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IP.com Disclosure Number: IPCOM000008558D
Original Publication Date: 1998-Mar-01
Included in the Prior Art Database: 2002-Jun-24

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John Oglesbee


This article describes a low cost high precision In operation, the UP software controls the con- resistive divider circuit arrangement for pre-scaling version process and switch logic control line such voltage input signals to a microprocessor controlled that two A/D readings are taken to sample each A/D conversion system. Many A/D converters have value of the input signal. The first reading is taken a usable input voltage signal range which is limited with the control line low, such that switches Sl and by the available power supply voltage, e.g., the S2 are both in the "A" position. The second reading range of input voltage signals to the A/D cannot is taken with the control line high, such that switch- exceed the power supply voltage to the A/D. es Sl and S2 are both in the "Et" position. Note that However, the actual range of many A/D input the switches act to reverse the positions of Ra and signals may exceed the available A/D power supply Rb in the pre-scaler divider during the two sample voltage by a factor which is typically between one readings. When the two sample readings are com- and two. The resistor arrangement described here pleted, the UP then calculates the mathematical may be used to divide any such A/D input signals average of the two readings. Because of the circuit precisely by two, thereby bringing the range of arrangement, when the average is calculated any most typical A/D input signals within the input divider error that occurs during the first reading is range of the A/D converter. The advantage of the exactly canceled by an equal error during the sec- proposed arrangement is that high precision divider ond reading. This can be verified mathematically by ratio performance can be obtained using non-preci- the expressions below, where Vx represents the sion components. A high precision voltage divider actual value of the unknown input signal, and Vy pre-scaler can thereby be accomplished using only a represents the actual value of the input to the A/D few low cost non-precision components, converter: A typical circuit arrangement for the pre-scaler resistor divider is shown in Figure 1. The switches (Sl and S2) indicated in Figure 1 are logic controlled analog multiplex switches. There are many common industry standard types of integrated circuits for performing this multiplex switching function. A typical such device might be the Motorola MC14053 integrated circuit. Resistors Ra and Rb in Figure I are approximately equal in value, and divide the input signal approximately by two. The actual voltage divider error is determined by the actual error in the matching of the values of Ra and Rb. A digital control line for the analog multiplex switches is connected such that both switches Sl and S2 are in the "A" position when the logic control line is low, and these switches are in the "B" position when the logic control line is high. The logic state of the control line is defined by UP software under program control.