Hi.
Assume we have a Cs (sampling cap.) and a Cf (feedback cap.) in our S/H circuit. Ciop is the input cap. of our Opamp and Co is the sum of Opamp output cap. and the input cap. of the next stage. Then we have
Cload = Co + (Cf *(Cs+Ciop)) / (Cf+Cs+Ciop)
So when you are simulating just the Amplifier in S/H, you should use the whole Cload as the load cap. of your circuit. But when you are simulating the S/H circuit, i.e. Cs and Cf exist in your circuit, you should use just Co as the load cap. of your circuit.
Regards,
EZT