PSN-L Email List Message

Subject: Re: OP-Amp with ADC only give positive values
From: GeoffHM gmvoeth@...........
Date: Fri, 11 May 2012 07:06:04 +0000


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), im 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 means you must like use B- as a common and limit the output between 0 
and +5 with zero floating at +2.5V
  [+2.5Volts = counts +/-((2^(16-1) - 1] converted to count zero,
But look at the counts and not the voltage because they many times do 
not agree exactly. Unless the output of the amp goes rail to rail
you need some kind of zero and span adjustment to realize
full scale results. Do not use a DC gain overall greater than
X10,000 or things get too difficult to handle. to sensitive.
You might even see pulses when you operate a microwave oven
seemingly from leaky microwaves coming out of the oven itself.
If your gain is like 100K or more.
Pay careful attention to the sensor construction, that's why
I use a commercial model instead of build my own.
I can not create a sensor which is as good as those you buy.
I guess you are writing your own program as well as building
your own circuits ?

  Too bad HEATHKIT did not build a seismometer kit.
  I bet some folks would enjoy building such a kit.

geoff


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