Reply by Steve at fivetrees●June 11, 20072007-06-11
"djordj" <djordj@despammed.com> wrote in message
news:mn.5ac67d7613d4d06f.73215@despammed.com...
> Steve at fivetrees expressed precisely :
>
>> We used the classic ratiometric approach: we used a 4-channel input
>> mux, and measured the input signal, a reference voltage, ground, and
>> either the cold junction for thermocouples, or the third wire for
>> RTDs. That allowed us to characterise everything except linearity.
>
> Can you give me any link about this approach?
> We used the classic ratiometric approach: we used a 4-channel input mux, and
> measured the input signal, a reference voltage, ground, and either the cold
> junction for thermocouples, or the third wire for RTDs. That allowed us to
> characterise everything except linearity.
Can you give me any link about this approach?
Thanks!
Reply by Steve at fivetrees●June 8, 20072007-06-08
"Paul Taylor" <paul_ng_pls_rem@tiscali.co.uk> wrote in message
news:pan.2007.06.08.20.29.01.108147@tiscali.co.uk...
> On Tue, 05 Jun 2007 16:56:20 -0500, Vladimir Vassilevsky wrote:
>
>> I am wondering of what could be a sensor which requires the ADC with
>> the
>> true 16-bit accuracy. For the sensor application, that sounds
>> unreasonable to me. Especially considering that the rest of
>> application
>> is handled by a small micro. Apparently there is a problem with the
>> concept.
>
> I worked with a 16-bit cirrus logic sigma-delta adc in a design with a
> non-linear gas sensor and 16-bits was _just_ enough. If needed, I
> could
> have gone up to the 23/24 bit device in the same footprint, for more
> resolution. Absolute accuracy wasn't the main issue (I could have got
> away
> with 12/13 bits for that), resolution was. I used it with an 8-bit
> micro.
My experience was similar (except with discrete integrating ADCs [1]).
Application was temperature control; we needed as much resolution as we
could get, with low noise and total monotonicity (mainly for the
derivative term), while absolute accuracy was somewhat less important -
around 0.25% of full-scale, IIRC.
[1] By "discrete", I mean that our cost and resolution constraints
didn't allow for buying in an ADC (this was some years ago). We used a
couple of methods: one was the classic dual-slope integrator (where we
measured time), the other was a voltage-to-frequency design (where we
counted pulses).
> The really good thing about the A/D, was that its programmable
> internal
> amplifiers had a gain stability against temperature of only a few
> ppm/degree celcius, which meant that I could get away with a cheapish
> reference, of about 5ppm (iirc).
We used the classic ratiometric approach: we used a 4-channel input mux,
and measured the input signal, a reference voltage, ground, and either
the cold junction for thermocouples, or the third wire for RTDs. That
allowed us to characterise everything except linearity.
Bottom line was that we were achieving something like 20- to 24-bit
resolution with components (in addition to the micro) costing a few tens
of pence. The tradeoff, of course, was speed - these were not flash
converters ;).
Steve
http://www.fivetrees.com
Reply by Paul Taylor●June 8, 20072007-06-08
On Tue, 05 Jun 2007 16:56:20 -0500, Vladimir Vassilevsky wrote:
> I am wondering of what could be a sensor which requires the ADC with the
> true 16-bit accuracy. For the sensor application, that sounds
> unreasonable to me. Especially considering that the rest of application
> is handled by a small micro. Apparently there is a problem with the
> concept.
I worked with a 16-bit cirrus logic sigma-delta adc in a design with a
non-linear gas sensor and 16-bits was _just_ enough. If needed, I could
have gone up to the 23/24 bit device in the same footprint, for more
resolution. Absolute accuracy wasn't the main issue (I could have got away
with 12/13 bits for that), resolution was. I used it with an 8-bit micro.
The really good thing about the A/D, was that its programmable internal
amplifiers had a gain stability against temperature of only a few
ppm/degree celcius, which meant that I could get away with a cheapish
reference, of about 5ppm (iirc).
Can't remember the exact cirrus part number now, but would look at those
again as the price was pretty good too.
Regards,
Paul.
Reply by Mark Borgerson●June 8, 20072007-06-08
In article <5Zk9i.16082$C96.15101@newssvr23.news.prodigy.net>,
antispam_bogus@hotmail.com says...
>
>
> Paul Keinanen wrote:
>
>
> >>I am working on a project where I need some 16 bits ADC to retrieve
> >>information from a sensor. I also need a small microcontroller such as
> >>a PIC, AVR or 8051, and I got surprising quotes for the ADC: Around $5
> >>(qty 1000), which is 5 times more expensive than the controller!
> >
> >
> > The problem with microcontrollers with on-chip ADC/DACs is that you
> > might not get the nominal 98 dB SNR due to the noise from the
> > controller.
>
> Besides, the high performance ADCs and the microcontrollers are the two
> different technologies. The MCUs with the good ADC/DACs usually contain
> two separate dies in one package. For that reason they are more
> expensive then the equvalent micro + equvalent ADC/DAC.
>
> >
> > Is DC accuracy (drifts) important in your application ?
> >
> > Is this ADC part of a control loop, in which case it would be
> > preferable that the ADC is monotonous.
