user10705: 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?)


user2881: 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)

user18662: The atod clock is set in the mode register. The clock is 42MHz/(PRESCAL#+1). One way of setting it is the use ADC->ADCMR = ADC_MR_PRESCAL(PRESCAL#), where PRESCAL# is some integer.  These symbols are defined in component_adc.h in the Arduino source code. 
 In the AVR data sheet, the recommended clock is MIN=1MHz, MAX=20 MHZ, corresponding to numbers between 41 and 1. Look in the data sheet, section 43.

10705: <p>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.)</p>

<p>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?)
<img src="https://i.stack.imgur.com/b08cX.png" alt="enter image description here"></p>
 2881: <p>You can change the prescaler (<code>ADCSR</code> register). Lowest scaler is 2, so it can never run at 20mHz. Also Atmel recommends a max ADC clock of 1MHz.</p>

<p>Arduino sets the ADC clock to 125 KHz (prescaler of 128)</p>

<p>So to match Arduino, you should select a prescaler of 8 ( set bits <code>ADPS1</code> and <code>ADPS0</code>)</p>
 18662: <p>The atod clock is set in the mode register. The clock is 42MHz/(PRESCAL#+1). One way of setting it is the use ADC->ADCMR = ADC_MR_PRESCAL(PRESCAL#), where PRESCAL# is some integer.  These symbols are defined in component_adc.h in the Arduino source code. 
 In the AVR data sheet, the recommended clock is MIN=1MHz, MAX=20 MHZ, corresponding to numbers between 41 and 1. Look in the data sheet, section 43.</p>
