## PSN-L Email List Message

Subject: Re: OP-Amp with ADC only give positive values
From: Bob McClure bobmcclure90@.........
Date: Tue, 8 May 2012 12:43:29 -0400

```You also need to apply minus 5 volts to the V- terminal. You cannot expect
negative output without it.

Bob

On Tue, May 8, 2012 at 12:31 PM, Brett Nordgren wrote:

> The problem is that your op-amp has a gain of about 100000000 at low
> frequencies.  It wil almost certainly go to maximum output in one
> direction.  I would expect that if the device isn't damaged by applying a
> voltage directly between the input pins, its output would simply snap
> between its maximum positive and maximum negative values as you operate the
> pickup coil.  You must have a resistor R1 added between the coil and the
> '-' input termal and another resistor R2 between the op-amp output terminal
> and its '-' input.  You will obtain a voltage multiplication equal to R2/R1
> with that circuit.  I would try R2 = 100K and R1 selected to get the gain
> factor you want.
>
> Op-amps will not work without having some way to feed back a signal from
> the output to the '-' input and a resistor to limit the input current.
>
> Regards,
> Brett
>
>
>
You also need to apply minus =
5 volts to the V- terminal. You cannot expect negative output without it.<=
/font>

BobOn Tue, May 8, 2012 at 12:31 PM, Brett Nordgren=
<brett3nt@.............> wrote:

The problem is that your op-amp has a gain o=
f about 100000000 at low frequencies. =C2=A0It wil almost certainly go to m=
aximum output in one direction. =C2=A0I would expect that if the device isn=
't damaged by applying a voltage directly between the input pins, its o=
utput would simply snap between its maximum positive and maximum negative v=
alues as you operate the pickup coil. =C2=A0You must have a resistor R1 add=
ed between the coil and the '-' input termal and another resistor R=
2 between the op-amp output terminal and its '-' input. =C2=A0You w=
ill obtain a voltage multiplication equal to R2/R1 with that circuit. =C2=
=A0I would try R2 =3D 100K and R1 selected to get the gain factor you want.=

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

Regards,
Brett

```