a simple electrocardiogram board

Posted on Feb 5, 2014

Following the same idea as the Chipstein and Cornell designs, the circuit uses an instrumentation amplifier (the AD620) to take the voltage difference between two spots on the body. A second amplifier (the CA3140) amplifies this differential signal. A potentiometer is used to siphon off current from the negative input of the second amplifier, and

a simple electrocardiogram board
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tuned to remove the DC offset resulting from the inevitable and unpredictable static voltage differences between any two spots on the body: unless removed, the DC offset causes the second amplifier to saturate its output at power or ground. Because we power the amplifiers using only the +5V and ground levels from an Arduino (or other micro-controller board), we need to split the supplies for the amplifiers, which require both a positive and a negative supply line. To do so, we use a voltage divider circuit to define a virtual ground at 5V/2 = 2. 5V, and then use +5V and Arduino ground as the positive and negative supply lines, respectively. An electrode held at ground electrode is also placed on the body at a third location. Low and high pass filtering are performed in-between amplification steps and during the second amplification step, and then a bank of three low-pass filters follows after amplification to remove additional 60 Hz noise. Those interested in understanding the exact frequency response of the system should do a full calculation (e. g. , using the Op Amp golden rules and the rules for frequency-dependent impedance of resistors and capacitors) or a Spice simulation, because the filters going into and through the second amplification stage cannot be treated simply as individual high and low pass filters in series. In addition to serving as the power source, the Arduino is also used to sample the amplified...

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