Amplifier input impedance

It is important to be able to calculate the system parameters from the model of the sensor. Let us determine the cut-off frequency for a piezoelectric sensor which has C = 400 pF and leakage resistance of 10 Gfi. The amplifier input impedance is 10 MS2. If we use the modified piezoelectric sensor equivalent circuit (figure 2.1(b)) for this calculation, we find that the cut-off frequency for this circuit is... 

Note that if we increased the amplifier input impedance by a factor of 100, we would lower the low-corner frequency to 0.40 Hz. 

The amplifier gain can be reduced by using a PMT load resistance or amplifier input impedance of more than 50 The increased load resistor in conjunction with the cable capacitance results in a pulse broadening without loss in amplitude. Load resistors of the order of 1 kf2 can be used. To avoid baseline drift in the amplifier, AC coupling is used. The coupling constant is selected as to minimise the baseline walk at higher pulse rates. Nevertheless, pile-up in the amplifier limits the useful pulse rate to typically a few 10 kHz. The general measurement setup is shown in Fig. 6.22. 

Operational amplifier. A linear, high-gain DC voltage or current amplifier with high input impedance, low output impedance, and the capability of producing a bipolar output from a bipolar input. 

Electronic instrumentation is available for the measurement of D.C. and A.C. voltage, current and power as well as impedance. Such instruments usually have higher sensitivities, operating frequencies and input impedance than is normally found in the electromechanical instrumentation described above. However, they may need to incorporate amplifiers and they invariably need power to operate the final display. Hence, an independent power source is needed. Both mains and battery units are available. The accuracy of measurement is very dependent on the amplifier, and bandwidth and adequate gain are important qualities. 

An example of the experimental setup for the measuring of extracellular action and resting potentials is shown in Fig. 4. All electrochemical measurements can be conducted at constant temperature inside a Faraday cage mounted on a vibration-stabilized table in a laboratory (Fig. 4). Ag/AgCl electrodes were connected to a voltmeter/pFl meter [Cole Palmer Microcomputer pFl-vision Model 05669-20, Fig. 4(a)] with high input impedance or a programmable electrometer/amplifier [Keithley-2000/20, Keithley-6517, or Keithley-6514, Fig. 4(b)]. An IBM-compatible microcompu-... 

When constituted of metals, thermopiles exhibit a very low noise, in particular only thermal noise if the voltage amplifier used for signal amplification has a very high input impedance. 

Another limiting factor is the bandwidth of the op-amp. On the factory specifications, the commonly used indicator is the gain-bandwidth product. The nominal dc gain is valid up to a cutoff frequency / which is typically 10 Hz. Above that frequency, the gain g is inversely proportional to the frequency. The product of gain and frequency, the gain-bandwidth product/a is typically 1 MHz. The input impedance of the amplifier increases with frequency ... 

Several types of reference electrodes are convenient for use in analytical electrochemistry. The use of high-input-impedance operational amplifiers in the reference electrode inputs of potentiostats ensures that very low levels of current are drawn from the reference electrode (see Chap. 6). This permits the use of reference electrodes that do not have to contain a large number of redox equivalents in order to ensure a constant reference potential and are therefore very small. Three reference-electrode designs that are convenient for use in analytical electrochemistry are shown in Figure 9.4. Saturated calomel and silver-silver chloride (of various concentrations of chloride) are among the most common commercially available or conveniently fabricated reference electrodes. 

Suppose now that a buffer amplifier (Section 6.11.2) is inserted between the pH sensor and the recorder. A buffer amplifier has a large input impedance (of the order of 10llQ), a low output impedance (about 10 (1) and a voltage gain of unity. The circuit is now represented by Fig. 6.71. 

Fig. 7.5 shows a design of an OA and Table 7.4 the characteristics of an ideal OA and of three real, easily obtainable, OAs. The two input potentials should be precisely equal in the absence of an applied potential (often externally adjustable), the current consumed by the OA should be zero, and its gain infinite. In other words, the input impedance of the amplifier should be infinite and the output impedance zero. As seen from... 

Electrometer amplifier— An electronic amplifier with an extremely high -> input impedance (Rln > 1014 Q). The device allows measurements of electrical voltages (potentials) at practically zero current. Early devices employed specially designed and selected vacuum tubes (electrometer tubes) operated in a mode with very low grid current. The development of field effect transistors of various types allowed the application of solid-state devices. Electrometer amplifiers are employed in - pH meters (and generally in so-called pi meters, where I stands for ion), all types of instruments for po-tentiometric measurements and in the reference electrode input of -> potentiostats. Because of the high input impedance electrometer amplifiers are sensitive towards electric interferences, consequently some potentiostats have their -> reference electrode input circuitry (essentially an electrometer amplifier) mounted in a separate housing to be attached as close as possible to the reference electrode in order to minimize external interference. 

The value depends on the voltage and Ri (the input impedance). In an ideal case (- electrometer amplifier, -> voltage follower) Ri is very large resulting in an input... 

Operational amplifier— An electronic device (available in numerous different forms, built with discrete components, in thick film or thin film technology, but mostly as an integrated solid state circuit IC). It is a an amplifier with ideally infinite input impedance, zero output impedance, response behavior independent of the rate of change of the input signal (amplification constant from DC to high frequency AC). It is schematically plotted as a triangle ... 

The operation of an oscilloscope can best be described by reference to Fig. 5, which shows a simplified layout of the controls of a commercial (Tektronix) digital instrument. The signal to be measured is applied to the input connector (BNC) of one of the vertical amplifier channels and must not exceed an upper limit of, typically, 400 volts if the scope input impedance is one megaohm and 5 volts for 50-ohm input impedance. The latter impedance is necessary for signal changes that occur rapidly, such as in the fluorescence decay measurements of Exps. 40 and 44. The lower limit of sensitivity is about 1 mV/division, so preamplification is sometimes needed if very low signal levels are to be measured. 

ISEs it is common practice to use potential measuring instruments with input impedances >10 Cl to ensure that there is no error in the potential measurement. Most modern pH/mV meters constructed with field-effect transistor-type input amplifiers fulfill this requirement. However, as the electrode surface area becomes smaller, the resistance of the ISE increases dramatically. Thus, for microsized electrodes, specially designed amplifier circuits with even higher input impedances are required to obtain accurate intracellular ion values and to help eliminate noise. In many instance, the micro-type measurements must also be made within the confines of a Faraday cage to reduce noise further by shielding the electrodes finm environmental noise. In automated clinical chemistry analyzers, confinement of the electrodes within the outer metal cabinet of the instrument serves a similar purpose. 

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