So you want to measure average current, or really get a detailed current trace? In the latter case, the sampling rate should be at least 10 times the signal frequency - then you get 10 measurements per wave. You should look at the data sheet whether the PSoC4 SAR ADC as a minimum sampling rate. You want to get the aqusition time to be as short as possible.
You have to have more than 10 times the signal frequency to reproduce the signal. Similarly acquisition time depends on source resistance and SAR input impedance. To calculate the acquisition time please refer page no 17 of the document provided below :< http://www.cypress.com/mwg-internal/de5fs23hu73ds/progress?id=gqruwXPjmXaFqwqhRzHYVDbo_OSUpLuZod2fVhSwlR0,&dl >. To measure the current high side current sensing method can be used.