I have single supplied sallen-key filter more that have nice 3MHz flat LPF response. I tried to compute the resistance directly by dividing the input voltage by input current which is the voltage and current of source but it seems not ok:
I know this "transient analysis" must give me impedance of this frequency. but this blue trace is weird trace!
I've also seen somewhere replaced voltage source by current source and simply plot input voltage bode diagram!!! but it's single supplied and we have constraint?!
Buffered RLC filter input impedance in transient analysis (According to "Andy aka"s nice answer): I think AC analysis show input analysis is ultralow, then we need buffer in input and output, isn't it? Frequency response that is very good for me: