Hard-core dry goods丨Online QA with engineers, a collection of common problems of the star device AD8232

Today, I will share with you some common problems of AD8232 for your reference when using this chip.

It is AC coupled. The AD8232 has AC coupling integrated in its internal structure.

The AC coupling system can remove the influence of the DC signal of the half-cell potential caused by the electrode. After removing the DC component, the amplifier at the back of the system can amplify the signal of interest with a large gain multiple. After the AC coupling signal is processed and amplified, the ADC with medium and low resolution can meet the design requirements, and many MCU integrated ADCs can meet the requirements.

The DC coupling solution is limited by the amplification factor. The signal amplitude after signal conditioning is relatively small, and a high-resolution ADC is required to achieve the same dynamic range. High-resolution ADCs are usually more expensive, larger in size, and consume more power.

AC coupling or DC coupling needs to be selected according to system requirements, there is no difference between good and bad. The power consumption of AC coupling design will be lower than that of DC coupling.

AD8232 is a single-lead solution that amplifies and filters the differential signal of a pair of two electrodes. A lead requires at least two electrodes, and a third electrode can be used as the right leg drive to suppress common-mode interference.

In summary, the AD8232 can only be used as a single-lead solution, using two or three electrodes.

Diagnostic ECG equipment is found in hospital clinics, bedside wards, ICUs, etc. It requires low noise, wide bandwidth, and very good signal quality.

AD8232 is mainly designed for heart rate detection and some ECG monitoring applications such as home ECG equipment, sports equipment, and emerging applications such as gaming and entertainment equipment, automobiles, notebooks, mobile phones, watches, etc. For diagnostic ECG applications, you can refer to ADI ADAS1000 series products:

http://www.analog.com/cn/products/application-specific/medical/ecg.html

The AD8232 Filter Design Tool can be downloaded from the AD8232 page. This tool helps users design bandpass filters through a graphical interface and helps users determine the values ​​of the required external resistors and capacitors.

At least a first-order high-pass filter is required, and a second-order or third-order high-pass filter can also be used. A low-pass filter is optional and recommended.

There are two design structures you can start with:

For applications where only heart rate is measured and the shape of the ECG signal waveform is not important, a narrower bandpass filter can be used. A filter between 7Hz and 25Hz is recommended. Signals with frequencies below 7Hz are attenuated, which can effectively suppress the effects of activity and is recommended for sports applications.

For applications where the shape of the ECG waveform is important, a 0.5Hz to 40Hz filter is recommended. Because some low-frequency signals can pass through the filter, this design is more sensitive to motion artifacts and baseline drift.

A two-order filter can be used to balance the number of components and distortion. A first-order filter uses fewer components and has a slower filter roll-off; if a steeper filter is required, an additional first-order RC can be added to use a third-order filter.

AD8232 is an AC-coupled solution that uses a high-pass filter to filter out DC and near-DC frequency signals. Because the cutoff frequency is relatively low, which can be 1Hz or lower, the settling time to remove the DC signal is relatively long. For example, after connecting the subject, the measurement may require a delay of several seconds. This happens every time the connection is lost or the electrode is not in good contact due to movement.

If the fast recovery function is used, the fast recovery circuit automatically detects signal loss and increases the cutoff frequency of the high-pass filter. This shortens the settling time. This function is achieved by controlling a small resistor in parallel with an external large resistor.

When the signal is established, the device automatically switches back to normal mode. The fast recovery function of AD8232 is completed automatically and does not require external controller to participate in control.

The right leg drive circuit can eliminate common-mode signal interference that affects biopotential signal measurement. To improve performance, a three-electrode solution is recommended, with the third electrode providing the output of the right leg drive amplifier.

In the two-electrode system, the right leg drive amplifier output is not connected to the electrode, but is connected to VIN+ and VIN- through pull-up resistors. The circuit performance is between the two-electrode and three-electrode solutions.

For heart rate monitoring applications with shorter lead wires, such as chest strap heart rate monitoring, the right leg driver amplifier .

One of the challenges in bioelectric signal detection is to suppress the influence of common-mode voltage from the human body, electrodes, and wires. This is why an instrumentation amplifier with a high common-mode rejection ratio is required.

The 50/60Hz power frequency electrical signal coupled by capacitor is the main source of common mode interference. The human body is a good conductor. When the human body is close to electrical equipment (such as hospital bedside monitoring equipment, surgical equipment, etc.) or the system lead wire is long, the interference signal will be enhanced.

One way to reduce interference is to drive the body to a known level instead of a floating level. This can be achieved by using a DC voltage driver, and performance can be improved by dynamically driving according to the common mode voltage. The common mode voltage changes of the two electrodes can be monitored and usually driven back to the body according to a reference voltage (usually half the supply voltage is chosen).

p.s. If you have any questions, whether it is AD8232 or other devices, you can ask questions in our Chinese technical forum~

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