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Input Voltage Scaling for an ADC

IP.com Disclosure Number: IPCOM000095283D
Original Publication Date: 1965-Nov-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

McCullough, JW: AUTHOR

Abstract

In order to obtain any desired full scale input range, loading resistors RL1... RLN are connected across the output of constant resistance ladder network 25. This functions as a reference voltage source for the ADC 10. An offset voltage is also introduced to enable 10 to convert to a digital result within +/- one-half the least significant digit of the exact analog voltage. This offset voltage is scaled for each input range through constant current source 30. This is possible because the equivalent source resistance of ladder 25 is reduced by the same factor due to the appropriate loading resistor RL1... RLN.

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Input Voltage Scaling for an ADC

In order to obtain any desired full scale input range, loading resistors RL1... RLN are connected across the output of constant resistance ladder network 25. This functions as a reference voltage source for the ADC 10. An offset voltage is also introduced to enable 10 to convert to a digital result within +/- one-half the least significant digit of the exact analog voltage. This offset voltage is scaled for each input range through constant current source 30. This is possible because the equivalent source resistance of ladder 25 is reduced by the same factor due to the appropriate loading resistor RL1... RLN.

For example, assume the largest scale factor to be 10MV per digit. The magnitude of current source 30 is made equal to the required offset voltage VO divided by the source resistance RS. If RS = 4K and VO = 5 MV, IC = 5 MV/4K =
1.25 mu a. When the scale factor is changed by connecting a loading resistance RL across the output of 25, the offset voltage is automatically changed by the same factor without changing 1C. If the scale factor is reduced to 1 MV per digit by making RL = RS/9, the resulting source resistance becomes RS/10 = 400 ohms. VO = 1.25 mua (400 ohms) =.5 MV which is the new offset voltage required.

Thus, the input scale factor is achieved by loading ladder 25. A proper offset voltage for the scale required is obtained automatically through source 30.

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