Need help with JFET/Op Amp amplifing circuit

Discussion in 'The Projects Forum' started by shmolky, Feb 3, 2010.

  1. shmolky

    Thread Starter New Member

    Feb 3, 2010
    2
    0
    I have built a circuit (see attached image) based on the LT1115 op amp to amplify the output from accelerometers (model ACH-01). They are amplifying signals received that are in the low kHz range.

    There are several problems, but the one I am most concerned with is that I have to unplug and replug in the accelerometers to get them to work after I've turned the power on. I've checked what I think is my drain current and it's normally between 10 to 20 uA (which is ok - if I understand the spec doc), but when I really move the accelerometers, the current can easily go up and over 100uA which (if I understand the spec doc) is beyond the saturation current and then they stop working until I replug them back in (a flat response or a little bit of higher frequency noise on the line). I'm sort of new to amplifiers... so if there's something I need to clarify I'm more than happy to do so.

    Another major issue is that I've got 8 of these amplifiers for 8 total channels and certain ones don't work when others are connected.

    They're powered in parallel from the filtered supply in the attached circuit. The best I could figure is that it was one of two things - either there wasn't enough power or there's some sort of grounding issue. I replaced the 9v batteries we were using with a PSU from a computer +/- 12v line to make sure it wasn't a lack of power issue. I know all the grounds are connected for the input signal and that ground is also connected to the output signal.

    Also, from time to time there's a strange higher frequency noise in the range of 60kHz or 300kHz.

    Any help is awesome as I've read from Horowitz and Hill and tried to find information on the internet, but it isn't quite soaking in.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Are the two C2's really in series with the power supplies? If so, that is your problem. The circuit works initially and quits when the capacitors charge up. Those caps are supposed to be bypasses, not i series. They are WAY too big; they are not needed as the two C1's are doing the bypass function.
     
  3. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    1. From the spec, they suggest that Rs should be 180K. This makes sense because the max Vgss is -1.8V. Therefore the quesient current is 10uA (1.8/180,000). The 20K referred to in the spec is the output impedance.
    2. C2 connected incorrectly. (already mentioned)
    3. Sensitivity is 9mV per g. To test; drop the device from one hand to the other... the output from the accel should be 7 to 11mV (9 typical).
    4. Your amp circuit gain is set for 100 so the output will be 0.9V for 1 g. Is that okay?
    Good Luck
     
  4. shmolky

    Thread Starter New Member

    Feb 3, 2010
    2
    0
    I will try changing Rs and see how that works. It was recommended to me to change Rhp (the 2meg resistor) and make it much smaller and I think it helped a little bit.

    About the two other capacitors, I believe they are there to rapidly supply current to the circuit. I don't understand it for these amplifiers (some high power circuit, sure), but it was explained to me by a professor that they provide power much more quickly than the batteries.

    I have also tried just using batteries with no capacitors and had the same problems with the amplifiers stop working. They always stop working for sure whenever the amplifiers have a large force subjected to them (I see Idrain go well over 100uA).
     
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