The double dual-slope multiplier: Elimination of offset

The double dual-slope multiplier is a multiplying analog-to-digital converter based on the dual-slope integrating converter. The ratio of the product of two unknown voltages to the square of a reference voltage is obtained in terms of the ratio of a measured time interval to a known time interval. Offset errors, which are the main source of error, are described and a modification which greatly reduces the error is proposed. The errors consist of scale factor, output, and linear feedthrough errors. The scale factor and output errors can be measured by a cycle consisting of two dual-slope measurements that use the reference voltage only. The feedthrough errors can be eliminated if the multiplying cycle is repeated with the order of the inputs reversed. The operation of the multiplier when the inputs are less than the offset is considered in detail. Since the decision making, memory, and calculations necessary for the control of the multiplier and the correction of the offset are considerable, the method is mainly applicable to precision instruments equipped with a microprocessor.