The problem is that your op-amp has a gain of=20
about 100000000 at low frequencies.  It wil=20
almost certainly go to maximum output in one=20
direction.  I would expect that if the device=20
isn't damaged by applying a voltage directly=20
between the input pins, its output would simply=20
snap between its maximum positive and maximum=20
negative values as you operate the pickup=20
coil.  You must have a resistor R1 added between=20
the coil and the '-' input termal and another=20
resistor R2 between the op-amp output terminal=20
and its '-' input.  You will obtain a voltage=20
multiplication equal to R2/R1 with that=20
circuit.  I would try R2 =3D 100K and R1 selected=20
to get the gain factor you want.

Op-amps will not work without having some way to=20
feed back a signal from the output to the '-'=20
input and a resistor to limit the input current.

Regards,
Brett


At 12:02 PM 5/8/2012, you wrote:u
>Hi.
>
>I=B4m struggling with this issue for some days:
>Using a "pickup coil" that i get from a car=20
>relay (310ohm), when i connect it directly to=20
>the ADC i have positive/negative values, but=20
>when i connect the "pickup coil" to a OP-Amp=20
>(OPA227) and the ouput from the amp to one ADC=20
>inputs i only have positive values.
>
>This is the results with a magnet using OPA227 :
>http://dl.dropbox.com/u/15802004=
/immm.png
>
>Rudimentary schematic (i=B4m not using any kind of=20
>resistor/capacitor in the amp)
>http://dl.dropbox.com/u/15802004=
/circ.png
>
>I really wish to use the OPA227 because it=B4s=20
>much more responsive than connecting the "pickup coil" directly to the ADC.
>
>
>Thanks.

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