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I've noticed that in analog signal acquisition hardware you see two paradigms: AC-coupled and DC-coupled. Typically AC-coupled hardware is less expensive, and basically has a 20 Hz high-pass filter to remove the DC component. DC-coupled hardware, which is good for reading sensor signals for example, tends to be in the hundreds or thousands of dollars, whereas AC-coupled hardware like computer sound cards is very inexpensive.

What are the electrical challenges in designing high-quality circuits for sampling DC-frequency analog signals? I'm trying to understand why there is this huge difference in cost.

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2 Answers 2

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Components are always a big cost. When you need high accuracy resistors, you can run up your BOM cost quickly. Add in some pricey op amps (which are also my favorite, unfortunately), and you can have a 100 dollar board in no time. If you want to match the high speed A/D technology of the AC coupled circuits you speak of and want to do it at a high resolution you also will be paying through the nose. Just the other day I was looking at an 18 bit, 1 MSPS A/D that topped 25 dollars. You could build a lot of cool stuff with that same 25 bucks.

Also, a key thing to remember is 1/f noise. As you go towards (absolute) DC noise goes through the roof. DC is a tough field, but I work with it everyday so if you have any other questions, give me a shout.

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high accuracy resistors aren't really an issue anymore; you can get 0.1% resistors for under 10c in moderate quantities (take a look at mouser or digikey). But you have point about ADCs and op-amps. –  Jason S Dec 11 '09 at 23:34
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True, but drift can be an issue as well. Those are a little more pricey. What I really meant were the highest quality resistors that are used for those designs. They might have come down in cost but use enough of them and they are are significant adder to the BOM. Maybe I'm wrong though. –  Chris Gammell Dec 12 '09 at 3:26
    
1/f noise is the problem that most people catch, it is the same reason it is so hard to explain to someone why they need a chopper amp. –  Kortuk Dec 12 '09 at 22:35

Being DC-accurate (or at least accurate for frequency content below a few Hz) is tough -- you need absolute references + amplifiers that have low offset errors.

In addition, the major market for high-accuracy AC sampling circuits is audio, so ADCs for AC circuits have additional economies of scale.

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+1 for economy of scale. –  davidcary Mar 28 '11 at 23:21

protected by W5VO Feb 26 '13 at 22:44

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