Look for one with a high secondary turn count, a 1:5000 model will do great. Then 100A in the primary (the single wire that goes through the clamp) gives you 20 mA RMS in the secondary.
R1 is the sense resistor, it gives a 0-1V peak signal. This circuit requires you to sample the ADC for at least 10 ms and get the highest value (the peak value of the wave), to get the actual value. R3 is just to protect the pin in case of current spikes, thus voltage spikes in the detector.
If you don't care too much about the actual value, it's quite easy to add a basic peak detector to it, and make the sampling a little easier.
With peak detection. Value of R1 amended to give a bit higher output voltage, compensating the voltage drop of about 0.5V across D1. C1 and R2 are a peak detector and smoothing circuit. Higher values for both C1 and R2 are fine, up to about 10µ and 100k.