The solution to both high-gain and high-input impedance problems is the instrumentation amplifier (INA). This circuit uses three opamps.
Figure 3 Click on image to enlarge
Use the inverting amplifier equations to derive an equation for the gain of the first stage. The input is the difference between the two points connected to positive of opamp and the output is the difference between the two outputs of the opamps. This gain of the first stage comes to Av¬ = 2(R1/R0) + 1. This is then given to the final stage which in itself is a differential stage. So finally the output from the circuit will take into consideration this too – thus the final gain will be [2(R1/R0) + 1] [R5/R3]. The main advantages of INA's are: ability to obtain high gain with low resistor values, extremely high input impedance, and superior rejection of common mode signals. Modern INA's are of the monolithic IC form with terminals for the R0 resistor which is usually variable and can be used to control the gain. You can see details about these ICs here.
As a conclusion, it should be noted that there are more complexities involved. There will be a filter connected to this amplifier as well as an isolation amplifier involved mostly to eliminate noise. There will also be circuitry to null out a dc offset introduced by the electrodes that are connected to the patients. We will try to introduce them in the coming weeks. For now this is just an introduction to a simple bioelectric amplifier and its concept as well as its implications in ECG. Visit the following sites to try designing these circuits: http://www.docircuits.com/circuit-editor/205 http://www.docircuits.com/circuit-editor/206 *60Hz noise is the noise introduced due to electromagnetic disturbance from other electronic appliances in the room along with the ECG machine. About the author: Shivananda Koteshwar (Shivoo) works as a director, PES Group, responsible for "The Amaatra Academy," and is a full-time professor in E&C Department in PES Engineering College.
In Let's Do Circuits!, Shivoo supports and encourages young engineers to find the joys of circuit design. Courtesy of EETimes India See related links:
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