I were watching a video about dual slope ADC's here on youtube


simulate this circuit – Schematic created using CircuitLab

I understand that the charging equation is $$ \frac{{V_{in}}\times{t_1}}{{R1}\times{C1}} $$

But bout discharging, the woman puts out something like this:

youtube shot

I do not understand how discharging equation is $$\frac{{V_{ref}}\times{t_2}}{{R1}\times{C1}}$$

The way I see it, at time t1, Vref is positive and Vo1 is positive, and inverting input is GND, so when we switch off S1 and switch ON S2, another current will flow through C1 and charge the capacitor more. I don't see how this will discharge the capacitor.

Am I missing something here? Would you please help me to understand this?

  • 2
    \$\begingroup\$ what would happen if vref and vin had opposite signs?? \$\endgroup\$
    – Neil_UK
    Aug 28, 2016 at 20:22
  • \$\begingroup\$ Well, that makes sense. I am equally mad at myself and the tutor on youtube. It must have been pointed out that the reference is negative potential. anyway, thanks. \$\endgroup\$
    – Alper91
    Aug 28, 2016 at 20:37

1 Answer 1


Maybe this is a better picture: -

enter image description here

Note the -Vref bit


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