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difference amplifier offset trimming

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

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  1. Hi John,
    May be this is the resistance (DC value). A impedance is a complex
    (frequency dependent) value. This is significant here. above 1 kHz the
    absolute value of your testimpedance will be much lower, I guess. If not,
    then you have to give up to kilovolt. Then you need a transformer very
    surely. You may try out a power transformer using the "primary" (110V side)
    as secondary. So for a 5V transformer you only need 50 V peak from your
    current regulator...

    This may be the reason for interrupted saturated coils I told about...

    Marte
     
  2. john

    john Guest

    Hi,

    I am confused about the following things

    1. What kind of transformer should I use, power transformer, current
    transformer, pulse transformer . Current ( range 10nA to 1mA) is the
    input to the transformer (primary) and current is the output of the
    transformer , my load is 100kohm.

    2. What should be the winding ratio?

    3. Input current waveforms are arbitray, continuous, pulses ( single
    or many ) and brust waveforms?

    4. I need the same replica of the input waveform. I suspect that the
    primary winding (inductor) will introduce high pass filtering, what
    should be done to avoid that?


    Regards
    John
     
  3. Hi John,
    How would it be, that you specify your needs complete. Elsewise we allways
    have to read in a big crystal ball...
    What I know:
    You want to stimulate a small tissue (retina of a rat), so your electrodes
    may be pretty small (needle electrodes?). You expect a impedance about 100
    kohm (which will be very frequency depending not only one value for all).
    Your current peaks will be up to 1 mA. The waveform is arbitrary. You didn't
    talk anything about duration repetition rate....

    A few infos more please then someone may help with concrete tips.

    Marte

    If you are interested in my help, then please write me the specification via
    email. here in this group the message may reach me or not. there is too much
    garbage arounud :-(
     
  4. john

    john Guest

     
  5. Winfield

    Winfield Guest

    John, learn transformer basics: e.g. primary voltages scale by the
    turns ratio, as do secondary currents. Impedance ratios are equal
    to the square of the turns ratios, primary load looks like secondary
    load times impedance ratio, in parallel with the reactance from the
    primary's magnetizing inductance (the secondary load impedance
    is of course due to the electrode skin resistance). The ratio is
    high
    at high frequencies, and can be ignored, but at low frequencies it
    determines the -3db cutoff frequency. To get a high magnetizing
    inductance you'll need more turns, as determined by the core's
    A_L parameter, which is specified in terms of nH/turns-squared.

    Once you learn these things and begin to understand transformers,
    you can either design your own, or understand how to evaluate a
    commercial data transformer, as has been suggested. It's likely in
    the latter case that you'll need impedance-measuring equipment,
    but it's also very useful in the former case. Either way it's
    valuable.
    I have found eBay to be a good source for old laboratory-grade
    high-performance HP vector impedance-measuring equipment, but
    for simplicity, ordinary ac-attenuation measurements may suffice.
     
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