Error voltage, current

There is very little loading effect of the current follower circuit. Because the input current source is connected between point S and arinmon and point 5 is kept at virtual common, the input-signal. source senses virtually zero resistance at its output terminals. The effective input resistance R, is the error voltage divided by the input current, i, that is, / , -- v ii.. Because - c,.M atid from Equation 3-4/, - -i jR,. we can write... 

Much of the modern direct indication equipment measures admittance or impedance by calculating the complex ratio of complex current and voltage, using sophisticated computing techniques. Apparatus of this kind is often equipped to accept inputs through y. 4. or 5 terminals, so that errors due to series and, or parallel stray impedances can be eliminated. 

Figure 8.45 Membrane conductance depending on the voltage applied to a CHO cell membrane. The stimulus was a 1 ms long rectangular current pulse. The figure shows an average (current and voltage) over a greater number of experiments with always new cells. Error bars would be on the order of 0.4 pS and are omitted for showing a nicer picture. It should be noted that CHO cells do not have an excitable membrane.

In the practical applications, the concentration of luminescent species is usually very low and consequently a photomultiplier is commonly used as a detector. If the emission can be efficiently focused into a small entrance, an avalanche diode can also be used. A reasonably fast detector is anyway needed also in the frequency-domain method due to the phase error induced by the detector. StiU the requirements are not as strict as in the case of prompt organic fluorophores. In the case of the most commonly used lanthanides, the upper limit of modulation is around 100 kHz, but still the reasonably error-free phase detection requires ca. 1 MHz bandwidth from the detector and accompanying current-to-voltage preamplifier. 

The sample temperature is measured by a miniaturized sensor, such as a thermocouple or a platinum resistance thermometer. The sensor voltage is then compared to a voltage set by the experimenter and an error voltage is used to drive current through the heater. For the best precision, three term control is used in which power is reduced as the set temperature is approached and optimum heating is provided to settle on the set temperature. 

If the resistance of the voltmeter is sufficiently high, effectively no current will pass between RE and WE. If the electrometer s resistance is too small a value, the current passing between these two electrodes will lead to errors in the potential difference measurements. In Figure 6.1, a battery and a variable resistor are used to set up a particular potential difference between WE and CE. However, in a modem lab, an electronic load is used instead of battery, variable resistor, and amperemeter. Potentiostat can also be used, which is a convenient electronic system to carry out a variety of electrochemical measuranents. The only disadvantage of using a potentiostat is that it is usually quite expensive. Note that most of the potentiostats are limited by a relatively low maximum current and voltage while an electronic load could have extended ranges of these parameters. 

In this method simultaneous readings of voltage at motor terminals and current are taken while using a d.c. source of supply and the resistance of Ihe windings is calculated. Current must be restricted to 10% of the rated current of the windings. Errors introduced into the measurement, by the resistance of leads and contacts must be compensated. 

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