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Old 7th February 2002, 01:25 PM   #11
tiroth is offline tiroth  United States
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One more question, then I'll go back to the books, I promise.

What are the consequences of running out of gain on a LPF circuit? Since the idea in an oversampling DAC is primarly to filter images that start in the hundreds of kHz, what happens when a device such as the venerable NE5532 is used? I see the open loop gain as

100 kHz: 42dB
1 MHz: 18dB

Is this sufficient to provide image rejection, or will the MFB concept break down due to insufficient gain?
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Old 18th May 2002, 11:53 AM   #12
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Default Use of NE5532

Hi tiroth,
Opamp bandwith should be at least 100<B>.</B>GAIN<b>.</B>Q<B>.</B>fn according to the Burr-Brown application note with the FilterPro program for a MFB section. Apart from that you will hear the poor quality of this opamp.
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Old 18th May 2002, 12:20 PM   #13
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Default Re: Ultra low noise filters

[QUOTE]Originally posted by tiroth
[B]I've been working some numbers, and I'd like to put some of them in front of the forum along with some questions. I hope some knowledgable folks will be able to help me out.

My situation is specific to op-amp based LPF, but applies equally to any kind of filter/buffer/transmitter. With today's ultra low noise opamps (AD797 at [0.9/sqrt(Hz)]) resistor noise can (usually) dominate even a simple circuit.


Parallel resistors (not w/w). Use bulk foils.
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Old 18th May 2002, 04:54 PM   #14
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1.) You do not need an elaborate filter after a '1704 at an 8x rate.

2.) You do not need an op-amp for these filters. Simple buffers will work fine, if not better as most any op-amp does not sound good as a unity gain block.

Trust me.

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Old 21st May 2002, 10:19 PM   #15
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You can always calculate from output to input sort of speaking. How low noise is "no signal"? Don't waste any time to make the noise in THEORY as low as possible. CD-audio has 98 dB theoretically and DVD around 110-120 dB. 120 dB S/N demands only 1 µV noise at 1 V. Not very extreme!

My advise is to use an opamp with noise in the area of 2-4 nV/Hz. OPA627 is not bad (but expensive...). 1 nV/Hz opamps demands signal sources with impedances of 100 ohms or less AND low resistor values in the feedback.
/Per-Anders (my first name) or P-A as my friends call me
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