Connecting output from instrumentation amp to a USB ADC?

Discussion in 'The Projects Forum' started by JimYoung, Jun 27, 2009.

  1. JimYoung

    Thread Starter New Member

    Jun 27, 2009
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    Hello Everyone,
    I have a circuit using an INA 128 instrumentation amp, and in my particular setup it outputs an AC signal with amplitude ranging from 0 - ~6v. Connecting my system to an oscilloscope works great, so everything is fine on that end.

    I also have a phidget 8/8/8 USB interface kit, which I have used before. It has several analog inputs. Each analog input has a ground reference (0v), a voltage source (+5vdc), and a analog measurement reference. Basically, from my (very basic) understanding, I can build a simple voltage divider circuit with these three wires and provide input.

    My problem is in how to connect these two systems together. I have read quite a bit of the textbook on this website (great help for a newbie like me!!), but im still not sure how to go about this.

    my instrumentation amp uses 2 9v cells in series, so I have considered replacing this with the 5v offered from the phidget board, although i am investigating how this will affect my gain. How can i keep the separate power sources, and still get my ADC board to read the voltage amplitude from the input signal?

    Any help would be greatly appreciated!
    Thanks!


    Jim
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    According to the datasheet, the INA128 has an output voltage range of +V -1.4v to V- +1.4v with a 10k Ohm load impedance. If you attempted to power it from +5v/GND, you would have an output range of 1.4v to 3.6v; not much.

    Better to leave it powered by your 9v batteries, which are providing +9v/-9v.

    Your amplifier's output is currently exceeding the range of your "phidget" ADC input, as far as I can tell - you've not provided much in the way of specifications. You also have not provided your current Rg value (gain resistor value for the INA128).

    Decreasing the gain of the INA128 by increasing the value of Rg would be much better than using a resistive divider on the output.
    G = 1+ 50k/Rg

    Without documentation on the "phidget", it'll be hard to say much beyond the two "grounds" needing to be connected together.
     
  3. JimYoung

    Thread Starter New Member

    Jun 27, 2009
    2
    0
    Thank you very much for the feedback.

    My gain resistor is at 120ohms, and there is a 100 pf capacitor in series to filter out some of the low frequencies. (~ gain 400)

    As you said, I can adjust this resistor to select my output range.. I think that my question is a little more fundamental.

    First, my (undeveloped) intuition says to share the ground between the two and just connect the output from the op amp to the phidget analog reference, ignoring the +5v it provides. However, I dont have the education to know if this is correct. Does it make sense to share the ground (- terminal in this case) of my 2x9v battery connection with the 0vdc on the phidget board?

    As for the internals of the phidget board, I dont have much details, as the manual was nothing.

    I just found a better manual on their website: http://www.phidgets.com/documentation/Phidgets/1018.pdf

    p 12 / 13. from looking at this, i want voltage to be formed across the analog input and ground, i assume? So, does my above approach make sense?

    i am trying to be very careful with the fundamentals before i start experimenting. Earlier in this project i mis-wired my bread board, putting the full 18v across the phidget board, frying my USB port.. so. yeah, careful :)

    Thanks again!!

    Jim
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK. Increase Rg to 180 Ohms, which should then give you an output range of 0v-4v instead of 0v-6v.

    Ahh ... that doesn't make sense, really. Do you mean that had a 100pF cap in series with Rg? Or in series with the output?
    Your gain was about 417 with Rg = 120 Ohms, the new gain should be roughly 279.

    OK. The phidget has three terminals per channel; ground, power, and analog input. The ground (black wire) should be connected to where your 9v batteries are connected together + to - (thus creating an artificial ground) and the output of your INA128 connected to the analog input (white wire). Leave the power (red wire) disconnected.
     
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