impedance and the input impedance of an ECG signal chain can be thought of as a resistor divider, so if the contact impedance is comparable to the input impedance of an ECG signal chain, it can result in the attenuation of the ECG signal. Since the bandwidth of interest for the ECG signal is from a low frequency of 0.05 Hertz or 0.5 Hertz to a high frequency of 40 Hertz or 150 Hertz, we are primarily interested in the impedance at low frequency, usually 10 Hertz.

    So at 10 Hertz, we would like the AC input impedance of the ECG signal chain to be much larger than the AC contact impedance. This is usually an easy constraint to satisfy for the AFE itself. Care needs to be taken to not add additional shunt components that could cause the input impedance to get lowered.

    The second undesirable effective of the contact impedance is the thermal noise introduced by the resisted part of the contact impedance. The portion of this noise that is within the ECG signal band results in an unavoidable reduction of the SNR of the ECG signal. However, the portion of this noise that is out of the ECG signal band should be band limited before conversion by the ADC to prevent additional aliased noise from affecting the SNR. For this reason, it is essential to have a low-pass filter before the ADC which acts like an anti-aliasing filter. Many of the TI AFEs have such a low-pass filter between the instrumentation amplifier and the ADC.

    Next, let's take a closer look at the mechanism of common mode interference and the role of the RLD, or right leg drive electrode, in suppressing it. The mains frequency of 50 or 60 Hertz can get capacitively picked up by the user wearing the watch. Such a pickup is assumed to be a common mode interference with the same signal appearing on both the positive and negative electrodes.

    The common mode signal on the electrodes has an undesirable effect, as we shall see in the following slides, and the right leg drive plays a role in suppressing this effect. The common mode signal at the electrodes can get converted into a differential signal at the ECG input pins. An imbalanced network results if the electrode contact impedance is mismatched between the positive and negative sides.

    Depending on the level of mismatch and the input impedance of the ECG signal chain, a fraction of the common mode interference can appear as a differential signal at the ECG input pins. This shows up as tones at the fundamental frequency of 50 or 60 Hertz and its harmonics at the output of the ECG signal and might require additional digital filtering to remove it from the ECG signal. The way to reduce the common mode to differential conversion is to have low contact impedance and to maximize the AC input impedance of the ECG signal chain.

    Next, we look at the role of the RLD, or right leg drive, electrode. The RLD amplifier drives the body through a third RLD electrode. The signal on the RLD electrode is inverted with respect to the common mode interference being picked up by the body, and its effect is to cancel or suppress the common mode pickup.

    The level of common mode cancellation signal on the RLD amplifier output is controlled through a feedback loop that attempts to make the common mode voltage at the ECG input pins equal to a DC reference voltage referred to as VCM. Any AC common mode signal at the ECG input pins results in a corresponding signal at the output of the RLD amplifier. The signal at the output of the RLD amplifier has an appropriate phase and amplitude to cancel or suppress the common mode signal picked up by the user.

    The loop gain of the feedback loop is provided by the gain of the RLD amplifier at the mains frequency. A larger loop gain means that the appropriate cancellation signal can be generated at the RLD amplifier output with a small residual signal at the ECG pins. In order to keep the RLD feedback loop stable, the TI analog front ends require an external capacitor at the output of the RLD amplifier. It is also to be noted that high contact impedance in the RLD electrode can reduce the effectiveness of the common mode suppression from the RLD loop. Therefore, it is essential to have good contact with the RLD electrode.

    Next, let's look at 2- and 3-electrode configurations. When only two electrodes are available, there needs to be a mechanism to set the common mode bias on the ECG input pins so that the instrumentation amplifier operates within a normal range of operation. Usually, large resistors are used to bias the input pins to the RLD bias. In an AC coupled configuration, the electrodes are connected to the ECG input pins through an AC coupling capacitance that serves as a DC block between the body and the electronics. In a DC coupled configuration, the bias resistors serve as a mechanism to set the DC bias at the ECG input pins as well as the DC voltage on the body.

    Note that the bias resistors need to be chosen to be large values so that they do not reduce the input impedance of the ECG signal chain. However, the larger the bias resistors, the less effective the RLD is in suppressing the common mode picked up by the body. The 3-electrode configuration overcomes drawbacks of the 2-electrode configuration by dispensing with the need for the bias resistors to set the DC bias.

    In a 3-electrode configuration, a pair of electrodes are used for the ECG input, and a third electrode is used for the RLD drive. This configuration provides the best drive for the RLD and maximizes suppression of the common mode interference. Also, no extra components are introduced at the input of the ECG signal chain, so the input impedance of the ECG signal chain is not reduced. It is also possible to realize a 3-electrode configuration with AC coupled electrodes.

    Next, let's compare the pros and cons of DC coupled and AC coupled interferences. DC coupling allows a direct connection to the electrodes without shunt components at the input of the ECG signal chain, thereby achieving a high input impedance. Also, through a strong RLD connection, the common mode pickup can be strongly suppressed, resulting in the best common mode rejection ratio. A DC coupled interference also allows both AC and DC biasing schemes for the detection of leads on and leads off.

    The DC coupled configuration suffers from one drawback. The electrodes can have a differential DC offset that can be several of millivolts. To handle the differential DC offsets of the electrodes, the gain of the instrumentation amplifier may need to be set to a relatively low value, resulting in a higher noise than would have been possible with a higher gain setting.

    The AC coupled configuration helps to block the DC offset, thereby allowing a higher gain setting in the instrumentation amplifier. However, the ECG input pins need to be biased through a resistor marked as R bias. A low value of R bias results in a reduced input impedance. On the other hand, a high value of R bias results in weaker RLD drive and therefore poor common mode rejection ratio.

    Also, the combination of the AC coupling capacitor, CAC, and the bias resistor, R bias, forms a high-pass filter. In order to pass the ECG signal band, the corner of this high-pass filter needs to be set to a low value in the range of 0.05 Hertz or 0.5 Hertz. However, such a low corner results in a large time constant for the filter. This can cause the initial signal acquisition to take a long time. Also, any change in the DC offset can result in a slow recovery.

    As mentioned earlier, the contact impedance can be a source of additional noise, which needs to be low-pass filtered before conversion by the ADC. The electrodes can also pick up other interferers that need to be low-pass filtered. If the analog front end does not have an inbuilt low-pass filter, then it might be required to add external low-pass filtering between the electrodes and the ECG input pins. An addition of such an external low-pass filter can result in a lowering of the input impedance of the ECG signal chain.

    To introduce the external low-pass filter without reducing the input impedance, an external buffer may be required. The buffer adds to power consumption of the overall system and is another source of noise. An internal integrated low-pass filter between the instrumentation amplifier and the ADC solves these issues. The low-pass filter acts as an anti-aliasing filter for the ADC, helping to filter out-of-band noise. With the low-pass filter integrated into the chip, a direct connection to the electrode is possible without the need of an external buffer.

    This concludes our introduction to the care-abouts related to interfacing with ECG electrodes in wearable devices. For more information, visit ti.com/medical. Thanks for watching.