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Transcript
Signal Conversion 73 3.4.5 Analog multiplexer When several signals need to be converted, it is necessary to either provide an A/D converter for each signal or use an analog multiplexer to direct the various signals to a single converter. For most biomedical signals, the required conversion rates are low enough that multiplexing the signals is the appropriate choice. Common analog multiplexers utilize either JFET or CMOS transistors. Figure 3.18 shows a simple CMOS analog switch circuit. A number of these switches are connected to a single Vout to make a multiplexer. The switches should operate in a break-before-make fashion to ensure that two input lines are not shorted together. Other attributes to be considered include on-resistance, leakage currents, crosstalk, and settling time. ø p-type Vin Vout n-type ø Figure 3.18 A simple CMOS analog switch. The basic functional block of a CMOS analog multiplexer. 3.4.6 Amplifiers The biomedical signal produced from the sensor or electrode is typically quite small. As a result, the first step before the A/D conversion process is often amplification. Analog amplification circuits can also provide filtering. Analog filters are often used to bandlimit the signal prior to sampling to reduce the sampling rate required to satisfy the sampling theorem and to eliminate noise. General For common biomedical signals such as the ECG and the EEG, a simple instrumentation amplifier is used. It provides high input impedance and high CMRR (Common Mode Rejection Ratio). Section 2.4 discusses the instrumentation amplifier and analog filtering in more detail. Micropower amplifiers
