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Instruments and measurement
Published in Stephen Sangwine, Electronic Components and Technology, 2018
Electronic meters are based on analogue-to-digital conversion, comparing a voltage to be measured against an internal reference voltage. These types of meter are usually multiple purpose and capable of measuring resistance as well as a.c. and d.c. voltage and current. They are known as digital multimeters or DMMs. Multiple voltage ranges are provided by switchable attenuators, switched automatically on more expensive auto-ranging instruments, or manually on lower-cost models. Current is measured by passing the current through a switch-selectable precision resistor and measuring the voltage developed across the resistor. A.c. measurements are usually accommodated by a precision rectifier circuit. All quantities to be measured by a digital multimeter are therefore converted to a proportional d.c. voltage, which is then converted to a digital reading by an analogue-to-digital converter (ADC). The result is then indicated, usually on a liquid crystal display. The ADC in most DMMs is a dual-slope integrator, which inherently measures the mean value of its input voltage. This type of ADC is illustrated in Figure 6.3 and operates in two stages. Initially the integrator is held at zero by the switch across the integrator capacitor, and the counter is held at zero. When the ADC is started, the analogue input is connected to the integrator, which ramps up as the counter increments towards its maximum value Nmax. The first stage of the conversion ends at time T1 as the counter overflows. During the second stage of the conversion, the integrator is connected to the negative reference voltage Vref and ramps down towards zero. At time T2, the comparator switches and the counter is stopped. The final value in the counter, N, is proportional to the time (T2 − T1) taken to ramp the integrator down to zero, which is proportional to the mean value of the analogue input during the first stage of conversion. Correct measurement of r.m.s. values for sinewave inputs may be achieved by designing the precision rectifier circuit to have a gain of π/2 or about 2.2. (This figure arises because the r.m.s. value of a sinewave is 1/2 times the peak-to-peak value, and the mean value of a rectified sinewave is 2/π times the peak value.) Correct readings will not be obtained for nonsinusoidal a.c. voltages and currents because of the different ratio of r.m.s. to mean value for nonsinusoidal waveforms.
Novel Resistorless, Cascadable Current-Mode Precision Rectifier Using CFDITA
Published in IETE Journal of Research, 2022
Full-wave rectifiers (FWRs) are employed in a variety of applications as basic building blocks, such as signal polarity detectors, RF demodulators, peak value detectors, averaging circuits, and various other non-linear applications [1]. The traditional rectifier circuits are based on diodes. However, they are not able to rectify the signals with a lower amplitude (below the knee voltage of the diode). This limitation of traditional rectifiers, being incapable of rectifying signals with lower amplitudes, motivates the researchers to develop high-resolution precision rectifier circuits. Subsequently, numerous precision FWR circuits, which employ a variety of current conveyors and operational transconductance amplifiers (OTAs), have been reported in the literature [2–19]. Among these circuits, the circuits of [2–8] operate in voltage mode, the circuits of [9–17] operate in current mode, and the rectifiers of [18,19] operate in mixed mode. A brief review of current-mode type rectifiers is given as follows. The rectifier circuits presented in [9–11] are realized using a current differencing transconductance amplifier (CDTA). The structure of [10] is resistorless. The circuit of [11] does not use any diode. In addition, the circuit in [11] has a greater operational frequency. Two rectifier circuits are presented in [12], one of the circuits utilized differential voltage current conveyor (DVCC), and other circuit utilized OTA. Both structures also use external resistors and diodes in addition to the current mode active element. The circuits in [13,14] have resistorless structures and utilized a second-generation current conveyor (CCII) as active block. The structure of [13] is realized using diodes whereas the structure of [14] is diodeless. The extra-X current conveyor (EXCCII)-based rectifier structures are presented in [15–17]. All these structures are resistorless and diodeless as well. Additionally, the operating frequency range of the circuit [17] is wider. Recently, a second-generation current controlled conveyor (CCCII)-based resistorless and diodeless rectifier structure is reported in [18]. The circuit has good input current dynamic range. More recently, a voltage conveyor (VC)-based rectifier circuit is presented in [19]. In addition to VC, the circuit also uses resistors and diodes for its realization. The circuit of [18,19] can operate in mixed mode.