Keywords: digital potentiometer, analog-voltage control, op-amp buffers
Let VFB = VREF = 1.25V (for the power supply shown), R1 = 10.0kΩ, and VOUT = 5.0V. To obtain the full adjustment range, calculate R2 with the wiper at mid-scale and VADJ = 0.625V. Rearrange Equation 1 and solve for R2:
VOUT = VFB + R2/R1 × (VFB - VADJ) (Eq. 1)
R2 = R1 × (VOUT - VFB)/(VFB - VADJ) = 60.0kΩ. (60.4kΩ is a standard 1% value.)To calculate the VOUT range and resolution of adjustment, set the wiper at its minimum position (VADJ = 0V). From Equation 1, the corresponding maximum VOUT is 8.75V. Then, set the wiper at the maximum position, where VADJ = 1.25V. Thus, the adjustment resolution for VOUT is (8.75V - 1.25V)/1024 steps = 7.32mV per step. (Note that the R2 value lets you raise or lower the voltage-adjustment range.)
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APPLICATION NOTE 4564,AN4564, AN 4564, APP4564, Appnote4564, Appnote 4564
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