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BCBaker
BCBaker
6/10/2013 4:48:52 PM
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Re: All current ADC
You are very welcome. I wish I had a similar blog as I started to work with these types of converters in my earlier years.

1. There are some ADC architectures that sample current instead of voltage. TI has a strong line that uses delta-sigma technology, however the input signal is current as you asked for. These converters have a front end acronym that is DDC.

2. Jitter does not seem to have an impact until you start to sample at the higher frequencies. The sampling clock determines the samples per second of the converter.

3. The anti-aliasing filter is definitely required with the SAR converter. The order of this anti-aliasing filter is higher than the delta-sigma converter. For instance, the delta-sigma converter only requires a 1st order filter (or maybe a 2nd order filter) to eliminate high  as the highest order for this type of converter. The SAR converter requires a 3rd to 7th o r8th order anti-aliasing filter; typically 4th or 5th order filter. The purpose of the anti-aliasing filter is to remove signals that have a frequency component higher that the nyquist frequency of the converter. If you are unable to remove these signals, the converter is reliably convert these signals into a lower frequency within the bandwidth of the digital conversion. The actual magnitude of these signals reduced per the input bandwidth of the converter. The signals are brought back into the converters bandwidth at the digital output with the formula fALIASED = |fIN - NfS| where N is equal to an integer between 1 and infinity, FIN is your input signal, and Fs is the ADC's sampling frequency.  Charge injection does play a role in the accuracy of the conversion, but this is different than any aliasing error in the output digital code.

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amrutah
amrutah
6/9/2013 5:54:36 PM
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All current ADC
Bonnie,

   Many thanks for writing the blog on ADC basics, I read through all the 6 parts and looking forward for next parts.

   I would like to know,

1> are there any architectures that use and sample the current instead of voltages? Because if the voltage starts to drop we can escalate the current and rise above the noise.

2> Does the jitter on the sampling clock affect us in any way?

3> In Δ-Σ architecture, there is oversampling and the need for anti-aliasing filter is needed, but in SAR where we are sampling the ADC at nyquist rate why do we need to have filter (do they have the additional roles like current limiting (R) and avoid charge injection (C) ?

Thanks.

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Scott Elder
Scott Elder
6/5/2013 6:30:37 PM
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Re: Extend 12 bit SAR-ADC
The Title of the thread was "extending 12 bit SAR".  My point was that you can extend resolution, but you can't extend accuracy with a SAR unless you get a lucky part.  I think many engineers confuse one with the other.

And you're right, somehow I dropped the 4096 denominator in my equation that was meant to normalize the fractional result relative to 12 bits prior to converting into 16 bits.  But I hope you get my point.

[went back and fixed the 4096]. 

 

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BCBaker
BCBaker
6/5/2013 4:55:50 PM
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Re: Extend 12 bit SAR-ADC
Scott,

Thanks for the formula. I can see how that works for at least the porton of the formula that is inside the parenthesis, which equals 4,094.5. The multiple of 2^16 does not make sense. Actually, the total formula equals 268,337,152 and not 65,512.

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BCBaker
BCBaker
6/5/2013 4:47:24 PM
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Re: Further specs
Jayaraman,

The beginning of this journey will start with the next installment of this article series. Expect it to come out the end of June or the first of July.

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Scott Elder
Scott Elder
6/5/2013 1:11:23 AM
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Re: Extend 12 bit SAR-ADC
Bonnie,

Say I take a 12 bit reading that is alternating between two 12 bit codes (because of introduced dither or noise) and repeatedly add them up to get a total.  And then divide that total by the number of times the 12 bit reading is added.  The result of the division is a fractional number.  I could turn that fractional number into a reading with more than 12 bits.  See the case below with 256 readings.  Do you agree?

(2^16/2^12)*(4095 x 128 + 4094 x 128)/256 = 65,512

If you agree, then my point is simply that this is a 16 bit number, but it is not neccesarily 16 bit linear unless the 12 bit DAC was 16 bit linear.  The process increased the number of bits in the answer, but it didn't necessarily improve the accuracy.

Did I write something that you think is inaccurate?

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Scott Elder
Scott Elder
6/5/2013 12:21:33 AM
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Re: Extend 12 bit SAR-ADC
Bonnie, I'm confused about your post.  What is it that you understand I am claiming?

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BCBaker
BCBaker
6/5/2013 12:15:56 AM
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Re: Extend 12 bit SAR-ADC
Scott,

Fascinating assertion. I know that you are bemoaning the fact that you 12-bit converter does not have 16 bit linearity in the on-chip DAC, however consider the manufacturer that develops a family of converters. The more common family of converters would be 8-bit, 10-bit and 12-bit converters. This manufacturer simply eliminates the LSBs at the output as the converter family goes lower in bits. The manufacturer charges the end customer the appropriate price in accordance to the other products offered in industry. This would be for the lower frequency SAR converter that provides an SPI serial output. Using your suggestion you could turn your 8-bit converter into a 12-bit converter with oversampling at the 8-bit price. But the place where I get lost is the fact (in my example) that you still get 8 bits coming out of the converter. How do you increase these 8 bits to 12 bits.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
6/5/2013 12:14:39 AM
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Master
Re: Further specs
Bonnie,

I agree fully that the involvement of analog parts in the path, multiplexer , sample/hold, etc influence the timing aspect. Surely, a detailed article on how one can work on these issues, would be welcome.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
6/5/2013 12:09:28 AM
User Rank
Master
Re: One's complement ADC
@Dirceu,

Innovative application of single supply amplifier and one's complement ADC. Of course, for a non-mcu system this would be very valuable.

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