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Working with small signals

  1. Feb 17, 2008 #1
    I'm sending in a sine wave with an A = 120mVpp, f = 200 Hz into a summing amplifier, in addition to a 3.0V DC offset going into the same node, with equal weights for the resistors. I'm doing this to add the sine wave and the offset together, into one signal, so I ultimately have a sine wave with an offset. I'm sending the summing amplifier's output to an inverting amplifier to invert the signals into the positive range.

    After some circuitry to work with the signal after I've applied the sine wave + offset, at the end I want to remove the DC offset to interface it to a DSP chip. I can't really get into specifics, since it's for my employer, but as of right now I have a high-pass RC circuit setup.

    My problem is that when I send in a 120mVpp sine wave, I get a 160mVpp sine wave as the output, without the DC offset. I'm trying to be as accurate as possible. I'm using metal film resistors that have 1% tolerance and LM324 op-amps (with +/- 15V). C_highpassfilter = .1uF, ceramic. R_highpassfilter = 1Meg.

    Anybody know the problem?
     
    Last edited: Feb 17, 2008
  2. jcsd
  3. Feb 17, 2008 #2
    Try increasing the value of the DC blocking capacitor to make it more transparent to AC.

    0.1 uF looks like 8 Kohm @ 200 Hz.
     
  4. Feb 17, 2008 #3
    I don't understand why your using a high pass filter to remove the DC offset. Can't you just use a large cap in the series path as a straight DC blocker?
     
  5. Feb 17, 2008 #4
    waht,

    I will try that. I have a .01uF that I can try.

    ---

    Corneo,

    Yes, I would suspect I need an R after the cap for the DC to flow to ground? I'm taking the output of the signal across R (after C) with respect to GND>
     
  6. Feb 17, 2008 #5
    No!!!, 0.01 uF will increase the reactance @ 200 Hz. If you increase the capacitance to 100 uF, the reactance will be around 8 ohms and will look like a short for AC. The resistor won't do much.
     
  7. Feb 17, 2008 #6
    Oops. :(

    See, the funny thing is that my coworker has the same config as me and his circuit works! it's only off by 10mV or so.
     
  8. Feb 20, 2008 #7
    How can I remove the noise that is generated by my circuit? I'm going to switch from a ceramic capacitor to a tantalum capacitor. Will I see much of a difference?

    Also, I don't have much experience, but if I have a low frequency signal 150Hz to 200Hz, in general, would you want a high or low capacitance?
     
  9. Feb 21, 2008 #8
    Impedance Cap = 1/(2*pi*f*c)

    So as C increase, impedance decrease. Or if f increase and c stays the same, impedance decrease (thats why people put cap filter to gnd to remove high frequency noise)
     
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