Design And Build A Simple DPM: Part II by Dennis L. Feucht
We continue and finish the simple DPM project, leaving the best for last -- the analog subsystem.
The Analog Part
The ADC is called a modified dual-slope converter because the input quantity, iX, is integrated all the time, while the reference is integrated only during the down slope of vC. The dual-slope converter was a major improvement over the single-slope converter, which measured voltage by timing how long iX took to charge C to some threshold voltage, VR. In this case, the accuracy of the measurement depends upon fc, C, and VR. For the dual-slope ADC, iX is integrated for TX. Then it is switched out and IR switched into the integrator, causing the capacitor voltage to ramp down to zero. At the zero-crossing is tX and the count is latched into the display.
