How to slow ADC clock speed to 1MHz on Arduino Due? I know that the ADC on the Arduino Due has a clock speed from 1 MHz to 20 MHz. I have two questions: 1. What is the default clock speed when I run a sketch, and 2. How can I change it? (I am using Arduino 1.6.4.)

The reason why I want to slow down the ADC as much as possible is to obtain the maximum source impedance. Is this logic correct? (i.e., if I slow down the ADC in the arduino code to kHz values, then the source impedance will reach MΩ values?)
You can change the prescaler (ADCSR register). Lowest scaler is 2, so it can never run at 20mHz. Also Atmel recommends a max ADC clock of 1MHz.

Arduino sets the ADC clock to 125 KHz (prescaler of 128)

So to match Arduino, you should select a prescaler of 8 ( set bits ADPS1 and ADPS0)