Output of Sample and Hold Circuit
In Phi1 CF is connected to ground on both sides and gets discharged. Cs is charged to Vin.

In Phi2 the OpAmp is in a feedback configuration. The difference of the voltage VDAC-Vin appears at the negative terminal of the opamp. For a positive voltage, in the first instant the output of the opamp will swing in the negative direction. Cs gets charged and the differential input voltage across the - and + terminals becomes smaller and smaller.

For infinite gain the difference between the + and - inputs of the opamp will become zero, the voltage VDAC will be stored onto CS, CF will hold a scaled version of VDAC-Vin it, depending on the ratio of Cs/Cf.

For finite gain a small voltage will remain across the input of the opamp. Again a scaled version of VDAC-Vin will be seen across Cf.

Happy calculations!