>
> I am wondering of what could be a sensor which requires the ADC with the
> true 16-bit accuracy. For the sensor application, that sounds
> unreasonable to me. Especially considering that the rest of application
> is handled by a small micro. Apparently there is a problem with the
> concept.
>
>
There are a lot of oceanographic variables that need something near 16-
bit resolution. When I was working with sensors to measure the optical
properties of seawater, our minimum standard was one part in 10,000
sensitivity and noise levels.
Mark Borgerson
Reply by Mark Borgerson●June 8, 20072007-06-08
In article <1181030631.316540.197960@g4g2000hsf.googlegroups.com>,
bruno.richard.fr@gmail.com says...
> Hi all,
>
> I am working on a project where I need some 16 bits ADC to retrieve
> information from a sensor. I also need a small microcontroller such as
> a PIC, AVR or 8051, and I got surprising quotes for the ADC: Around $5
> (qty 1000), which is 5 times more expensive than the controller!
>
> Does anyone have an idea about how I can get some low cost ADC-
> Controller solution? I need only few dozens of samples per second, so
> some of you may have nice tricks to do that (op-amps, capacitor charge
> time stuff and the like).
>
> Thanks, Bruno
>
>
You haven't said what type of sensor and whether its output is
ratiometric to the power supply. If it is not ratiometric, then
your ADC needs a precision reference and that will bump up the
ADC cost significantly.
Mark Borgerson
Reply by Hershel●June 8, 20072007-06-08
On 7-Jun-2007, =?ISO-8859-1?Q?Hans-Bernhard_Br=F6ker?=
<HBBroeker@t-online.de> wrote:
> > I do analog stuff at 16 bits all the time, where 12 or even 10 bits
> > would
> > have matched the sensor. It helps sell the product (which is more
> > important then
> > saving a buck).
>
> You appear to have rather unusual customers. For most of us out here,
> saving a buck is *way* more important that giving marketing a
> meaningless bullet point to brag about. FWIW, you could be put out of
> business by a copycat who saves that buck, and then *pretends* to have a
> 16-bit ADC in there --- nobody could tell the difference anyway.
Not everybody here is designing consumer products.
It's more of an industry preference then a customer preference. I've got a
small number of competitors in a specialized area of industrial control.
When I visit with a customer, I generally know which of my competors
were there the week before, and (in this case) the resolution of their ADC
or whatever.
The math is really simple. If you spend and extra buck on a device that
sells for $2K with a 70% PM, and you sell 5% more, then you make
more money.
>
>Hershel wrote:
>
>> I do analog stuff at 16 bits all the time, where 12 or even 10 bits would
>> have matched the sensor. It helps sell the product (which is more important
then
>> saving a buck).
>
>You appear to have rather unusual customers. For most of us out here,
>saving a buck is *way* more important that giving marketing a
>meaningless bullet point to brag about. FWIW, you could be put out of
>business by a copycat who saves that buck, and then *pretends* to have a
>16-bit ADC in there --- nobody could tell the difference anyway.
When I was working at Mattel making toys, saving a penny was *huge*.
When I was at Parker Aerospace making test fixtures, the engineering
costs made the difference in cost between a 12-bit and a 16-bit ADC
disapear in the noise; I just picked the part with the best specs.
I never worked for Rolex, but I imagine that for them giving
marketing a meaningless bullet point to brag about is more
important than saving a buck.
--
Guy Macon
<http://www.guymacon.com/>
Reply by ●June 7, 20072007-06-07
Hershel wrote:
> I do analog stuff at 16 bits all the time, where 12 or even 10 bits would
> have matched the sensor. It helps sell the product (which is more important then
> saving a buck).
You appear to have rather unusual customers. For most of us out here,
saving a buck is *way* more important that giving marketing a
meaningless bullet point to brag about. FWIW, you could be put out of
business by a copycat who saves that buck, and then *pretends* to have a
16-bit ADC in there --- nobody could tell the difference anyway.
Reply by Meindert Sprang●June 7, 20072007-06-07
"Andrew Smallshaw" <andrews@sdf.lonestar.org> wrote in message
news:slrnf6g45h.gpb.andrews@sdf.lonestar.org...
> On 2007-06-07, Meindert Sprang <ms@NOJUNKcustomORSPAMware.nl> wrote:
> > "Andrew Smallshaw" <andrews@sdf.lonestar.org> wrote in message
> > news:slrnf6e6hp.rr4.andrews@sdf.lonestar.org...
> > As long as your captured image lies within the dynamic range of an 8 bit
> > converter, it is of course rediculous to use a 16 bit converter just to
give
> > you the dynamic range for calculations.
>
> A simple example. Let's say I have an image of something or other
> and the background sky is not true black due to the effects of
> skyglow caused by street lighting. I decide to improve my image
> by removing that and making the sky black by adjusting the contrast
> so that pixels below a certain value are scaled to make them darker.
Wait a minute, you already have an image you say. That can of course be
processed in 16 bit for better results. But that has nothing to do with the
dynamic range of the original video signal captured from the sensor. If that
signal has a S/N ratio of less than 8 bit, it brings you zip when sampled
with a 16 bit converter. But you are free to extend the word size of the
already digitized image in order to give you more room for calculated
results.
Meindert