Ic1a Op Amp
Figure 1.11 Schematic diagram of a capacitive active bioelectrode. Biopotentials are coupled to buffer IC1A through resistor R1 and the capacitor formed by the biological tissues, aluminum oxide dielectric, and aluminum electrode plate. Operational amplifier IC1A is configured as a unity-gain buffer. IC1B drives a shield that protects the input from current leakage and noise. Resistors R3 and R2 reduce the gain of the shield driver to just under unity to improve the stability of the guarding circuit. C1 limits the bandwidth of input signals buffered by IC1.
Op Amp Circuits
The most commonly used op-amp is IC741. The 741 op-amp is a voltage amplifier, it inverts the input voltage at the output, can be found almost everywhere in electronic circuits. Opamp IC1A effectively subtracts the reference voltage (V REF) from a bias voltage (V BIAS) that’s generated by R6 and R7. Therefore, the output of IC1A is a dc potential that lies above. This circuit accomplishes DC restoration using a CA5260 dual op amp (IC1a, IC1b) coupled with a sample-and-hold circuit based on the 74HC4053 switch (IC2). VIN, consisting of the input video signal and a DC offset (VDC), is routed to the non-inverting input of the EL8102 (IC3). Op amps IC1a and IC1b carry a pure sinusoidal signal that alternates symmetrically around a direct voltage of 3 V, whereas that of IC1c alternates around 0 V. This means that this op amp can handle an amplification of x 2.2 much better than the earlier two.
3Hard anodization Super is a process licensed by the Sanfor Process Corporation (United States) to Elgat Aerospace Finishing Services (Israel) and is described in Elgat Technical Publication 100, Hard Anodizing: 'Super' Design and Applications.
contamination. IC1B, also a unity-gain buffer, is fed by the input signal, and its output drives a shield that protects the input from leaks and noise. Resistors R3 and R2 reduce the gain of the shield driver to just under unity in order to improve the stability of the guarding circuit. Capacitor C1 limits the bandwidth of input signals buffered by IC1A. The circuit is powered by a single supply of ±4V dc. Miniature power supply decoupling capacitors are mounted in close proximity to the op-amp.
IC1A and IC1B are each one-half of a TLC277 precision dual op-amp's IC. Here again, the selection of op-amps from the TLC27 family has the additional advantage that ESD protection circuits which may degrade high input impedance are unnecessary because LinCMOS chips have internal safeguards against high-voltage static charges. Note that this circuit shows no obvious path for op-amp dc bias current. This is true if we assume that all elements are ideal or close to ideal. However, the imperfections in the electrode anodiza-tion, as well as in the dielectric separations and circuit board, provide sufficient paths for the very weak dc bias required by the TL082 op-amp.
The circuit is constructed on a miniature PCB in which ground planes, driven shield planes, and rings have been etched. The circuit is placed on top of a 1-cm2 plate of thin aluminum coated with hard anodization Super used as the bioelectrode. A grounded conductive film layer shields the encapsulated bioelectrode and flexible printed circuit ribbon cable, which carries power for both the circuit and the signal output.
Figure 1.12 presents a prototype bioelectrode array designed to record frontal EEG signals measured differentially (between positions Fp1 and Fp2 of the International 10-20 System), as required for an experimental GLOC detection system. One of the bioelec-trodes contains the same circuitry as that described above. The second, in addition to the buffer and shield drive circuits, also contains a high-accuracy monolithic instrumentation amplifier and filters. Such a configuration provides high-level filtered signals which may be carried to remotely placed processing stages with minimal signal contamination from noisy electronics in the helmet and elsewhere in the cockpit.
A miniaturized version of the circuit may be assembled on a single flexible printed circuit. Driven and ground shields, as well as the flat cables used to interconnect the electrodes and carry power and output lines, may be etched on the same printed circuit. As shown in Figure 1.13, the thin assembly may then be encapsulated and embedded at the appropriate position within the inner padding of a flight helmet. Nonactive reference for the instrumentation amplifier may be established by using conductive foam lining the headphone cavities (approximating positions A1 and A2 of the International 10-20 System) or as cushioning for the chin strap.
