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METHOD FOR INTERPOLATING INTERMEDIATE VALUES IN A CLOSED LOOP SYSTEM

IP.com Disclosure Number: IPCOM000006675D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2002-Jan-22
Document File: 2 page(s) / 101K

Publishing Venue

Motorola

Related People

Dan Schroeder: AUTHOR

Abstract

As a specific example (Figure 2), it may be required to display the power output of an amplifier to the nearest watt, but the A/D used to detect the power level can only resolve 4-watt steps. In the example shown, the power output is dependent on the gain of the amplifier. The gain of the ampliier is adjustable. To adjust the gain, the microprocessor outputs an S-bit word to the S-bit D/A converter. The analog level output from the D/A is used to control the gain of the ampliier. As the gain is changed, the output from the amplifier is also changed. The output level is detected by the level detector and sent to the internal 6-bit A/D. The microprocessor drives a digital level meter that displays the output level. Note that because the D/A has higher resolution, several steps of D/A level adjustment may t&e place before any AID input change is noted. It is this added control capability that allows the intermediate values to be interpolated.

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MO7WROLA INC. Technical Developments Volume 17 December 1992

METHOD FOR INTERPOLATING INTERMEDIATE VALUES IN A CLOSED LOOP SYSTEM

  As a specific example (Figure 2), it may be required to display the power output of an amplifier to the nearest watt, but the A/D used to detect the power level can only resolve 4-watt steps. In the example shown, the power output is dependent on the gain of the amplifier. The gain of the ampliier is adjustable. To adjust the gain, the microprocessor outputs an S-bit word to the S-bit D/A converter. The analog level output from the D/A is used to control the gain of the ampliier. As the gain is changed, the output from the amplifier is also changed. The output level is detected by the level detector and sent to the internal 6-bit A/D. The microprocessor drives a digital level meter that displays the output level. Note that because the D/A has higher resolution, several steps of D/A level adjustment may t&e place before any AID input change is noted. It is this added control capability that allows the intermediate values to be interpolated.

  For this example, assume that the level detector and the gain control functions are linear and of the same gain such that for every four S-bit D/A step increases, the 6-bit A/D increases one step. Further, assume that the 6-bit A/D resolution allows the output level to be displayed in 4 watt increments. Since the D/A has two more bits, or 4 times the precision of the A/D, the S-bit D/A can change the output level in 1 watt increments.

  To determine the output level to the nearest watt, the S-bit control signal is incremented one and two steps from the initial set point, and then decremented one and two steps from the initial set point. The control step at which the 6-bit A/D increments or decrements is then used to determine more accurately what the output power level is at the initial set point. The chart in Figure 3 shows how the power level is interpolated. Shown are the D/A value, the A/D value, the indicated power out- put without interpolation, and the interpolated power output indication.

by Dan Schroeder

  In many microprocessor controlled products, the microprocessor is required to output an analog level that is used as a control voltage. This is done using a digit...