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difference amplifier offset trimming

Discussion in 'Electronic Design' started by john, Apr 17, 2007.

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  1. john

    john Guest


    I am using texas instrument's part number INA2133U and OPA 131 to
    build the constant current source. The following link will take you to
    the circuit and data sheet ( page 13, figure 16). The inverting input
    of the amplifier is grounded.

    I tested the circuit for Leakage current and with zero input voltage ,
    load resistor of value 100kohm, R = 50kohm and rail voltages of +/-
    15volts, I am getting current in the range of 1 to 10 micro amp, which
    is undesirable for my application. The questions are as follows

    1. Do I need offset trimming like the data sheet discussed it on page
    number 9?
    2. Is there any other way to stop this leakage current?

    The desirable leakage current range is less than 1 micro amp.

    Please advice!

  2. How long would it take to solder in a trimpot and find out?

  3. Winfield

    Winfield Guest

    john <wrote:
    I calculate that your measured output "leakage" current should be
    less than 0.075nA, based on the maximum offset voltages of the
    ina133 of 0.45mV, and the opa131 of 0.75mV, summed and divided
    by 50k, plus 0.05nA maximum bias current from the opa131. Solve
    that big discrepancy, before thinking about zero-trimming circuits.
  4. john

    john Guest


    I connected the load of 100kohm resistor at the output and measured
    the voltage across the resistor using voltmeter. Is it a good method?

  5. Is everything well shielded? Do you have AC pickup (75pA
    is a pretty weak current, implying high-Z, subject to pickup
    and rectification)? Have you looked at the various signals
    with a scope probe? It's not the "schematic" part of a circuit
    that can get you into trouble, as much as the parasitic part.
  6. john

    john Guest


    What if the 50Kohm resistor is replaced by 5kohm resistor? will the
    leakage current go higher?

  7. The offset-voltage contribution will be 10x higher, yes,
    but it's still under 240nA, low compared to your value.
  8. john

    john Guest

    I tested the circuitry again and I am getting the leakage current with
    5kohm resistor of approx. 240nA. Is there any way that I reduce it
    less than 0.075nA.

  9. You can program up to 1 or 2 mA with a 5k resistor, and the
    0.24uA zero-current error you mention observing is 1/8000 of
    that, which is pretty good. If you add a little zeroing (which
    can be on the input-voltage side) you may be able to improve
    that by 10x or more. This gets you into the 10ppm area, but
    the matching errors of the difference amplifier may also begin
    to cause trouble and need attention.

    If you want 75pA zero accuracy in a 2mA full-scale circuit,
    that's only 0.04 ppm. To attempt to achieve that you'd need
    an entirely different approach, a true current source made with
    precision electronics, and perhaps with range switching, etc.

    What are you trying to do?
  10. john

    john Guest


    If you want 75pA zero accuracy in a 2mA full-scale circuit,
    that's only 0.04 ppm. To attempt to achieve that you'd need
    an entirely different approach, a true current source made with
    precision electronics, and perhaps with range switching, etc.

    So, the current source that I am using is not a true current source.
    Would you advice that what is a true current source and how can I
    build it?

    What are you trying to do?
    I am trying to build a system that can stimulate Lab. rat's retinal
    tissues. Thats why I am concerned about leakage current.

  11. Those that use the collector of a transistor or the drain of
    a FET for the output current, with an opamp providing the
    programming control.
    Aha! Then do you need really 2mA fullscale? If you use a
    higher resistor value then offset voltage is less of an issue.
    Maybe you can have a range-switching scheme?

    But I wonder if your concern is over leakage to ground, which
    could be eliminated with a battery-operated current source?
  12. Hi Winfield,
    No, in long term stimulation there would be a problem with dc leakage
    current, too. Like in all good TENS the OP should stimulate biphasic and
    dc-free. This is easyly done tith a output transformer.

  13. john

    john Guest

    Can you advice any circuit diagram or FET ?
    How a battery operated current source can not have a leakage current?

  14. john

    john Guest

    No, in long term stimulation there would be a problem with dc leakage
    Can you advice the transformer setup? Will the transformer be able to
    handle frequencies in the range of 1kHz to 40Khz.
    current amplitudes between 1uA to 1mA?

  15. Winfield

    Winfield Guest

    I like the transformer suggestion. If you feed it with a current
    source, the output will also be a current but without any DC
    component. A 1 to 40kHz range is easy, you can get three
    or even four decades of frequency range if necessary. There
    is little practical low limit to the currents that can be handled.
  16. Hi John,
    With such "easy" things you can use simple audio transformers. May be ISDN
    transformer will work too. just try out. This is not a big thing.
    If you want to do this work "science" like, then go to a manufacurer's site
    and use their transformer calculation tool for their cores.

  17. Hi Winfield,
    That ist the typical way to do this in transcutan stimulation (TENS). Other
    way is to couple out via capacitor. But a transformer is prefereable. Often
    it is seen in a saturated coil design. Just interrupting a few mA flow
    through a primary coil and plug the secondary winding to patient. But this
    one is not prefereable for use in battery powered devices.

  18. john

    john Guest


    I found the following transformer

    What do you think about it... Is it appropiate to pass peak to peak
    1nA to 1mA of AC current. I am also confused aboout grounding the
    primary and secondary of the transformer. Would they have same ground?

  19. Hi John, This is only a inductor, no transformer. There is only one winding. You need
    two. If you want good and stable signals then I would use a little bigger
    cores. What is the impedance that you expect to drive (including
    electrodes)? The core dimension is depending at voltage-time-product not
    only the current.

  20. john

    john Guest

    Hi Marte

    The impedances are between 100Kohm to 1Mega ohm.

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