Automated Impedance Analyzers

There are a number of automated and semiautomated impedance analyzers on the market. Although these are intended primarily for network and network component analysis, they have a limited applicability for measurements in electrochemical systems. 

Generally, this class of ac analyzer operates with a so-called autobalance bridge. The desired signal (comprising both ac and dc components) is applied to the 

the unknown impedance can be determined directly from the value of the range resistor, Rr, and the attenuation factors a and b imposed by the null detector to achieve the null condition. 

The inhinsic disadvantage of this method is its two-terminal nature the facts that a dc potential cannot be applied to the electrode of interest with respect to a suitable reference electrode and that the potential e, across the specimen varies during the balance procedure. Since the in-phase and quadrature null signals usually are derived from a PSD, instruments of this type are limited at low frequencies to approximately 1 Hz due to the instability of analog filters with longer time constants. 

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All automated hematology analyzers are comprised of flow cytometers, which count and size red cells, white cells, and platelets, and photometric hemoglobinometers, which measure hemoglobin concentration. All analyzers produce CBCs and most also produce MPV, RDW, and PDW. Automated CBC analyzers use either the basic aperture impedance method or one of the light-scattering methods to count and size cells. Automated analyzers that... 

S. Doerner, T. Schenieder and PR. Hauptmann, Wideband impedance spectrum analyzer for process automation... 

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. 

In general, direct methods can be used to acquire impedance data significantly more rapidly than bridge methods. This is particularly true for digitally demodulated, phase-sensitive detectors, for which only a single cycle is required. Nevertheless, in unstable systems, such as rapidly corroding specimens, acquisition rate is an important consideration, and a major criticism of PSD methods is that these must be performed frequency by frequency. Fortunately, this often is not a serious hindrance when such equipment is automated. In the past decade, a number of experimenters have used automated frequency response analyzers as digitally demodulated, stepped-frequency impedance meters. Typical of this class are the Solartron 1170 and 1250 series frequency response analyzers (FRAs). 

Analysis of Lissajous figures on oscilloscope screens was the accepted method of impedance measurement prior to the availability of lock-in amplifiers and frequency response analyzers. Modern equipment allows automation in applying the voltage input with variable frequencies and collecting the output impedance (and current) responses as the frequency is scanned from very high (MHz-GHz) values where timescale of the signal is in micro- and nanoseconds to very low frequencies (pHz) with timescales of the order of hours. 

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