Small 2-Lead ECG

This small 2-Lead ECG analog frontend breadboard compatible was developed by Matthew Shieh (external link)

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Documentation presented here is also available for download as pdf files.
Data sheet - ECG2_AM1 DataSheet.pdf (1.25 MB)
Schematics - ECG2_AM1 Schematic.pdf (41.88 KB)
Bill of materials - ECG2_AM1 BOM.pdf (33.95 KB)

Features

• Single Supply Operation
• Quiescent Current: 1.57mA at 3.3v
• External Reference Input
• Internal Reference at VDD/6
• 100k , Protected Inputs
• Built in 60Hz Notch Filter
• Built in Antialiasing Filter
• Servo Feedback DC Removal
• Buffered Output for direct ADC
interface
• Small Size 1.0”x0.71”
• Breadboard 0.1” pitch compatible

Applications

• Medical Instrumentation Testing
• Off-the-shelf Component
• Data Acquisition

Description

The ECG2_AM1 is a fully functional implementation of a 2-lead electrocardiogram (ECG) analog frontend.
It consists of input protection, amplification, 60Hz notch filtering, antialiasing, and buffering for direct connection to an analog-to-digital converter (ADC). The module has been designed to offer the user a low noise, low offset, and low power solution in a small package that is compatible with 0.1” pitch breadboards for easy prototyping. A minimum of external components, such as decoupling capacitors and power regulation, are all that are needed for its proper use. For more control, a reference pin is provided so that the user may dynamically adjust the virtual ground of the output signal. If this control is not desired, the system intrinsically generates a reference voltage that
positions the output signal at about 1/6 the differential voltage between the positive and negative
power supply rails. An internal bias is also provided to the inputs so that a no input signal condition will result in an output with a minimum chance of saturation from stray interference. Single and dual rail power supplies ranging from 2.7v (±1.35v) to +5.5v (±2.75v) can be used with the module. Current consumption over the allowed voltage gamut ranges from 1.514mA to 1.688mA assuming room temperature. Temperature tolerance is specified between -20°C to 80°C.

Notice

This module has not been approved for use in the diagnosis and monitoring of any physiological
activity and/or illness. Thus, this module is provided “as-is” and all necessary precautions must be taken with its use.

Figure 1: ECG module compared to a US quarter dollar

Absolute Maximum Ratings

Supply Voltage…………………………………...+6v
Signal Input………………………………....V- to V+
Reference Input……………………………..V- to V+
Output Short Circuit…………………….. continuous
Operating Temperature…………….... -20°C to 80°C
Storage Temperature………………...-40°C to 120°C

Electrostatic Discharge

This module is dependent upon the protection offered by the semiconductors that it uses. Therefore, care
must be taken in that the user handles and uses the module under appropriate conditions.

Pin Designations

Figure 2: Top copper pads and board outline of 2-lead ECG Frontend module. Not to scale.

Table 1: Pin Designation

Functional Block Diagram

Typical Characteristics

All measurements are taken at 25°C, single supply 3.3v, and with a greater than 1M, load (10:1 probe) unless otherwise specified.

Application Examples

Quick Setup with Internal Reference

As shown in the figure below, very few components are needed to quickly set up and run the ECG module. The two capacitors used are for decoupling the power supply and further suppressing any noise present on the internal voltage reference. The two orange wires, one from the “OUT” pin and the other from the power supply ground, are attached to an oscilloscope. The two white connectors are vertical friction lock PCB headers used to interface ECG electrode leads (not pictured) to the module from commercial electrode pads.

Figure 14: Very basic ECG module setup with internal reference

Friction Lock PCB Headers: http://www.mouser.com/Search/Refine.aspx?Keyword=22-05-3031 (external link)

ECG Leads: http://www.orsupply.com/product/-Lead-ECG-Snap-Set-Leadwires/490/category/Medical-Monitoring- (external link)

Supplies/108/category/ECG-Supplies/749/category/ECG-Leads-EKG-Lead-Wires/208
ECG Electrode Pads: http://www.buyemp.com/product/11229750.html (external link)

Figure 15: Suggested ECG electrode placement – Image courtesy of “Gray’s Anatomy” by Henry Gray

Quick Setup with External Reference

In some cases, it is advantageous to be able to dynamically control the reference voltage on which the amplified ECG signal sits on in order to add additional functionality such as base line drift compensation and QRS saturation avoidance. Modification of the board itself, which consists of removing capacitor C5, may be needed depending upon the voltage source used and how fast the reference voltage must change during operation. In any case, a low impedance voltage source that is capable of sourcing at least 1mA must be used to override the internal voltage reference. Shown in the figures below are three typical ways in which an external reference may be generated.

