Voltage-measuring device high-impedance

While scopes are usually used as voltage measuring devices and therefore have high input impedances, there are probes available which, by clamping around a wire, measure the ac current. This can be useful in monitoring the current flowing in NMR transmitter coils. 

The basic measurement technique for intensity-modulated photovoltage spectroscopy (IMVS) is the same as for IMPS. In principle, IMVS measurements can be made for any constant current condition, but in practice it is usual to make measurements under conditions where the net current is zero. In the case of a photoelectrochemical solar cell, this corresponds to the open-circuit condition, and a high impedance voltage amplifier is used to ensure that a negligible current is drawn from the illuminated device. The output of the voltage amplifier is fed to the FRA, and the remainder of the set up is the same as for IMPS (cf. Fig. 12.26). 

The potentiostat device used to measure the potential difference between the RE and the WE has very high internal input impedance so a negligible current is drawn through the RE. Since the voltage diference in the three-electrode cell is measured between the stable RE and WE and current is supplied by the CE, the measured impedance will only be influenced by the properties of the WE interface and the properties of the analyzed media resistance R between the WE and the RE. The influence of the media resistance between the WE and the CE on the voltage measurement is eliminated. 

As Figure 8-10 illustrates, two voltage-measuring reference electrodes are connected through a high-impedance input device that in principle should reject all current flow through these electrodes. This input impedance consists of the capacitance with a typical value of 10 pF in parallel with... 

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