KCC's quizzes AQQ260 about a technical article correction

1. "The time it takes for the integrator to trip the comparator is proportional to the unknown voltage (T_INT/V_IN)". This is wrong. Also the relation T_INT∝V_IN on the right side of fig. 1b is not correct.

2. Correction: "The time it takes for the integrator to trip the comparator is inversely proportional to the unknown voltage (T_INT ∝ 1/V_IN)". And on the right side of fig. 1b we need to put T_INT ∝ 1/V_IN.

Hum, missing the negative sign?  Depends on the convention in the document, but the Vint should be proportional to minus Vin ( 180 degree pahse shift). Not sure about how Tint is defined though, since it appears only in the graph and not in the schema.

Thanks Gaetano! Indeed the error is indeed the time here is inversely proportional to Vin. OK, you propose the correction in the sentence. But the circuit, as it is, will not work as a single slope ADC! Changes must be done on the circuit for sure... Do you have an idea?

That's somehow true: there is a sign problem. But there is also an other more fundamental mistake... And that is present on both the text description and on the schematics!

If Tint=R*C, then ref time is Tref=Tint*Vref/Vin.
Time is not proportional but inversely proportional to Vin.

(And maybe a small note, the voltage is not integrated, but here the current is integrated, which depends on Vin)

The first stage integrator is inherently inverting, and thus the output (Vint) would start at zero and slope negative.  The comparator will never trip.  You could add another inversion stage (after the integrator).  Inverting the Vref would also work if you had negative supplies in the system.

Sure viktor_vojta , you have found the main mistake which is Tint is not proportional to Vin. Then you can say, as corrective action, to correct the text, it's inversely proportional to Vin. But our problem is not solved: as it is now, the circuit cannot work as a single-slope ADC! Something must be corrected in the circuit as well (and not only the text)!

Hi TimW , thanks for your reply! Yes, there is indeed an inverting problem in the integrator. But there is also a fundamental mistake both in the text and in the circuit: as it is the structure cannot operate as an ADC! 

Indeed, if Vin, constant, is doubled, Vout will reach saturation twice as fast, so Vin and the time, Tin, is thus inversely proportional to Vin, as long as the OPAMP are not in saturation, Simple simulation confirms it (not with ideal OPAMP since ideal OPAMP have no saturation limits).
So kind of changing the proportion for its inverse, the only correction that I can see is to add the possibility that the OPAMP reaches saturation? Else, the graph and the numerical simulation both agree.

In order for it to function as an ADC, there needs to be circuitry added to count the elapsed time to trip the comparator.