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Dual Slope ADC

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Aside from comparators, the most important analog part of a dual slope ADC is an integrator. The circuit is:

Integrator

Vout is the integral of Vin, or actually

integral

Any time one wants to reset the integral to 0, one simply shorts the capacitor. Notice the negative sign -- positive input voltages give negative output voltages.

If we want to digitize a postive voltage, we'll also need a reference voltage that's negative (and, of course, if we want to digitize negative voltages, we need a positive reference). We integrate the unknown voltage Vin(t) for a fixed time T. For now, let's assume the potential is constant in time. The integral will be -VinT/RC. Now disconnect the input potential. Since the input is 0, the integral holds constant. Next, connect the input to the reference voltage (opposite sign to Vin) and integrate. What will happen? The integrator will ramp towards 0. How will we know when we get to zero? Multiple choice, choose the right component:

 

   
 

 

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