Working with small signals

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In summary, the conversation revolves around troubleshooting a circuit that is supposed to remove the DC offset from a sine wave signal using a high-pass RC filter. The problem is that the output signal is not accurate and is still affected by the DC offset. Suggestions are made to increase the value of the DC blocking capacitor and to use a large capacitor in the series path as a straight DC blocker. The conversation also touches upon the use of different types of capacitors and their effect on the circuit's performance. The concept of impedance and its relationship with capacitance and frequency is also discussed.
  • #1
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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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  • #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.
 
  • #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?
 
  • #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>
 
  • #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.
 
  • #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.
 
  • #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?
 
  • #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)
 

Related to Working with small signals

1. What are small signals in scientific research?

Small signals refer to low amplitude or low intensity signals that are typically difficult to detect or measure. They can be found in various scientific fields, such as physics, engineering, and biology, and can provide valuable information about the underlying processes or phenomena being studied.

2. How do scientists work with small signals?

Scientists work with small signals by using specialized techniques and equipment that are sensitive enough to detect and measure them. This may include using highly sensitive instruments, amplifying the signal, or averaging multiple measurements to improve the signal-to-noise ratio.

3. What are the challenges of working with small signals?

Working with small signals can be challenging due to their low intensity and susceptibility to noise. This can make it difficult to distinguish the signal from background noise and can require careful calibration and control of experimental conditions.

4. How do scientists minimize noise when working with small signals?

Scientists can minimize noise when working with small signals by using techniques such as shielding, filtering, and averaging. Shielding involves physically blocking external sources of noise, while filtering involves removing unwanted frequencies from the signal. Averaging involves taking multiple measurements and combining them to improve the signal-to-noise ratio.

5. What are the applications of working with small signals?

Working with small signals has a wide range of applications in scientific research. It can be used to study subtle changes in physical systems, detect tiny biological signals, and measure small changes in environmental conditions. It also has practical applications in industries such as telecommunications, where small signals are used to transmit and receive information.

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