On 5/8/2012 7:23 PM, Tiago A. wrote:
> I forgot the important info that the ADC accepts only 0-5V, but on page 18
> of the datasheet (http://www.ti.com/lit/ds/symlink/ads1211.pdf), using
> Vbias pin 7..it allows the ADC accept negative voltage. But i dont know if
> it will help solving my problem.
>
> About connecting Pin24 to REF 2.5V i did that several times, but since
> didint solve my issue, i put it back to AGND. But if you recommend
> connecting to REF i will do that.
>
> I wish to start with the basic (simple amplification), i´m only doing some
> tests..later when i have the complete sensor i will build a more complete
> (and complex) circuit. Using this schematic:
> http://pw2.netcom.com/~shammon1/compressed/PSN_PreAmp.jpg
> Can i remove the second OPA227? If so, can i "cut" the circuit from C1
> capacitor?
>
> I usually set the ADC to output in 16bits, but if external amplification is
> not enough i can raise the resolution or/and use the internal PGA up to
> 16x, but with this coil and magnet, its still not enough..the amp is giving
> great response, but incorrectly.
>
> So i still need -5V supply in the OPA227? This one will be a bit tricky..
>
> Thanks for the answers.
>
>
> 2012/5/8 Ben Bradley
>
>>   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 Nordgrenwrote:
>>
>>> 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
>>>
>>>
>>>
>>
>
It seems to me a matter of selecting reference points or GROUNDS.
If you use a split supply you can select three different reference points.
Zero to B+ (B- will be ground)
Zero to B- (B+ will be ground)
Zero to +/- B (Center point being ground in a split supply)

You can achieve a similar result using a voltage divider circuit
with a single voltage supply circuit.
Some op amps do not swing all the way to B+/-
in that case you might do best to increase the voltage of B+/- to where
saturation and cutoff gives you a full swing (span) you are looking for.
So like if the opamp cuts of +/- 0.7 volts on either side.
And, you are looking for a swing of +/- 5.00 volts.
You make the supplies have a value of +/- 5.7 volts.
Its a matter of grounds and spans and zeros and can get
complex when making circuits have compatible grounds without destroying
each other.


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