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'[PIC] ADC interface to hiZ source'
2012\04\27@133221 by alan smith

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I've done some searching around, with some mixed results on the proper interface.

Since the source is 0-10V but 20Mohms (according to the customer), at minimum I plan on a 100K/100K divider to range it to 0-5V, and a zener clamp for the ...just in case...since this is an industrial application.

Now, I was thinking...and this is where I've seen it both ways....that I really want to use a voltage follower on the input, that feeds the divider?  This gives a lower impedance input to the PIC's ADC input.  But I've seen it where some have said...not necessary...

So? what are the reasons for the opamp (other than stated for impedance interfacing) or just the divider network?

2012\04\27@135122 by Bob Ammerman

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If the source impedance is really 20Mohm, a 100K/100K divider isn't going to work. It will load the source extremely.

I am guessing the source impedance is really much, much less than 20Mohm. I would verify this with your customer before proceeding.

-- Bob Ammerman
RAm Systems

{Original Message removed}

2012\04\27@140859 by alan smith

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What I meant was to put the divider on the output side of the voltage follower, thats lower impedance.



{Original Message removed}

2012\04\27@143404 by Bob Blick

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How many bits of accuracy do you need? If the source is really 20M then
it could be tough to get very much accuracy. On the other hand, maybe
the customer is really asking that your interface have a 20M input
impedance, much easier.

Best regards,

Bob


On Fri, Apr 27, 2012, at 10:32 AM, alan smith wrote:
{Quote hidden}

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2012\04\27@173157 by alan smith

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12bits of accuracy (18F2423) and yes the interface I believe needs to be high impedance.

Im leaning toward just having an opamp voltage follower on this

2012\04\27@174404 by Bob Blick

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On Fri, Apr 27, 2012, at 02:31 PM, alan smith wrote:
> 12bits of accuracy (18F2423) and yes the interface I believe needs to be
> high impedance.

Just to clarify, your interface having 20M input impedance is OK?

> Im leaning toward just having an opamp voltage follower on this

Yes that sounds reasonable. You might look towards one that is
specifically called a buffer.

Best regards,

Bob


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2012\04\27@182009 by Harold Hallikainen

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In my opinion, there are two things that set the maximum source resistance
driving a PIC ADC. The first is the hold capacitor charge time. If you are
measuring a near DC voltage and not using the PIC input multiplexer to
read multiple ADC inputs, this should have no effect. The source
resistance and the input hold capacitor form a low pass filter, but does
not introduce a steady error.

If the input is multiplexed, this becomes an issue. You'll see crosstalk
between inputs as it takes time for the input holding capacitor to change
voltage from one channel to another. If the signals are slow moving, a way
around this is to have a capacitor on the PIC ADC multiplexer input. The
ADC input hold capacitor charges from this larger capacitor with minimal
voltage change.

The second effect of high source resistance is voltage error due to bias
current. I do not believe there's a workaround on this. If you need fewer
bits resolution, you can round the ADC result to get rid of the error. If
you need the full ADC resolution, I think you have to drive with a
relatively low resistance source, like the voltage follower suggested by
the original poster.

Harold


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2012\04\27@184222 by IVP

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> What I meant was to put the divider on the output side of
> the voltage follower, thats lower impedance

Unless there's a reason to save power, use much lower than
100k/100k, down in the low k to be in line with PIC spec

An op-amp can drive pretty hard, but check that the load you
choose is OK. 1k/1k should be fine for both amp and PI

2012\04\27@202558 by Denny Esterline

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On Fri, Apr 27, 2012 at 3:20 PM, Harold Hallikainen
<spam_OUTharoldTakeThisOuTspamhallikainen.org>wrote:

> In my opinion, there are two things that set the maximum source resistance
> driving a PIC ADC. The first is the hold capacitor charge time. If you are
> measuring a near DC voltage and not using the PIC input multiplexer to
> read multiple ADC inputs, this should have no effect. The source
> resistance and the input hold capacitor form a low pass filter, but does
> not introduce a steady error.
>
> If the input is multiplexed, this becomes an issue. You'll see crosstalk
> between inputs as it takes time for the input holding capacitor to change
> voltage from one channel to another.
>
>
>
It's been forever and a day ago since I read that section, but I thought
the multiplexer
specifically discharged the sample & hold capacitor between conversions
specifically
to prevent this type of cross talk.

Or is my memory going like my hair?

-Denn

2012\04\27@220704 by Spehro Pefhany

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5t 01:32 PM 4/27/2012, you wrote:
>I've done some searching around, with some mixed results on the
>proper interface.
>
>Since the source is 0-10V but 20Mohms (according to the customer),
>at minimum I plan on a 100K/100K divider to range it to 0-5V, and a
>zener clamp for the ...just in case...since this is an industrial application.
>
>Now, I was thinking...and this is where I've seen it both
>ways....that I really want to use a voltage follower on the input,
>that feeds the divider?  This gives a lower impedance input to the
>PIC's ADC input.  But I've seen it where some have said...not necessary...
>
>So? what are the reasons for the opamp (other than stated for
>impedance interfacing) or just the divider network?


As Bob has said a couple times, you need to verify that the _input_ impedance looking into your input
has to be 20M vs. the actual _source_ impedance being 20M. One requires a 20M input impedance (obviously)
the other a 2G Ohm input Z for 1% loading (or 20G for 0.1% loading). BIG difference.

