Not correct I'm afraid!
Your original post said you wanted to convert current to voltage but normally, a photo-detector circuit has an voltage output not a current output. Your design may be different but most would take the output from a photo-detector, amplify it from the few mV it produces up to say 5V maximum then use the ADC to measure it.
If you take the output from a photo-detector diode or transistor directly it will be too small to measure accurately. For example a PIC ADC, assuming you use internal reference and all 10 bits resolution can only measure voltage in steps of about 4.8mV. There are 1023 steps up to maximum but if the detector only produced say 10mV your ADC would only return values of 0, 1 or 2. You would need the amplifier to scale the photo detector output up to a value nearer 5V in full brightness.
Where does the Varactor fit into this?
Brian.