Dual Slope Integrating ADCs

[David]

Hi all,
I have just come across dual scope integrating ADCs and I am having a little trouble trying to understand them...any simple explanations out there? Is the input signal captured before it is integrated? If so, what is the point of the integration?

Dave

[Zad]

I think the initial integration is there for two reasons, firstly it averages the signal. This of course helps remove any noise present. Secondly it allows a certain amount of scaling to maximise the use of the dynamic range available. The only time I have used it was on a CCD sensor, where it is exceptionally good because it integrates the entire charge on each CCD cell.

[alm]

This seems a decent explanation.

The basic idea is to use a capacitor of constant (at least for the duration of the cycle) but not necessarily known capacitance. You charge this with the unknown voltage through a resistor for a certain amount of time, then disconnect the input signal and connect it to a known reference voltage of known value of opposite polarity that discharges it through the same resistor, and measure the time it takes before it returns to zero. The net change in charge is zero (you start with an empty cap and end with an empty cap), so the charge during the two cycles is equal. You can calculate the unknown voltage from the time it took to discharge it.

The advantage is that it's quite easy to detect 0V and measure time accurately, and component values like R and C are not critical. The integrating feature gives good noise rejection. The disadvantage is that's quite slow, since you have to discharge fairly slowly to get good accuracy. There are various smart ways to improve this, by using multiple ramps. The dual slopes are there to negate the effect of a change in capacitance value (it's hard to make capacitors really accurate or stable).

[David]

Ahh I think it has just clicked!...by knowing the time constant and using the reference voltage you know the voltage at any particular time through the discharge so when it reaches zero you can simple calculate it.