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Scott Elder
Scott Elder
6/20/2013 5:18:16 PM
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Less than perfect means more power and noise
I've been studying this problem for a while now.  Think about all of the power and money that is used in an analog design just because things don't perfectly match.

All of these calibration circuits that have to fix the mismatch in a simple differential pair at the front of an amplifier.  Like auto zeroing and chopping.  Yes, chopping gets rid of 1/f noise, but then you get lots of chopping noise in return.  So we go add a few more power hungry filter circuits to solve that.

Same for resistor mismatch around an instrumentation amplifier to reject the common mode noise.  More noise, more power, more money.

So you're right that you don't need perfect anymore.  But if you don't have it, be prepared to pay for it.

Last time I checked, an 18-bit SAR was over $25 in moderate volume.  And that's for a part made from 30 cents worth of materials.  Sure is a lot of money for having to fix something that's not perfect.

 

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Netcrawl
Netcrawl
6/20/2013 6:51:28 PM
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Re: Less than perfect means more power and noise
@Bill great article, the fierce competitition in the industry demands that a company must develop complex products in the most cost-effective manner and the most fastest way, and speaking of cost-effective, how do you get a cost-effective design? 

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D Feucht
D Feucht
6/22/2013 8:56:19 PM
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Re: Less than perfect means more power and noise
@Bill - The fraction of purely analog designs like those Williams liked to do is diminishing. Thirty years ago, I was using a 6502 uP to do two-point calibration of a thermal energy meter DAS which used an LM331 VFC for A/D conversion. Two-pt cal is common and need only assume that the process is linear for compensation of both offset and gain or slope errors.


@Scott - With a cheap uC and an external op-amp integrator, it is not hard to achieve 14 or more bits of conversion with a sigma-delta algorithm. This is often adequate resolution and linearity is not much of a problem with inherently monotonic sigma-delta conversion. The biggest benefit of the uC is that it allows abandonment of all those awful trim pots! Some variable capacitors can also be eliminated with dynamic compensation.

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Scott Elder
Scott Elder
6/22/2013 11:33:35 PM
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Re: Less than perfect means more power and noise
@D Feucht

I here you, but if you need the result in 1us from a sleep state so the average power is a few microwatts, you'll need something alot faster and more expensive than a 1st order SD ADC.

I think there will be a resurgence in SAR converters once this IoT thing gets into full gear.  SAR converters are about as low power as you can get since they can wake up, make a 16-bit measurement, and shutback down to zero power in one micro second.  And most IoT devices will probably be mobile battery powered devices.

But who knows.....for sure.

 

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amrutah
amrutah
6/23/2013 1:05:46 AM
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Re: Less than perfect means more power and noise
I agree with Scott here.

  Though the process and technologies are changing (shrinking) the perfection to the analog parts is brought by growing the size i.e. reducing mismatch, burning more current to increase accuracy and reduce noise and each of these directly translates to increase in cost.

  We still need good and perfect analog, with the stirdes we are making into space and inside human bodies.  But we are losing the people who know it.

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Brad Albing
Brad Albing
6/23/2013 10:13:33 PM
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Re: Less than perfect means more power and noise
Scott - very good point on the very fast cycle time as a method of saving power. If you really can power up the device, take a measurement, process/store the data, and shut back down, that's the way to go in ultra-low power apps. Rather than just trying to make the worlds lowest power SAR.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
6/27/2013 6:35:54 AM
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Master
Re: Less than perfect means more power and noise
@Brad,

Hit-and-run would be a good idea, I agree, but, if there are intemediate stages like the sample and hold, then this timing needs to be added to the process. As you said, instead of wasting energy on making an impossibly low power device, one could do well with a rapid cycling device with so called 'imperfections' .

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