Input impedance

Every electronic component has a characteristic impedance. When a signal is transmitted from a unit with a high-impedance output to a low-impedance input, there is going to be a loss in the signal unless an impedance-matching device is used to couple the two units. Manufacturers of preamplifiers and amplifiers quote the impedance of the input and output for their products. Coaxial cables have an impedance between 90 and 100 ft. 

Figure 3.2 Experimental configuration to include a high-impedance input.

The electrical potential of the reaction mixture is measured with a platinum electrode and a Hg/Hg2S04 reference (available from Rainin). The output can be recorded on a single-channel strip chart recorder, with the platinum electrode attached to the positive terminal and the reference to the negative, or the recording can be made via a high-impedance input to an A/D board (analog-to-digital converter) on a computer (see Chapter 3). 

A prime requirement for an appropriate transducer is that it be compliant at its input. CompHance in this sense means that the input energy required for proper operation of the transducer, and hence a correct measurement of the controUed output, does not significantly alter the controUed output. A transducer that does not have this compHance is said to load the controUed output. For example, a voltmeter must have a high-impedance input in order that the voltage measurement does not significantly alter circuit current and, hence, alter the voltage being measured. 

As shown above, potentials of several electrodes can be simultaneously acquired. Figure 5.26 shows a potentiometric system with eight high-impedance inputs allowing up to eight selective electrodes and a reference electrode to be connected. The impedance adapter board is connected to an A/D converter that is in mrn interfaced to a computer via its serial port. 

The end-point detector is essentially a low impedance input vacuum-tube voltmeter with a meter relay in series with the indicating meter. The meter relay is used to terminate the generating current at or slightly before the end point, while the indicating meter registers the rela-... 

The piezo-electric film used in this device as a microphone. The brand name is called Kynar Piezo Film, and is manufactured by the Penn wait Corp. We find that it is a useful, inexpensive material which,can act as a speaker or a microphone, as well as a stress and impact sensor. Small pieces give a large electrical signals into high impedance inputs (order of volts) in response to "fingertip stresses. We expect it to work in the 1.5 Tesla field of the magnet. 

So are the two states 0 and M an accurate representation of die signals in a circuit How should the designer construct a design that accurately models in simulation the occurrence of a high impedance input signal yet still synthesizes the required two-state logic operation ... 

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... 

In many instances time-consuming sample preparations such as filtrations, distillations or extractions can be avoided by employing ion-selective electrodes. From an instrumental standpoint, no expensive, sophisticated instrumentation, i.e. integrators, compensators for non-linear calibration curves, etc. are needed. After the appropriate calibration, the concentration can be read directly from a voltmeter with a high impedance input. 

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