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Old 28th February 2002, 05:27 AM   #1
tiroth is offline tiroth  United States
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Default Audio Measurement ADC input stage

I'm considering building an ADC or using a stock sound card as an input in order to take advantage of computer-based measurement tools. How have others solved the input protection problem?

(While "always use an attenuator" is an option, I would prefer to have both a "protected" input as well as a bare "hi quality" one)

What about the circuit below, shamelessly adapted from the Bitscope plans? The bandwidth of interest would be 10-100kHz, and 103dB S/N with very low distortion would be considered minimal. I am not sure that the 82pF cap serves any purpose at these frequqncies. I've seen designs with zeners but I would think these to be far too noisy. Are there other alternatives? I am afraid to go active in what will be my measurement set.
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Old 19th March 2002, 05:58 PM   #2
Kay is offline Kay  Germany
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Default Input Stage

Use Led's instead of 1N4148.
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Old 19th March 2002, 06:56 PM   #3
tiroth is offline tiroth  United States
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Lower noise?
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Old 19th March 2002, 08:47 PM   #4
Kay is offline Kay  Germany
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Default LED

>lower noise
Yes, you see LED's in current sources for improvement.
And you get lower distortions, but it's dependend for the level you can accept or you need at your input.
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Old 20th March 2002, 08:42 AM   #5
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The reason LED's are used in current sources is due to their voltage drop of around 2.0 - 2.2v (depending on colour), not 0.6v for a diode (plus it looks cool having a led glowing away on the PCB).
Does anyone have any definitive proof of the noise contribution of LED's over normal doides?
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Old 20th March 2002, 09:03 AM   #6
sonnya is offline sonnya  Denmark
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I have not been able to find any proof of LED versus normal diodes (Not Zeners), but would like to hear if any papers exists on this matter.

I think using LED as OVP is a bad idear. They are not as fast as ~1n4148 or schottky (only .2V drop) or fast recovery diodes.

I would go for the 1n4148 because of the lower capacitance across it.
For the cap 82pF across 10k: Depending on the size of the inputcapacitance of your ADC i would think it only has a little impact above 50 - 100KHz. Try run without first.

Sonny
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