Issue 133 August 2001
MSP430 News Flash:

Recognizing the Flexibility of Reprogramming

Start • MSP430F1121 • Comparatively Speaking • Battery Monitor • Dynamic Inputs • RC To The Rescue • E(OR I)IN • It's Only The Beginning • Sources & PDF

DYNAMIC INPUT
I think of an ADC as a device having some internal circuitry where a reference is adjustable so multiple comparisons can be made to an unknown input. (The successive approximation converter is only one variety of ADC, albeit possibly the most well-known.) This process of zeroing in on a value equal to that of the unknown input takes time. If the reference is infinitely adjustable, then the number of comparisons (amount of time you can spend doing the comparisons) is related to how close you can come to the actual value (bits of resolution). One comparison can determine whether the input is above or below half of a reference. Two comparisons can narrow that value again by half, and so forth. Another way of thinking about it might be to imagine the reference as an incremental potentiometer (the number of increments, x, resolution, is usually a power of 2). Crank that puppy from zero to VREF until the comparator switches state and then read off the incremental decoder for a digital value of the unknown input. This process can be simulated using a timer/counter and a charging and discharging capacitor, which makes it closer to another style of ADC, the dual slope converter.