Bob's idea is not correct, because the A/D converter only has 0-to-5V 
inputs. The top drawing in which the coil's wires are connected directly 
to the plus and minus inputs is "correct" or at least more nearly 
correct, but no doubt the inputs could use some sort of bias (DC 
reference voltage).  The A/D sees the difference between the voltages on 
the positive (pin 1) and negative (pin 24) inputs. Furthermore, the 
op-amp need some biasing and gain-setting resistors to work properly.

Amplifying the coil's signal and connecting the amplified signal to this 
A/D converter is substantially more complicated than indicated by either 
of these drawings. Filtering is also needed, and at least some of it is 
better done in analog (between the amplifying circuit and the A/D 
converter) than in the digital domain.

The circuits Stephen posted will work, but the negative input, pin 24, 
needs to connect to a source of 2.5V instead of ground. This can be done 
just by connecting together  pin 24, the REFin and REFout pins.

The data sheet shows an INA118 on several schematics such as one on page 36:
http://www.ti.com/lit/ds/symlink/ads1211.pdf
The INA118 may "look" like an op-amp in the circuit, but it's an 
instrumentation amp made of several op-amps, and the biasing and gain 
setting are done differently.


On 5/8/2012 12:43 PM, Bob McClure wrote:
> 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
>
>


  
    
  
  
    Bob's idea is not correct, because the A/D converter only has
    0-to-5V inputs. The top drawing in which the coil's wires are
    connected directly to the plus and minus inputs is "correct" or at
    least more nearly correct, but no doubt the inputs could use some
    sort of bias (DC reference voltage).  The A/D sees the difference
    between the voltages on the positive (pin 1) and negative (pin 24)
    inputs. Furthermore, the op-amp need some biasing and gain-setting
    resistors to work properly.

    

    Amplifying the coil's signal and connecting the amplified signal to
    this A/D converter is substantially more complicated than indicated
    by either of these drawings. Filtering is also needed, and at least
    some of it is better done in analog (between the amplifying circuit
    and the A/D converter) than in the digital domain.

    

    The circuits Stephen posted will work, but the negative input, pin
    24, needs to connect to a source of 2.5V instead of ground. This can
    be done just by connecting together  pin 24, the REFin and REFout
    pins.

    

    The data sheet shows an INA118 on several schematics such as one on
    page 36:

    http://www.ti.com/lit/ds/symlink/ads1211.pdf

    The INA118 may "look" like an op-amp in the circuit, but it's an
    instrumentation amp made of several op-amps, and the biasing and
    gain setting are done differently.

    

    

    On 5/8/2012 12:43 PM, Bob McClure wrote:
    
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 <brett3nt@.............>
          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