This invention relates to a double integrating-type analog-to-digital converter and, more particularly, to a double integrating-type analog-to-digital converter used in an electronic weigher.
An analog-to-digital converter (referred to hereinafter as an "A/D converter") of the double integrating type is suitable for slow measurements at high accuracy and therefore finds use in instruments such as digital voltmeters and electronic weighers. The double integrating-type A/D converter has a control circuit composed entirely of hard-wired logic and therefore is devoid of freedom in terms of converting speed and resolution. This means that the construction of the circuitry must be modified whenever there is a change in the conditions under which the converter is used. Accordingly, double integrating-type A/D converters of this kind are produced in a wide variety of models but in small lots, with attendant high manufacturing cost. Another consequence of such production is the complexity involved in making the arrangements for the manufactured lots. Furthermore, since there is a decline in the conversion precision of a double-integrating A/D converter in cases where the level of the input signal is low, it is necessary to adjust the analog signal input level at a stage prior to the A/D converter in order to obtain a prescribed conversion precision over the full range. In addition, since the capacity of a counter constituting the prior-art double-integrating A/D converter described above imposes a limitation upon the maximum output count, the latter cannot be raised at will.
A variety of electronic weighers having different weighing capacities and weighing accuracies are available. In order to avoid the high manufacturing costs entailed by producing a large variety of weighers in small lots, a conventional practice is to use an identical load cell as a weight sensor and an identical double-integrating A/D converter for converting an output analog signal into a digital signal, with the intention being to utilize common parts and reduce the total number of components. However, when an attempt is made to change the value of the weighing capacity without causing a decline in the resolution of the double-integrating A/D converter, it becomes necessary to adjust the input level of the A/D converter. In the prior art, this means the provision of an amplifier circuit designed for the particular method. For example, in a case where three electronic weighers having respective weighing capacities of 5 kg, 10 kg and 20 kg are to be manufactured using a load cell designed to give an output voltage of 20 mv when weighing 20 kg and a double-integrating A/D converter having a maximum input voltage of 2 v, amplifier circuits having amplifications of 100, 200 and 400 are required for the electronic weighers having the weighing capacities of 20 kg, 10 kg and 5 kg, respectively. Thus, according to the prior art, amplifier circuits conforming to the various weighing capacities must be provided. There is need of an improvement which will afford a simplification in this respect.