Reference voltage sources

Each of the possible ways for analog-to-digital conversion requires some sort of reference, usually a reference voltage. The performance of the reference voltage source has a major influence on the overall performance of a data acquisition system. 

Zener diodes are reverse biased diodes. At a particular voltage the reverse current increases sharply and the voltage at this point is virtually independent of the current through the diode. Zener diodes are available for a wide range of voltages (3 to 200 V). The temperature coefficient varies with Zener voltage from negative to positive values and crosses zero at approximately 5.6 V. For very low requirements it can be sufficient to use just a normal Zener diode. 

Most of the commonly used reference elements are integrated circuits which hold all the necessary auxiliary circuitry to insure temperature stability and the proper output voltage. Extra temperature stability is achieved by integrating a heater on the semiconductor device. 

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Larger button cells with rated capacities in the range 35-lOOOmAh are manufactured for direct mounting on printed circuit boards where they are used as standby power sources for CMOS RAMs, reference voltage sources, etc. Fig. 9.10 shows the construction of these cells and the position of the terminal/mounting pins. Projected discharge curves for Catalyst Research Corporation cells are given in Fig. 9.11. 

Except for the most exact measurements, zener diodes in a constant-temperature oven are generally used now as reference voltage sources in electrical equipment. They are stable and virtually immune to mechanical trauma. 

A schematic diagram for a simple pH meter is shown in Figure 5.13. It is nothing more than a conventional potentiometer circuit. The precision slide wire (PSW) is calibrated in units of pH rather than in volts or ohms. A stable power supply is used, which may be a battery or a regulated electronic supply. The reference voltage source may be an unsaturated standard cell or a zener diode electronic reference. 

Silver-zinc batteries are used, in addition, in photoelectric exposure devices, flashlights, instruments, reference voltage sources and missile telemetry, space launch vehicles, aerospace, portable equipment and military applications. 

Catalyst Research Corporation supply the Lithiode range of disc-shaped lithium—iodine power sources with PC bound mounting for CMOS-RAM as standby power sources and for reference voltage sources and other low-current applications where the cell is the main power source. 

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

Mossbauer spectra are usually recorded in transmission geometry, whereby the sample, representing the absorber, contains the stable Mossbauer isotope, i.e., it is not radioactive. A scheme of a typical spectrometer setup is depicted in Fig. 3.1. The radioactive Mossbauer source is attached to the electro-mechanical velocity transducer, or Mossbauer drive, which is moved in a controlled manner for the modulation of the emitted y-radiation by the Doppler effect. The Mossbauer drive is powered by the electronic drive control unit according to a reference voltage (Fr), provided by the digital function generator. Most Mossbauer spectrometers are operated in constant-acceleration mode, in which the drive velocity is linearly swept up and down, either in a saw-tooth or in a triangular mode. In either case. 

A memory sweep is initialized when the MCA receives a start pulse from the function generator at the trigger input (7). The start pulse is synchronized with the sweep of the reference voltage (Vr) for the Mossbauer drive. It opens the first MCA channel when the source velocity passes through the minimum (cf. Fig. 3.3). After this start trigger, a train of 512 channel advance pulses follows with exact delay times of about 100 ps each. On receiving such a channel advance pulse, the... 

Fig. 4.10 Capillary electrometer. The basic component is the cell consisting of an ideally polarized electrode (formed by the mercury meniscus M in a conical capillary) and the reference electrode R. This system is connected to a voltage source S. The change of interfacial tension is compensated by shifting the mercury reservoir H so that the meniscus always has a constant position. The distance between the upper level in the tube and the meniscus h is measured by means of a cathetometer C. (By courtesy of L. Novotny)...

Alternatively, one may control the electrode potential and monitor the current. This potentiodynamic approach is relatively easy to accomplish by use of a constant-voltage source if the counterelectrode also functions as the reference electrode. As indicated in the previous section, this may lead to various undesirable effects if a sizable ohmic potential drop exists between the electrodes, or if the overpotential of the counterelectrode is strongly dependent on current. The potential of the working electrode can be controlled instead with respect to a separate reference electrode by using a potentiostat. The electrode potential may be varied in small increments or continuously. It is also possible to impose the limiting-current condition instantaneously by applying a potential step. 

Figure 4-5 Influence of Temperature on the AFC Cell Voltage Source Figure 6, p. 889, reference (19).

It has also been demonstrated that the sensing electrode can be maintained at a predetermined voltage of 1.0 V by electrically biasing (constant voltage source) the sensing electrode vs. a stable counter-reference, such as Pt/Hp, H+ or PbOp/PbSO, H+ (1, 2,3). With the former, the sensing electrode is electrically biased 1.0 V above the Pt/Hp,H+ potential (0.0 V) and with the latter 0.7 V below the PbOp/PbSOi, H+ potential (1.7 V). Both of these counter-reference electrodes exhibit good reversibility, but reliability and life are not adequate. 

Fig. 5.8 Schematic diagram of polarographic (or voltammetric) circuits for two-electrode (a) and three-electrode (b) systems. WE(DME) indicator or working electrode (dropping mercury electrode in the case of polarography) RE reference electrode CE counter electrode DC voltage (V) DC voltage source Current (/) current measuring device.

To understand the electrical behaviour of the LAPS-based measurement, the LAPS set-up can be represented by an electrical equivalent circuit (see Fig. 5.2). Vbias represents the voltage source to apply the dc voltage to the LAPS structure. Re is a simple presentation of the reference electrode and the electrolyte resistance followed by a interface capacitance Cinterface (this complex capacitance can be further simulated by different proposed models as they are described, e.g., in Refs. [2,21,22]). In series to the interface capacitance, the insulator capacitance Cj will summarise the capacitances of all insulating layers of the LAPS device. The electrical current due to the photogeneration of electron-hole pairs can be modelled as current source Ip in parallel to the... 

Single-beam instruments These consist of a radiation source, a monochromator, and two cells for the reference and the sample solutions, which are alternately inserted in the light path also a detector, an amplifier, and a reading device. These instruments require a stable voltage source to prevent errors arising from variations in the beam intensity. Also, differences between cells (mainly irregularities in the walls) are not easily compensated for. 

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