High Resolution AD Converter Noise

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A customer is using a ready off the shelf AD Converter board using 8
differential channels with a resolution of 16 bit @ 100 khz. The circuit
contains 8 parallel switched capacitor SAR (no analog mux) from Analog
Devices AD976. It is using the internal reference and the input circuit is
a INA103 from Burr Brown. With gain=1 the input range is +-10Volt.

Now customer complains about unknown noisy results (of about 20 LSB) using
that board. To examine this, I use the board at my table in clean
environment. After connecting all differential ends to ground, I can easily
see 10 diffrent numbers at a simple 100mS refreshed display what is equal
to an error of 10 LSB. With a gain of 1, the INA103 can hardly cause this
noise.

My last AD design was long ago with only 12 bit resolution at lower speed
but I was able to see with a 10gear poti any number at the screen without
its neighbour and it was even possible to adjust the poti to the edges
where the result toggled to the upper or to the lower neighbour.

I can hardly believe the AD976 is poor becouse the noise is much above the
INL what is specified with +-2LSB max. I have no idea how the given
"transition noise" of 1 LSB is defined in the data sheet. The gausian
histogramm shows only 7 codes for DC code uncertainty. 4 of them are
extremly rare and only 2 of them make about 2/3 of the results.

Assume all, I think the board layout or grounds are poor and we meassure
some crosstalk of the bus or the close DC/DC converters. Therefore it would
be nice to hear your experience with AD convertes at similar resolution and
maybe someone already examined a 16 bit or above reference board with its
own design?


Re: High Resolution AD Converter Noise
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Switched capacitor ADCs require a quite strong drive to
overcome these input capacitances.

Rene
--20%
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Re: High Resolution AD Converter Noise

...
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would
and

the power supply for the INA103 & the AD976 had to be really clean for a
fast 16bits.
you talk of DC/DC convertors; are they powering the op-amp & converter..?
there should be a linear regulator between them and good filtering and
analog ground. and the analog and digital parts on the board should be on
separate location on the board..



Re: High Resolution AD Converter Noise
Check for power supply noise. Power supply noise rejection figures
given for op-amps often apply at low frequencies and noise from (e.g.)
DC-DC converters appears on the output. Are you limiting the bandwidth
of the signal out of the op-amp? That might be worth a try.


Look on the bright side - 10DN of peak-to-peak noise is only the bottom
four bits and is 3.6 DN rms.  You've got 12 good bits of data.

Cheers
TW


Re: High Resolution AD Converter Noise

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No, it is not. Having a cap over the feedback doesn't
limit the noise bandwidth. A bandwidth limit has to
be external, eg as RC following the OPAmp.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Re: High Resolution AD Converter Noise
I didn't say anything about putting caps on feedback.
Perhaps I should have said "limiting thebandwidth of the  signal into
the ADC"

TW


Re: High Resolution AD Converter Noise
BTW...this may come in handy for some, but I had a nosiy ADC to deal
w/. I spent 2 weeks on it and tried every single source. It turned out
that one of the ground pins on the SOIC package had a cold solder joint
that looked fine even under a microscope. Just running the soldering
iron over the connection reflowed the solder and everything was fine.

The bad connection on one of the ground pins, meant high inductance to
ground..basically lots of local ground bounce.


Re: High Resolution AD Converter Noise
snipped-for-privacy@despammed.com says...
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If the board is off-the-shelf, please specify which board it is and
whether the schematic diagram is available on line.  In the best case,
there may be someone here who has experience with that particular board.
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The DC-DC converters can be a problem---especially if they are running
their oscillators at a few hundred KHz and you are collecting data
at the same rate.


If you don't need 100KHz sampling rates, you could oversample and
use the average to reduce noise.  This will work if you are seeing
noise with a Gaussian distribution.  If the noise is nearly synchronous
due to the DC-DC converters,  oversampling may not work as well.


I've used the AD974 (AD976 with 4x front end mux) and gotten about
2 LSBs noise (standard deviation of 1000 samples)  with a nice
Gaussian distribution of the samples.

Mark Borgerson



Re: High Resolution AD Converter Noise
You can always use a properly designed common mode choke filter on the
supply lines to make sure you dont get noise around frequencies of
interest.


Re: High Resolution AD Converter Noise
On Monday, in article

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How long are the wires connecting all these differential inputs from the
onboard circuitry, via the the normal connector on an off the shelf board
to your connection scheme?

Leaving aside noise inside a PC, what is the possibility of noise in the
wiring externally.

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I would actually consider rerunning the the experiment with a special
connector to the off the shelf board with connector having each input
shorted to itself (i.e. +ve to -ve) or via a 10R resistor, then shielding
the connector with a connection to frame ground at least.

With these low values on a 16bit convertor without knowing the exact lengths
connections and intermediate connections and other influences small levels
of noise can be an issue.

What gain settings are the conversions done at? Because quite a lot of
these cards have front end gain amplification sometimes as high as x1000.

My other issue is connecting all the differential inputs together leaves
possiblities for ground loops defeating the purpose of a differential
input. You have made a differential input effectively into a single ended
input with 0V input.

Does the card have an EMI screened A/D converter section on the board?

This is from my working over 10 years ago with PC add on cards for
A/D converters and video cards for a well known company in that field
at the time.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: High Resolution AD Converter Noise

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See http://www.keithley.com/data?asset46%40 which is a white paper on the
subject of dealing with noise issues in measurement systems. Also worth
looking at is "Low Level measurements Handbook - 6th Edition". Also, if you
can get on one of their free seminars on the topic you will find them quite
worthwhile.

--
********************************************************************
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Re: High Resolution AD Converter Noise
On Wednesday, in article

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The original poster may well do. My comments are based on how many add on
cards have "screw terminal panels" that are PCBs on at least 0.5m of often
unshielded cable to the off the shelf card. Then people ground all the
"screw terminals" and with at least a 1m ground loop, wonder why they
get so much noise on low level readings with 16bit A/D. Once I remember
a customer complaining of the noise levels being very high, when he
was doing something similar at a gain level of x100 (effective 16bit range
of +/-0.1V).

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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