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Update on: High gain current amplifier has excess noise

Discussion in 'Electronic Design' started by alan, Aug 17, 2005.

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  1. alan

    alan Guest

    I'm building a high gain current amplifier with a LT1793 and a 10GOhm
    resistor. It has the following problems:

    I measured the frequency response by blasting in white noise thru
    another large resistor. The frequency response is relatively flat up to
    about 100 Hz, has a very slight rise, and then falls off at a corner
    freq ~200Hz with a rolloff that looks a little bit faster than 1 pole.
    I tried that "RCR trick", but all that did was make the freq response
    fall off more slowly, more like 1 pole. (This "trick" is supposed to
    cancel out the parallel capacitance of the feedback resistor) It also
    makes the noise of the amp fairly flat (with freq) so that at higher
    frequencies, the S/N gets worse. I have tested this amp with and
    without an additional 80pF worth of input capacitance (to simulate the
    system this will be measuring) and the results are about the same.

    So I don't know why the frequency response of this amp drops off while
    the noise does not. Is there some parasitic capacitance elsewhere, or
    is it because this amp is running high gain, and I need to use some kind
    of composite amp config to improve the freq response?

    Thanks
     
  2. Isn't that a voltage amplifier with a 10GOhm feedback resistor?
    That is almost a wide open amp, no wonder noise is high.
    What is your required bandwidth of your design?
    Put a filter on it to cut the bandwidth down, noise is proportional to BW.
    What is the noise spec of your amp?
    Get a better amp.
     
  3. John Larkin

    John Larkin Guest


    Well, two or more cascaded opamps have a lot more gain-bandwidth than
    one.

    John
     
  4. alan wrote...
    Be careful doing that, Alan, what was the parasitic capacitance of
    your "another large resistor"? For example, a 10G resistor with
    0.1pF of self capacitance just looks like a capacitor above 160Hz.
    Have you remembered to include the effects of e_n-Cin noise? With
    8nV and 80pF this noise swamps the Johnson noise of a 10G resistor
    above f = sqrt(4kT/R) / 2pi en Cin = 315Hz. You can improve this
    by changing to an opamp with lower e_n, such as an AD743, assuming
    you don't mind 150pA rather than 3pA of bias current. Or you can
    make a discrete JFET input stage. For example, I'm using Toshiba's
    2sk170, which has under 1nV of noise, 8x better than your LT1793.
    Its gate leakage is less than 1pA for drain voltages under 8V.
    Keep in mind that the e_n-Cin noise density increases as f, which
    means it looks flat above a single-pole rolloff (this compares to
    white noise, which only increases as sqrt f). With a 10G resistor
    your e-n-Cin noise dominates above 300Hz, and 0.1pF of uncorrected
    parasitic capacitance means your 10G would rolloff above 160Hz, so
    your signal would go down but your noise would be flat above 300Hz
    until the opamp's 4MHz bandwidth limit hits at 900Hz (see below).
    Your modest f_T = 4MHz opamp has enough gain-bandwidth to extend the
    frequency response of an 80pF input node with 10G feedback resistor
    to about f = sqrt(f_T / 2pi Cin Rf) = 892Hz, assuming you can keep
    the 10G resistor's parasitic capacitance to under 0.018 pF, that is.
    Hah! Or you can use the R-C-R trick.

    Alan, did you tell us your bandwidth and current-noise specs?
    What are you working on?
     
  5. alan

    alan Guest

    I used a 100Meg, and tested it against a Keithley 427 to make sure the
    freq response was flat.
    Yes. And I also found out that for 10G, the LT1793 gave the lowest
    overall noise in the hundreds of Hz range.


    Or you can
    How does one make such a stage?
    While testing the noise only, I didn't add any input capacitance, so the
    e_nCin noise wouldn't rise above the Johnson noise until at much higher
    frequencies. Therefore the noise was flat, but the freq response was
    still dropping with 1 pole, even with the RCR trick.
    I tried and failed. See above. The freq response still falls off with
    1 pole at ~300ish Hz. Where did you come up with the f=sqrt formula? I
    guess my system rolls off only half an order of magnitude below that, so
    can I just build a composite amp by adding a 10x non-inverting amp
    inside the feedback loop? OR do I limit the Lt1793's gain inside the
    feedback loop and and another high gain amp? I guess I just have to
    look at the product of the open loop gains and make sure it is stable?
    I saw one of your composite amps in your response to "so what is the
    RCR trick", but it has both amps with switched input terminals (to
    un-invert each other?) and the second op-amp doesn't look like it has
    any DC gain.
    892 Hz is fine for bandwidth. Current noise should be as low as
    possible at a few hundred Hz.

    STM amp.
     
  6. alan

    alan Guest

    Never mind, I botched up my understanding of the composite amp. What
    resistor and cap values are you supposed to use inside the composite amp
    anyway?
     
  7. alan

    alan Guest

    BTW, I built a simple composite amp by inserting a 10x stage inside the
    feedback loop. It didn't prevent the roll-off, so I don't think the
    problem was with f_T
     
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