EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Obi_Kwiet on November 19, 2015, 04:29:35 am
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I'm trying to throw together a quick and dirty anti-aliasing filter to play around with Freedom K64F's built in ADC and DSP functions. The plan is to feed it an audio signal, and see how it does, but quality isn't important at this point.
I have some dual opamps LM358s sitting around, so I biased the input to the mid-point of the ADC (~1.65V), used one of the op-amps as a unity gain buffer, and fed that into the reference filter in this design calculator: http://sim.okawa-denshi.jp/en/OPttool.php (http://sim.okawa-denshi.jp/en/OPttool.php)
The problem is that the output across all frequencies pretty much uniformly has a gain of maybe -25dB. I probed the circuit and found that everything looks great at the input node of the low pass filter, so the LM358 is working. I'm powering it with +12V and Ground, but the signal isn't going anywhere near ground.
Does anyone who knows the LM358 well have any suggestions before I tear down and try another approach?
Thanks!
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Schematic?
Is the input AC coupled and the op-amp biased to VCC/2, or the input referenced to a virtual ground?
Tim
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See attachment. Hopefully that will make things easier to see at a glance.
Like I say, I probed at pin 1 and the signal is doing exactly what it should be there. Nice pretty unity gain, biased around 1.65v, no where near ground.
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Low pass filter needs to be referenced to your mid-point.
As it is, at DC it's got a gain of -1, and your audio is at VCC/2. Multiply that by -1 and your output is at ground.
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Doh! I should have noticed that! Negative gain applies to the DC component as well! |O This is what I get for copy-pasting a topology without taking the time to understand it. It's been too long since I've worked with hardware.
Thanks!
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Pin 5 must be referenced to 1.65V as well.
Randy