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Topic: Weird data from Analog Input (Read 3938 times) previous topic - next topic


Hi all,
I have a problem with Analog Input on my Arduino Uno. I have A0 port to AD620N in-amp (powered from Arduino - +5V from USB), which amplifies signal from MPX2010DP sensor (powered from external 12V power supply). Readings I get from A0 port are far from I expect - they tend to oscillate from around 400mV to 4000mV (with the same frequency as sine signal I get when A0 is not connected to anything), when, standard, simple voltometer shows "steady" 1700+-10mV - and I don't know why.

I've tried connecting AD* to the same power supply as MPX* (power supply has +5V and +12V rails with two ground lines), disconnecting external supply, changing AD* - but no success.

I would be very thankful for any help/ideas why this may be happening.

Thanks in advance.


What gain resistor are you using on the IA?

Do you have an oscilloscope handy?
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I am using 47 Ohm resistor (for very high gain, since changes in pressure I'm going to detect are very small) - but changing it does not change the reading form A0 - it still oscillates.

Unfortunately, I do not have an oscilloscope, only very simple voltometer.


If you have a second Arduino you could make a (simple) scope - http://arduino.cc/blog/2010/10/06/xoscillo-cheap-arduino-based-oscilloscope/ -
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)


I do not have second Arduino either. I have tried connecting IA to other analog inputs, but with the same results - oscillating input (with nothing connected, it is simple sine wave).


If you short -IN and +IN on the AD620 do you get 0 volts out?

Are you using long wires between the pressure sensor and the AD620?  They might act as antenna and shielded twisted-pair cable might be a better choice.

Are you sure the 12V to the sensor is well filtered?
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Can you photograph or sketch your whole circuit?  We're guessing otherwise.

The AD620 will struggle work from a single 5V supply - its inputs both need to be very close to +2.5V  (about 2V to 3.5V) or it'll saturate.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


I'll post the sketch as soon as I get home. What baffles me is that readout from voltometer works (or looks like it is) OK- it changes from 1700mV to 1600mV (or 1900mV) as I change pressure on sensor inputs.

I can try and connect AD620 to 12V power, but I don't know if Arduino AIn can handle its output.


Little selfbump, here is my board, +5V comes from Arduino and AD620 is grounded by Arduino.

I have shorted IN+ an IN- and I do not get 0 volts, I get around 1.6V. Cables are quite short (max 5cm).


There's your problem!  You are hooking one side os a differential output to both sides of the amp!

Shouldn't you connect Vout+ (Pin 2) to +IN and Vout- (Pin 4) to -IN?
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OK, i forgot so specify that MPX image is wrong here - in database I found it has only one output. I have VOut+ connected to VIn+ and VOut- to VIn-. My bad ^^'' But, nevertheless, when I shor the AD620 i do not get 0 volts...


You don't seem to be providing a reference voltage on the REF pin of the IA.  Have you tried grounding that pin?
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I have grounded that pin, output still oscillates but with different pattern (i get "peaks" from time to time), and when shorted i still get 0.5V volts on IA. Voltometer reports changing pressure better, and "peaks" change in amplitude when I change pressure)


Is there something plugged into the AOUT jack?  If not it appears to be wired to short DOUT to Ground. :(
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May 27, 2012, 11:31 pm Last Edit: May 27, 2012, 11:35 pm by SurferTim Reason: 1
This is the datasheet at Analog Devices.

Note carefully in the specifications the output swing (page 4) and the input voltage range (page 3). Neither goes to the ground rail.

edit: I think that IC was designed for a dual power supply. Maybe there is a better op-amp for this?  :)

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