Thank you!

In other words - simple integrator can't accurately reproduce rising and falling lines of triangle because of limited bandwidth, so it has some additional impedance Z=1/(2Pi*f_{BGW}*C) at a point of OP-amp input? I understood you correctly?

In this case, for my circuit (with simple integrator), equation will be:

U_{x}R_{ref}(1-Z/R_{x})

Duty Cycle = ---------------------------------- , where Z, R_{x}, R_{ref}, U_{ref} are constants, dependence of Duty Cycle from Ux is nonlinear, OK

U_{ref}(R_{x}-Z(1+R_{x}/R_{ref}))+ZU_{x}

In my case, Rx and Rref are different and Ux is connected constantly.

But the circuit (attachment) of Prema multimeter (with the same integrator circuit) works another way. Here there is only one resistor and input connects with Ux and Uref in turn.

I think, this is a simple double-integrating ADC, with fixed period of charge (Tx), and measured discharge period (Ty) proportional to Ux (algorithm of measuring duty cycle in this case will be absolutely nonlinear).

t_{x}I_{x}=T_{y}I_{y}, T_{y}=T_{x}(I_{x}/I_{y})

Currents will be: I_{x}=U_{x}/(R+Z), I_{y}=U_{ref}/(R+Z)

The denominators are the same and cancel, so equation for Prema ADC:

T_{y}=T_{x}U_{x}/U_{ref}, linear, does not depend on GBW.

I now don't speak about extra high section frequency (or low RC) of differentiator. In two-OPs integrator circuit, amplifier's inverse inputs connected directly for DC.

That's why, maybe Prema ADC uses this circuit for some other purposes?