Timer to make sampling time constant

Hello my friends, I'm new with arduino, so, i trying to use timer's to make the sampling time constant. For example, the time the a/d conversor expends is 108us to 116 us, is changes a lot, so i need this time to be constant.
i'm planning to use a timer with 108 us to interrupt at 108 us.

Sorry about my english...
thanks a lot for your help!!!!!

You cannot make the conversion time constant - the best you can is call for an analogRead at a constant rate.
Say every 120uS to be sure it is done, using a timer, or blink without delay:

unsigned long currentMicros;
unsigned long previousMicros;
unsigned long elapsedMicros;
unsigned long duration = 120; // multiple of 4 microseconds
unsigned int analogVal;
void setup(){
void loop(){
currentMicros = micros();
elapsedMicros = currentMicros - previousMicros;
if (elapsedMicros >= duration){
analogVal = analogRead (A0); // analogRead is a "blocking' call"? Nothing else happens during the conversion time?
// if so, change this to start the analogRead and then respond to the end of conversion interrupt.
Serial.print (analogVal); // likely to spit out too fast and fill serial buffer on PC
// perhaps save 100 values in an array and then do something with them?
} // end time pass check
// do other stuff while waiting for next duration to pass.
} // end loop

The hardware provides several ways to take analog samples at fixed intervals. For example on a 16 MHz Arduino UNO you can set the ADC clock prescaler to 128. That gives you an ADC clock of 125 kHz or 8 microseconds per cycle. It takes 13.5 cycles to do a conversion in Free Running mode so you would get a sample and interrupt every 108 microseconds.

125 kHz is under 200 kHz, the fastest you can get accurate 10-bit samples. You can change the prescale to 64 to get a 250 kHz clock at the expense of noise in your samples. That would give you a sample and interrupt every 54 microseconds.

The maximum clock rate for the ADC is 1 MHz so you could get samples at 27 (prescale = 32) and 13.5 (prescale = 16) microseconds with increasing noise.

If you want a specific rate you can use Timer1 to trigger a sample at whatever rate you can set Timer1 for. The timer rate has to be slower than the sample rate for the chosen pre-scale.