Quick Setup with Heart Beat (QRS) Indicator

Minimal functionality of the ECG module can be evaluated without the need for an oscilloscope by using a very simple 1-bit ADC. The threshold of the 1-bit ADC, or comparator in this case, can be statically and/or dynamically set such that the occurrence of a QRS complex can be easily detected amongst other electrical activity and noise. In this example, a LED was used to indicate the occurrence of the QRS complex. It is important to note that when watching the LED and feeling for a pulse, there is a slight lag due to the coupling between electrical activity and the propagation of physical action. To elucidate, there is a delay because it takes time for the ventricles to move from a state of isovolumic contraction to maximum ejection where upon the initiation of a physical pulse is felt.

Figure 19: ECG Module Interfaced with a Comparator for QRS Detection – The comparator used in this example is TI’s TLV3012 which contains an internal reference that is set to 1.242v. The voltage divider at the + input of the comparator is used to situate the input signal such that it will appropriately vary around the voltage reference.

Figure 20: QRS Detection Schematic – The resistor values used in the schematic were calculated assuming a Vdd = 3.3v and a reference of 1.242v. The comparator reference can alternatively be sourced from one of the three suggested possible solutions laid out in figures 16 through 18.
Comparator: http://focus.ti.com/lit/ds/symlink/tlv3011.pdf (external link)

Parameter Setting

All resistors are 0402 sized thick-film. All capacitors, with the exception of C1, C2, and C3, are 0402 sized and consist of either C0G or X7R (EIA Class-1 or Class-2) dielectric material for signal path or decoupling respectively. Capacitors C1, C2, and C3 are 1206 sized and contain C0G dielectric. Resistor values are in Ohms, and capacitor values are in Farads unless otherwise specified.

Virtual Ground

Although the module is fully capable of intrinsically setting its virtual ground to approximately Vdd/6, assuming single supply, there may be instances in which the user or controlling system needs to dynamically vary it. Dynamic variation is made possible by providing a low impedance voltage source to the “REF” pin of the module as illustrated in the “Quick Setup with External Reference” example. Note that the removal of C5, a decoupling capacitor, may need to be done to avoid instability of the voltage source (op-amp) and/or severe bandwidth limiting. In the case where an external reference is not needed, C5 can be left alone to reduce noise that is normally present on the internal reference due to variations in the power supply and Johnson noise generated by the resistors. In addition, one can attach additional capacitance to the “REF” pin for further noise reduction.

Gain

The overall gain of the module is set at default to about 1000v/v. The bulk of this gain is contributed by op-amp U4B that sits on the bottom side of the board. The ratio of two nearby resistors, R16 and R17, dictate the amount of gain that this op-amp provides to the module. A change in gain provided by U4B requires either a change in value for R16 or R17. Alternatively, resistor R7, which dictates the gain of the instrumentation amplifier, may be changed as well. Selection of which resistor to change is primarily dictated by whether one wishes to increase or decrease the gain of the overall system at the expense of some other parameter(s). One example would be if one were to increase the gain by decreasing the value of R17. An advantage of making such a change would be that Johnson noise contributed by R17 itself would become lower by the square-root of the resistor value in regards to spectral density. A Disadvantage, however, would be that the gain stage would present a lower load impedance to the notch filter stage. Note that changing the value of R16 will also change the antialiasing filter’s cutoff frequency since the gain stage also functions as an antialiasing filter.

Antialiasing Filter Cut Off Frequency

The antialiasing filter’s cut-off frequency can be changed by either varying the value of C4 and/or R16. Changes to R16 will affect the overall gain of the system because it also functions as the feedback resistor in the inverting amplifier. If changing the value of C4 is not possible, the consequences of setting R16 can be mitigated by setting the value of R17 to compensate for any changes made to the gain.

Notch Filter Center Frequency

The built in notch filter’s center frequency can be changed by either varying two sets of resistors or a set of capacitors. The two sets of resistors (R6, R11) and (R1, R2) must be equal in sum to one another for the filter to function correctly. Similarly, capacitors C1 and C3 must be equal in value to one another for correct functionality. Selection of whether to vary either the two sets of resistors or the set of capacitors is dependent upon availability of the value(s) needed and their effects on the entire system. For example, varying the two sets of resistors will change
the Q-factor.

Notch Filter Q Factor

The Q-factor, or quality factor, of the notch filter may be set by changing a set of resistors (R8, R12). Generally speaking for a notch filter, a higher Q-factor results in a narrower (“sharper”) notch and vice versa. A narrower notch may be desired if one wishes to preserve as much energy in the frequencies surrounding the center frequency as possible. A smaller Q-factor may be desirable if the frequency to be rejected has some small variance associated with it. In any case, one should try to select a high enough Q-factor such that very little energy of the surrounding frequencies is removed while maintaining filter response