interfacing into ADC of microcontroller

[banh]

how do we go about interfacing an analog input into the microcontroller ADC pin?
- current limiting? (depending on the max current allowed at the pin)
- capacitors to filter out noise?
- i've seen a circuit that before going into the pin, the analog signal goes through a resistor (quite small about 22 Ohm). THe resistor is said to provide "damping effect". Really dont understand what this means..
...

 

[banh]

"damping effect" is not used for ADC, isn't it?..

 

[glenjoy]

The signal goes through a 22 Ohm resistor either to match the input impedance of the ADC or to minimize the current sink at the ADC input.

But I think the 22 Ohm might be useless at all, ADC inputs are high impedance inputs to avoid loading the signal source.

The capacitors are of no use at all, it will add a delay on the change of the analog signal because it will store the voltage level therefore not getting the actual reading present.

Current limiting is not really an issue on ADC's, the best to do is to follow the maximum voltage input allowed on the ADC pin, if you think your input exceeds or is less than the requirement, you can always use a signal conditioner IC to match the said requirements.

Glen

 

[andy1]

I think in general having a resistor in series with an input gate is to prevent sudden change in current from damaging the gate, especially that input is interfaced to the rest of the world and who knows what might be comming in.

In general, you need a low pass filter and possibly an input buffer or amplifier if your analog signal is weak. You have to figure out the bandwidth of the incomming analog signal and determine what kind of low pass filter you need.

In some case, you need to isolate the incomming analog signal using optocoupler or transformer if absolutely needed.

Layout of ground between the analog and digital section has to be carefully planned out so that the digital circuits do not interfere with the analog circuit.

 

[IanP]

There are no written rules on how and what to connect to ADC (in microcontrollers) pins.
As mentioned before good thing is to have a buffer.

Wheather a buffer is there or not, I always use small (10-100Ω) resistor and a diode (such as 1N4148) to limit input range: ADC pin (anode) to Vref or Vcc (cathode).

 

[banh]

In general, you need a low pass filter

is it to get rid of the noise? sth like decoupling?

Wheather a buffer is there or not, I always use small (10-100Ω) resistor and a diode (such as 1N4148) to limit input range: ADC pin (anode) to Vref or Vcc (cathode).

sorry, i dont get you. btw, a so small resistor helps anything?

 

[glenjoy]

A small resistor will not do anything, better use a signal conditioner to have a proper impedance match.

Glen

 

[IanP]

The role of that small resistor is to limit current that may flow through protective diode and because of input impedance (usually »10kΩ) this value is not critical.
Also, for the reason that ADC have high input impedance no impedance matching is required..they "read" voltage, no current..

 

[andy1]

Using a low pass filter is to eliminate some high frequency noise and also eliminate aliasing if alias is a problem with your design.

You need to find out the bandwidth of your signal first before designing the lowpass filter.