If it's 20M looking into your output, you could just use two 10M resistors in a voltage divider and
a low Ib amplifier running off your (presumably) 5V Vdd. You will need to keep your PCB clean and dry,
including care with the type of flux used, but it's not that bad (the buffer amplifier sees a 5M
impedance, so 1nA leakage will cause 5mV of error or 1 bit on a 10-bit ADC with 5V span. No further
"protection" should be necessary, up to a few hundred volts or the rating of the resistors, whichever
comes first.

Otherwise, I think that you will need at least a 10V supply for the buffer
and then a voltage divider. If you want the input voltage defined for
disconnected input, then either 20M or 2-20G to whatever voltage you want it
to approach (eg. ground).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2012\04\27@224543 by Harold Hallikainen

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> It's been forever and a day ago since I read that section, but I thought
> the multiplexer
> specifically discharged the sample & hold capacitor between conversions
> specifically
> to prevent this type of cross talk.
>
> Or is my memory going like my hair?
>
> -Denny

Choosing a random chip
(http://ww1.microchip.com/downloads/en/DeviceDoc/39646c.pdf ) on pdf page
281, there is mention of "discharge" at the end of the ADC cycle, but I
believe this is the discharge of a capacitor array used in a sort of
charge balancing ADC instead of the discharge of the input holding
capacitor. In practice, I designed a DMX lighting controller about 15
years ago. The PIC ADC read slide pots. Each channel also had a "bump
button" that ran the lights to full when the button was pressed. This was
done using a SPDT pushbutton switch. The PIC ADC input connected to the
common terminal of the switch. The NC contact connected to the pot wiper.
The NO terminal connected to +5V. It turned out that these switches had
flakey NC contacts that would sometimes not close. In that case, adjusting
one pot affected the next scanned channel (I scanned down, so pot 13 would
affect light 13 and 12 if the switch on 12 was bad).

So, that's my experience, but I'd like to hear others!

Thanks!

Harold




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2012\04\28@035628 by Jan-Erik Soderholm

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Denny Esterline wrote 2012-04-28 02:25:
{Quote hidden}

That will give you a value with an error towards whatever level
is used for the "discharge" (instead). I do not see how that is
better then a value that is somehowe baised on interference
from another channel.

If you have two channels with, say, 3.0V and 3.1V on each, and you
*do* have problems with aquisision time, you will get a larger
error if you always discharge to 0V between each channel switch.

Besides, I do not remember reading about that discharge for
each conversion either. :-)

Jan-Erik.


> Or is my memory going like my hair?
>
> -Denn

2012\04\28@074651 by Isaac Marino Bavaresco

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Em 28/4/2012 04:56, Jan-Erik Soderholm escreveu:
{Quote hidden}

Once I designed a digital echo chamber. It used two audio channels, one
for input and another for output.
In the first design there was some crosstalk between channels and the
sound never faded completely, even with muted input and high decay rate.

It was not two A/D channels but the effect can be the same for some
applications.


Isaac


'[PIC] ADC interface to hiZ source'
2012\05\01@024128 by Andries Tip
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> That will give you a value with an error towards whatever level is used for the "discharge" (instead). I do not see how that is better then a value that is somehowe baised on interference from another channel.
>
> If you have two channels with, say, 3.0V and 3.1V on each, and you
> *do* have problems with aquisision time, you will get a larger error if you always discharge to 0V between each channel switch.
>
> Besides, I do not remember reading about that discharge for each conversion either. :-)
>
> Jan-Erik.

The discharge phase is not implemented on the smaller PICs (well, maybe on some newer devices, I don't know them all).

For example, the PIC18F4550 *does* have such a discharge phase (see datasheet DS39632D-page 267):

 21.7  Discharge

 The discharge phase is used to initialize the value of
 the capacitor array. The array is discharged before
 every sample. This feature helps to optimize the
 unity-gain amplifier as the circuit always needs to
 charge the capacitor array, rather than charge/discharge
 based on previous measurement values.


-Andries

2012\05\01@083932 by Harold Hallikainen

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{Quote hidden}

Is "the array" the input sample capacitor? I think the ADC uses an array
of capacitors in a charge balance mechanism. It is, I think, like an R2R
ADC, but is instead C2C. I don't think the capacitor being discharged is
the input sample/hold capacitor.

Harold


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2012\05\02@023112 by Andries Tip

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I think you're right Harold. Sorry for the confusion. I should have read the datasheet better.

-Andries

-----Oorspronkelijk bericht-----
Van: EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu [piclist-bouncesspamspam_OUTmit.edu] Namens Harold Hallikainen
Verzonden: 01 May 2012 14:39
Aan: Microcontroller discussion list - Public.
Onderwerp: RE: [PIC] ADC interface to hiZ source


{Quote hidden}

Is "the array" the input sample capacitor? I think the ADC uses an array of capacitors in a charge balance mechanism. It is, I think, like an R2R ADC, but is instead C2C. I don't think the capacitor being discharged is the input sample/hold capacitor.

Harold


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2012\05\02@084704 by Harold Hallikainen

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> I think you're right Harold. Sorry for the confusion. I should have read
> the datasheet better.
>
> -Andries
>

It's not super-clear. They mention the capacitor array once, in this
sentence about discharge.

Harold


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