Author Topic: ADC Fluctutations.  (Read 5498 times)

nikhilpandey

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ADC Fluctutations.
« on: January 10, 2011, 10:50:11 PM »
Dear Experts,

I am using ADC to read my sensors.
Its a LED basically, i am trying to read the capacitance value which i get turned up through a process.
I am using Arduino Mega, everything is up and running, no problem with the circuit.

The few things creating nuisance are,
1. whenever i plug the power ON, on my laptop... ADC starts fluctuating pretty badly... there is absolutely no fixed pattern.

2. if the system is connected to my laptop and i am reading the values, if in between i move or tap my laptop, readings change for a while and go back to normal eventually.

3. If i try to plug the power adapter (given by arduino guys to provide board power)on Arduino, the ADC values again fluctuate with no fixed pattern.

4. If there is any appliance like a monitor or CPU sitting right next to me and if its power is turned on, my ADC goes crazy.

5. I did read plenty of forums and have order one spectrum analyzer to find the bug, i did try to have a capacitor, but the problem is that, since my actual sense singal is also AC, the capacitor rejects my signal along with the disturbance and i get only 0 in my readings.

6. I did try to switch on appliances around me and put my laptop on adapter, and then short the analog input to see if the fluctuations go away. And, the result is that they did go away so that means my ckt is picking up power line noise or some kind of other interference from somewhere.

7. All these interference are pretty different in nature, e.g. if i plug in adapter on my laptop it varies roughly +/-100.. if i plug in the adapter for Arduino Mega board, the readings are randomly up n down with shift of +/- 50.
And, if i keep it nearby to an opened up monitor(i opened it because i have to work on that.) then the shift is fierce.. more +/- 1000.


Please help me with it.

P.S. the whole circuit works great if i am running my laptop on battery and there is no other appliance running closer than a foot to me.
Also, i need to have this circuit put up inside the monitor hence i cannot have this interference.
« Last Edit: January 10, 2011, 10:54:04 PM by nikhilpandey »

subpilot

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Re:ADC Fluctutations.
« Reply #1 on: January 13, 2011, 12:27:38 AM »
Is the shield of the sensor cable grounded? An un-grounded shield cable can induce all kinds of noise in the ADC input. Also if the sensor cable is very long, you want to ground the shield only on one side, preferrably on the sensor side in order to prevent ground loops, which are also a source of noise.
Hugo

nikhilpandey

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Re:ADC Fluctutations.
« Reply #2 on: January 13, 2011, 04:05:15 AM »
Actually there is no sensor cable as such.. i am using normal connecting wires to read my sensor (a LED).

Actually i was able to beat it up.. i introduced a metal sheet in between my sensor circuit and monitor, it took almost a lot of noise out.

But again, i am getting a bit of it...