Why is My 120mVpp Sine Wave Outputting at 160mVpp?

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Discussion Overview

The discussion revolves around the unexpected output of a sine wave signal, where a participant reports an output of 160mVpp instead of the expected 120mVpp after processing through a circuit involving an op-amp and a high-pass RC filter. The conversation includes considerations of circuit design, component values, and noise reduction strategies.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes sending a 120mVpp sine wave into a circuit and receiving a 160mVpp output, raising concerns about the circuit's behavior.
  • Another participant suggests increasing the value of the DC blocking capacitor to improve transparency to AC signals.
  • A different participant questions the use of a high-pass filter for DC offset removal, proposing a large capacitor as a simpler DC blocker.
  • One participant expresses confusion about the capacitor values, indicating that a smaller capacitor would increase reactance at the given frequency.
  • Another participant notes that their coworker has a similar circuit that works correctly, suggesting potential differences in implementation.
  • A participant inquires about noise reduction strategies and the impact of switching from a ceramic to a tantalum capacitor.
  • One participant provides a formula for impedance related to capacitance and frequency, discussing the implications for filtering high-frequency noise.

Areas of Agreement / Disagreement

Participants express differing opinions on the appropriate capacitor values and methods for removing DC offsets, indicating that there is no consensus on the best approach to resolve the output discrepancy.

Contextual Notes

Participants mention specific component values and configurations, but there are unresolved assumptions regarding the circuit design and the impact of various components on the output signal.

Who May Find This Useful

Individuals interested in circuit design, signal processing, and noise reduction techniques may find this discussion relevant.

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I'm sending in a sine wave with an A = 120mVpp, f = 200 Hz into a http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar5.html" 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?
 
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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.
 
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?
 
what,

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>
 
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.
 
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.
 
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?
 
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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