Analog input circuit with combination sample and hold and filter

Details Analog input circuit with combination sample and hold and filter Analog input circuit with combination sample and hold and filter

1984-10-16

1987-03-17

Miller, Stanely D.

Electrical transmission or interconnection systems

Nonlinear reactor systems

Parametrons

307352, 307543, 328167, 328151, H03K 1726, H03K 1760

Patent

active

046510346

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to an analog input circuit which samples and holds predetermined components extracted from input signals.


DESCRIPTION OF THE PRIOR ART

A conventional circuit of this type is represented by an analog input circuit shown in FIG. 1 which is used for digital protective relays. In FIG. 1, reference numerals 1a to 1h denote filters for removing noise components that are undesirable for relay judgement; numerals 2a to 2h denote sample and hold circuits which are provided for the filters 1a to 1h, and which effect the sampling simultaneously; numeral 3 denotes an analog-to-digital converter circuit, and numeral 4 denotes a multiplexer for selecting the input for the analog-to-digital converter circuit. Symbols (a) to (h) denote analog input signals, and symbol (p) denotes a digital output signal. The analog input signals (a) to (h) may be signals from various systems such as detection transformers and detection current transformers utilized in a power system, and which are subjected to a level conversion, current and voltage conversion, and wave-form shaping.
The operation will be described below. Voltage or current wave-forms introduced from a system contain DC components or harmonic components produce errors in determining whether or not an accident in the system being monitored has developed. Therefore, only required components are extracted through the filters 1a to 1h, and are held simultaneously by the sample and hold circuits 2a to 2h. The voltage levels which were just present are produced by the sample and hold circuits 2a to 2h for only a predetermined period of time. The multiplexer 4 successively scans these channels and connects them to the analog-to-digital converter circuit 3. A series of operations is initiated in the holding condition of the sample and hold circuits 2a to 2h, and the channels are connected to the analog-to-digital converter circuit 3 as they are scanned successively in a time-sharing manner. This operation is complete at the moment when all of the signals of the respective channels have been converted into digital signals in a time sharing manner.
The conventional analog input circuit has been constructed as described above. Therefore, if the number of channels increases, the numbers of filters and sample and hold circuits increase correspondingly, resulting in an increase in the number of parts. In most cases, furthermore, the filters and sample and hold circuits are provided with circuits for adjusting their offset and gain depending upon the amplitude of the input analog signals. Therefore, these adjusting functions are often provided in a duplicated manner.


SUMMARY OF THE INVENTION

The present invention provides an analog input circuit which employs two stages of voltage-follower circuits connected in series via a switch are used as an operational amplifier to adjust the quality factor Q of an active filter, wherein a holding capacitor is provided at the output terminal of the switch, and wherein the holding capacitor assumes the same potential as the output of the active filter when the switch is closed and when the switch is opened, the holding capacitor maintains the output voltage of the active filter.
According to the analog input circuit of the present invention, two stages of voltage-follower circuits are utilized both as an operational amplifier for the sample and hold circuits and as an operational amplifier for the filters. Therefore, the number of parts can be decreased, and the adjusting functions are not duplicated, and the manufacturing cost can be reduced, and space can be saved, and reliability can be increased.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional analog input circuit adapted to digital protective relays;
FIG. 2 is a block diagram of an analog input circuit according to an embodiment of the present invention which is adapted to a digital relay;
FIG. 3 is a diagram showing the structure of the analog input circuit of FIG. 2;
FIGS. 4 and 5 are diagrams showing

REFERENCES:
patent: 3838346 (1974-09-01), Cox
patent: 3839650 (1974-10-01), Wood
patent: 4063182 (1977-12-01), Besson
patent: 4199697 (1980-04-01), Edwards
Association for the Promotion of Japanese Electronics Industries, "Analog IC Applications Handbook", 10-9-76.

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