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Author Topic: a LOT of noise on the a2d port...  (Read 1287 times)

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Offline arjesTopic starter

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a LOT of noise on the a2d port...
« on: February 19, 2009, 01:24:49 AM »
We are doing a2d conversion from a homemade sensor (it is picking up noise at different times). The sensor is a emf detector, with center frequency at 10khz. The sensor is picking up noise at random intervals, even when we are not close to the source of the emf field. We noticed that as we touch the metal chassis, the noise goes away and the a2d conversion outputs the expected results.

We are using 10 bit conversion that varies from 0-200 when the noise is present. We have also verified the ouput of the sensor, it is rectified and it outputs values from 0 volts  - 5 Volts


I would greatly appreciated any advice anyone may have regarding this... We are at a loss as to why this is happening.

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Re: a LOT of noise on the a2d port...
« Reply #1 on: February 19, 2009, 03:44:14 AM »
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We noticed that as we touch the metal chassis, the noise goes away and the a2d conversion outputs the expected results.
This very much sounds like a grounding problem. Perhaps you don't have a common ground, or you did but a wire came loose.

Now when you say metal chassis, what wiring is touching it? A DC motor by any chance?

There is also the chance the metal is acting like an antennae and picking up signals.

Try taking a wire connected to ground and touch it to the metal chassis and see what happens.

Offline arjesTopic starter

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Re: a LOT of noise on the a2d port...
« Reply #2 on: February 19, 2009, 10:45:46 AM »
We thought so too. We did try hooking an alligator clip from the gnd on the axon to a screw on the chassis.

Let me give some more details as that post was at 3AM when we were trying to figure out what was wrong.

We made two of these sensors, one works great though the amplification is a bit low (just need to redo one of the stages with a smaller resistor). The other responds to the 10kHz square wave signal its supposed to pick up but the a2d fluctuates significantly.

The motors are 12V DC grearhead motors, but they are off when it we were testing the a2d. Additionally before we started having problems the a2d red less then 100 with motor noise (so the filter used to work well at least). We have attached the circuit to a function generated and a oscilloscope and it is properly filtering the square wave to its component 10kHz sine signal and it is clipping at +7V as it is designed to amplify a signal that is smaller then the function generator can generate.

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Re: a LOT of noise on the a2d port...
« Reply #3 on: February 20, 2009, 09:09:00 PM »
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We made two of these sensors, one works great though the amplification is a bit low (just need to redo one of the stages with a smaller resistor). The other responds to the 10kHz square wave signal its supposed to pick up but the a2d fluctuates significantly.
Are both the exact same sensor/schematic? Both share the same ground? Whats your amplification multiple?


Try disconnecting the motor wiring and see what happens.

 


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