Figure 1.12 Block diagram of a capacitive bioelectrode array with integrated amplification and filter circuits designed to record frontal EEG signals. One of the bioelectrodes contains the same circuitry as Figure 1.11. The second also contains a high-accuracy monolithic instrumentation amplifier and filters. (Reprinted from Prutchi and Sagi-Dolev [1993], with permission from the Aerospace Medical Association.)
Figure 1.12 Block diagram of a capacitive bioelectrode array with integrated amplification and filter circuits designed to record frontal EEG signals. One of the bioelectrodes contains the same circuitry as Figure 1.11. The second also contains a high-accuracy monolithic instrumentation amplifier and filters. (Reprinted from Prutchi and Sagi-Dolev [1993], with permission from the Aerospace Medical Association.)
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Ic1a Op Amp Diagram
Detects the amount of salt contained in liquid foods, Three-level LED indicatorThis circuit was designed to detect the approximate percentage of salt contained in a liquid. After careful setting it can be useful to persons needing a quick, rough indication of the salt content in liquid foods for diet purposes etc. IC1A op-amp is wired as a DC differential amplifier and its output voltage increases as the DC resistance measured across the probes decreases. In fact, fresh water has a relatively high DC resistance value that will decrease proportionally as an increasing amount of salt is added.
IC1B, IC1C and IC1D are wired as comparators and drive D5, D4 and D3 in turn, as the voltage at their inverting inputs increases. Therefore, no LED will be on when the salt content of the liquid under test is very low, yellow LED D5 will illuminate when the salt content is low, green LED D4 will illuminate if the salt content is normal and red LED D3 will illuminate if the salt content is high. D1 and D2 are always on, as their purpose is to provide two reference voltages, thus improving circuit precision.
At D2 anode a stable 3.2V supply feeds the non-inverting inputs of the comparators by means of the reference resistor chain R8, R9 and R10. The 1.6V reference voltage available at D1 anode feeds the probes and the set-up trimmer R4. One of these two red LEDs may be used as a pilot light to show when the device is on.
Circuit diagram:
R1________________470R 1/4W Resistor
R2,R5______________10K 1/4W Resistors
R3,R6_____________220K 1/4W ResistorsR4__________________5K 1/2W Trimmer Cermet
Ic1a Op Amplifier
R7________________680R 1/4W Resistor
R8__________________2K2 1/4W Resistor
R9,R10,R11,R12,R13__1K 1/4W Resistors
C1________________100µF 25V Electrolytic Capacitor
D1,D2,D3______3 or 5mm. Red LEDs
D4____________3 or 5mm. Green LED
D5____________3 or 5mm. Yellow LED
Ic1a Op Amp Calculator
IC1_______________LM324 Low Power Quad Op-amp
P1_________________SPST Pushbutton
Probes_________________ (See Text)
B1___________________9V PP3 Battery
Probes:
- It was found by experiment that a good and cheap probe can be made using a 6.3mm. mono jack plug. The two plug leads are connected to the circuit input by means of a two-wire cable (a piece of screened cable works fine).
- The metal body of the jack is formed by two parts of different length, separated by a black plastic ring. You should try to cover the longest part with insulating tape in order to obtain an exposed metal surface of the same length of the tip part, i.e. about 8 to 10mm. starting from the black plastic ring.
- In the prototype, three tablespoons of liquid were poured into a cylindrical plastic cap of 55mm. height and 27mm. diameter, then the metal part of the jack probe was immersed in the liquid.
- Wait at least 30 seconds to obtain a reliable reading.
- Wash and wipe carefully the probe after each test.
- To setup the circuit and to obtain a more precise reading, you can use a DC voltmeter in the 10V range connected across pin #1 of IC1A and negative supply.
- Set R4 to obtain a zero reading on the voltmeter when the probe is immersed in fresh water.
- You may change at will the threshold voltage levels at which the LEDs illuminate by trimming R4. Vary R8 value to change D4 range and R9 value to change D5 range.
- P1 pushbutton can be substituted by a common SPST switch.