Instrumentation amplifier using an opamp
An instrumentation amplifier is a kind of differential amplifier with additional input buffer stages. The addition of input buffer stages makes it easy to match (impedance matching) the amplifier with the preceding stage. Instrumentation is commonly used in industrial test and measurement application. The instrumentation amplifier also has some useful features like low offset voltage, high CMRR (Common mode rejection ratio), high input resistance, high gain etc. The circuit diagram of a typical instrumentation amplifier using opamp is shown below.
A circuit providing an output based on the difference between two inputs (times a scale factor) is given in the above figure. In the circuit diagram, opamps labeled A1 and A2 are the input buffers. Anyway, the gain of these buffer stages is not unity because of the presence of R1 and Rg. Op-amp labeled A3 is wired as a standard differential amplifier. R3 connected from the output of A3 to its non-inverting input is the feedback resistor. R2 is the input resistor. The voltage gain of the instrumentation amplifier can be expressed by using the equation below.
Voltage gain (Av) = Vo/(V2-V1) = (1 + 2R1/Rg ) x R3/R2
If need a setup for varying the gain, replace Rg with a suitable potentiometer. Instrumentation amplifiers are generally used in situations where high sensitivity, accuracy, and stability are required. Instrumentation amplifiers can be also made using two opamps, but they are rarely used and the common practice is to make it using three opamps like what is shown here. The only advantages of making an instrumentation amplifier using 2 opamps are low cost and improved CMRR.
A high gain accuracy can be achieved by using precision metal film resistors for all the resistances. Because of large negative feedback employed, the amplifier has good linearity, typically about 0.01% for a gain less than 10. The output impedance is also low, being in the range of milli-ohms. The input bias current of the instrumentation amplifier is determined by the op-amps A1 and A2.
A simplified instrumentation amplifier design is shown below. Here the resistances labeled R1 are shorted and Rg is removed. This results in a full series negative feedback path and the gain of A1 and A2 will be unity. The removal of R1 and Rg simplifies the equation to Av = R3/R2.
Practical instrumentation amplifier using an opamp.
A practical instrumentation amplifier circuit designed based on uA 741 op-amp is shown below. The amplifier operates from +/-12V DC and has a gain of 10. If you need a variable gain, then replace Rg with a 5K POT. Instead of using uA741 you can use any opamp but the power supply voltage must be changed according to the op-amp. A single LM324 op-amp Ic is a good choice. Out of the four opamps inside the LM324, three can be used for IC1, IC2, IC3 and the remaining one can be left alone. This reduces the PCB size a lot and makes the circuit compact. The supply voltage for LM324 can be up to +/-16V DC.
An instrumentation amplifier is a differential amplifier optimized for high input impedance and high CMRR. An instrumentation amplifier is typically used in applications in which a small differential voltage and a large common mode voltage are the inputs.
ECG Machine Working Principle
How to measure heart activity?
Heart activity can be recorded in two ways:
1. Electrocardiography (ECG, EKG)
ECG records the electrical activity generated by heart muscle depolarizations, which propagate in pulsating electrical waves towards the skin. Although the electricity amount is in fact very small, it can be picked up reliably with ECG electrodes attached to the skin (data unit: microvolt, uV). The full ECG setup comprises at least four electrodes which are placed on the chest or at the four extremities according to standard nomenclature (RA = right arm; LA = left arm; RL = right leg; LL = left leg). Of course, variations of this setup exist in order to allow more flexible and less intrusive recordings, for example, by attaching the electrodes to the forearms and legs. ECG electrodes are typically wet sensors, requiring the use of a conductive gel to increase conductivity between skin and electrodes.
2. Photo-Plethysmography (PPG).
Throughout the cardiac cycle, blood pressure throughout the body increases and decreases – even in the outer layers and small vessels of the skin. Peripheral blood flow can be measured using optical sensors attached to the fingertip, the earlobe or other capillary tissue. The device has an LED that sends light into the tissue and records how much light is either absorbed or reflected the photodiode.
Why combine ECG with other sensors?
Of course, biometric data based on heart rate alone offers valuable insights into subconscious arousal in response to emotionally loaded stimulus material. However, solely based on ECG or PPG data we can‘t be sure whether the arousal was due to positive or negative stimulus content.
Why? The change in heart rate is in fact identical. Both positive and negative stimuli can result in an increase in arousal triggering changes in heart rate.
In other words: While ECG/PPG are ideal measures to track emotional arousal, they are not able to reveal emotional valence, that is, the direction of an emotion. The true power of ECG/PPG techniques unfolds as these sensors are combined with other biometric sources such as facial expression analysis, EEG, and eye tracking.
Physiology and function of the heart:
Before we dig deeper into the fundamentals of ECG, let’s briefly recap on heart physiology and function:
- The heart has four chambers. The upper two chambers (left/right atria) are entry-points into the heart, while the lower two chambers (left/right ventricles) are contraction chambers sending blood through the circulation. The circulation is split into a “loop” through the lungs (pulmonary) and another “loop” through the body (systemic).
- The cardiac cycle refers to a complete heartbeat from its generation to the beginning of the next beat, comprising several stages of filling and emptying of the chambers. The frequency of the cardiac cycle is reflected in heart rate (beats per minute, bpm).
- The heart operates automatically – it is self-exciting (other muscles in the body require nervous stimuli for excitation). The rhythmic contractions of the heart occur spontaneously, but are sensitive to nervous or hormonal influences, particularly to sympathetic (arousing) and parasympathetic (decelerating) activity.
Cardiac parameters of interest:
- Heart Rate (HR). HR reflects the frequency of a complete heartbeat from its generation to the beginning of the next beat within a specific time window. It is typically expressed as beats per minute (bpm). HR can be extracted using ECG and PPG sensors.
- Inter-Beat Interval (IBI). The IBI is the time interval between individual beats of the heart, generally measured in units of milliseconds (ms). Typically, the RR-interval is used for the analysis.
- Heart Rate Variability (HRV). HRV expresses the natural variation of IBI values from beat to beat. HRV is closely related to emotional arousal: High-frequency (HF) activity has been found to decrease under conditions of acute time pressure and emotional stress. Also, HRV seems to be significantly reduced in individuals reporting a greater frequency and duration of daily worry, as well as in patients suffering from post-traumatic stress disorder (PTSD). For IBI and HRV analysis, ECG sensors are recommended as they are more sensitive to certain signal characteristics which PPG sensors cannot pick up.