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'[PIC]: A/D input impedance > 10K'
2002\02\20@210007 by Spehro Pefhany

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Hi all:-

What would the consequences be of going to 20K on the midrange 10-bit A/D
input? I've read the reference manual and the data sheet. If the input is
bypassed with a 0.1uF or something of that ilk, it seems to be primarily
limited by the worst case leakage current on the input vs. guaranteed
accuracy. I'm willing to lose 1 bit or 1.5 bits in the A/D, and high
temperature operation is not required.

Any thoughts on this?

I'd normally put an op-amp in there without much thought, but the quantities
are such that thought will be well rewarded if a few cents can be snipped.

Thanks for any comments & Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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2002\02\20@214725 by Dave Dilatush

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Spehro Pefhany wrote...

>What would the consequences be of going to 20K on the midrange 10-bit A/D
>input? I've read the reference manual and the data sheet. If the input is
>bypassed with a 0.1uF or something of that ilk, it seems to be primarily
>limited by the worst case leakage current on the input vs. guaranteed
>accuracy. I'm willing to lose 1 bit or 1.5 bits in the A/D, and high
>temperature operation is not required.
>
>Any thoughts on this?
>
>I'd normally put an op-amp in there without much thought, but the quantities
>are such that thought will be well rewarded if a few cents can be snipped.

The only other error I can see you getting, other than the drop across
your 20K resistor due to input leakage, would be a scale factor error
that can occur if you have the input bypassed and are sampling it at a
high rate.  But if you're not looking for full 10-bit accuracy I can't
see this as a big issue.  
Dave

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2002\02\21@015133 by Vasile Surducan

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face
On Thu, 21 Feb 2002, Dave Dilatush wrote:

> Spehro Pefhany wrote...
>
> >What would the consequences be of going to 20K on the midrange 10-bit A/D
> >input? I've read the reference manual and the data sheet. If the input is
> >bypassed with a 0.1uF or something of that ilk, it seems to be primarily
> >limited by the worst case leakage current on the input vs. guaranteed
> >accuracy. I'm willing to lose 1 bit or 1.5 bits in the A/D, and high
> >temperature operation is not required.
> >
> >Any thoughts on this?
> >
> >I'd normally put an op-amp in there without much thought, but the quantities
> >are such that thought will be well rewarded if a few cents can be snipped.
>
> The only other error I can see you getting, other than the drop across
> your 20K resistor due to input leakage, would be a scale factor error
> that can occur if you have the input bypassed and are sampling it at a
> high rate.  But if you're not looking for full 10-bit accuracy I can't
> see this as a big issue.
>
 You need also  ( according to datasheet ) to increase also the minimum
required aquisition time. It's possible ( not tested yet ) to keep the
full 10 bit accuracy. The leakage current from datasheet is +/-500nA.
I think is a mistake or an absolute maximum rating.
Vasile

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2002\02\21@052110 by Alvaro Deibe Diaz

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About the minimum aquisition time, I've measured and made a graph of the
binary value
(0-1023) from the AD conversion, charging Chold from 0 to 5V and dischargin
from 5 to
0V, using simple series resistors of 100, 470, 1K, 4K7 and 10Kohms. This was
in a
16F877, but I think this would be very simmilar in other PIC's. In real
applications,
Chold would charge and discharge faster, because the voltage in the AD pin
will be
between 0 and 5.

You can get a jpg of the graph in my ftp:

ftp://piclist:.....piclistKILLspamspam.....xurel.eps.cdf.udc.es/pub/tad.jpg

As you'll see, the TAD is not the same charging and discharging. There is a
constant
slope first stage, and an exponential-like second one.

Hope this helps,

Alvaro Deibe Diaz

{Original Message removed}

2002\02\21@061409 by Michael Rigby-Jones

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The only issue I can see apart from a slight loss of accuracy is that the
acquisition time will have to be be increased.  That may or may not be a big
deal depending on your timing requirements of course.

Mike

> {Original Message removed}

2002\02\21@090804 by Olin Lathrop

face picon face
> What would the consequences be of going to 20K on the midrange 10-bit A/D
> input? I've read the reference manual and the data sheet. If the input is
> bypassed with a 0.1uF or something of that ilk, it seems to be primarily
> limited by the worst case leakage current on the input vs. guaranteed
> accuracy. I'm willing to lose 1 bit or 1.5 bits in the A/D, and high
> temperature operation is not required.

I can see two issues with increasing the A/D source impedance beyond the 10K
specified by Micrcochip.

The first, as you pointed out, is leakage.  That will cause a higher offset
voltage, especially at the high end of the temperature range.  You can't
make the offset voltage go away, but it will be repeatable at the same
temperature for reasonable periods of time.  If you really needed to, you
could calibrate to an external reference, and adjust accordingly in
software.

The second is acquisition time.  A higher impedance will increase the time
constant for charging the internal capacitor the that A/D read the voltage
from.  The simple solution for this is to wait longer between selecting the
channel and starting an A/D conversion.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, EraseMEolinspam_OUTspamTakeThisOuTembedinc.com, http://www.embedinc.com

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