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Lock in amplifier usage

  1. Nov 15, 2007 #1
    i am using a lockin amplifier for the 1st time other than a canned college lab demo.
    i have a 5mV signal from a strain gage that is driven with 10Vpp at 1kHz or whatever i choose.
    the 5mV is increased to 5.01mV when i add some weight to the arm the
    gage is weighing.
    i wanted to increase sensitivity of this reading with the LIA since there is noise in the next digit, i.e.
    i read 5.01mV +- 0.01mV.

    But i seem to go out of range (overload) before reading any weight change.
    do i need to get a 0V average signal first, and put this into
    the LIA?

    the 4 strain gages are in a full wheatstone bridge configuration.
    The 10V 1kHz signal is sent to the bridge and to the LIA 'reference' input,
    5mV 1kHz bridge output sent to to LIA signal in. I adjust the LIA phase for max output,
    zero the signal with the internal LIA offset, and increase sensitivity until i overload.
    then i back off the sensitivity so theres no overload. at this point i see no change in output
    with change in weight. the LIA sensitivity is generally 30mV at this point, whereas
    the signal I want to measure is 0.01mV or less, so it seems natural i wont get a change in
    output. the only thing i can think of is i must 'balance the bridge' to get 0.00V output, and
    amplify this to higher levels e.g. at the 0.01mV level.
    is this right?
  2. jcsd
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