Arduino analog pins

How many sensors can be used by a arduino uno and mega separately?

Assuming you mean analog signals. As many as the number of analog pins on those boards. This can be extended by using an analog multiplexer and additional digital pins. The total number of channels is number of analog pins minus one plus log2 of the number of digital pins used to select the input channel, e.g. 8 additional analog channels through a multiplexer will require 3 digital pins for channel selection.

Does each sensor can use the ADC of arduino simultaneously?

There is a single ADC module which takes approx. 112 us to convert an analog signal to 10-bit digital number. There is no simultaneous conversion of multiple channels.

If not how to use all the sensors simultaneously if possible?

All sensor can be connected but only one at a time is sampled and conversion to digital number.

Cheers!

All AVR- and ARM-based Arduinos have a single ADC, connected to multiple input channels. If you require simultaneous sampling of multiple sources then you will need to use one or more external ADCs.

If not how to use all the sensors simultaneously if possible?

a few ways:

1) use external adc;

2) use a sample-and-hold circuitry on the sensors;

3) find a mcu that has more than one adc modules; some pic24/dspics have that. some atxmegas have that. and some arm chips have that as well, to list a few.

4) use multiple avrs, each for a sensor;

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