# Working with small signals

1. Feb 17, 2008

### DefaultName

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" [Broken] 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 by a moderator: May 3, 2017
2. Feb 17, 2008

### waht

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. Feb 17, 2008

### Corneo

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. Feb 17, 2008

### DefaultName

waht,

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

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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. Feb 17, 2008

### waht

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. Feb 17, 2008

### DefaultName

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. Feb 20, 2008

### DefaultName

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. Feb 21, 2008

### edmondng

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)