Searching \ for '[EE]: Signal source impedance for ADC in AT90S8535' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/io/atod.htm?key=adc
Search entire site for: 'Signal source impedance for ADC in AT90S8535'.

Exact match. Not showing close matches.
PICList Thread
'[EE]: Signal source impedance for ADC in AT90S8535'
2001\11\02@120800 by wzab

flavicon
picon face
Hi All,

I have to design a simple circuit using the ADC in AVR AT90S8535.
I remeber the strong requirements for signal source impedance in PICs,
but the only info in 8535 datasheet was "RAIN Analog Input Resistance = 100
MOhm (typ.)". Does it mean that the S/H in 8535 features the internal
voltage follower or something like this?
I'd appreciate any pointers.
--
                               TIA & Regards,
                             Wojciech M. Zabolotny
       http://www.ise.pw.edu.pl/~wzab  <--> spam_OUTwzabTakeThisOuTspamise.pw.edu.pl

http://www.debian.org  Linux - free OS for free people!

--
http://www.piclist.com hint: To leave the PICList
.....piclist-unsubscribe-requestKILLspamspam@spam@mitvma.mit.edu


2001\11\02@123515 by David VanHorn

flavicon
face
At 06:06 PM 11/2/01 +0100, Wojciech Zabolotny wrote:
>Hi All,
>
>I have to design a simple circuit using the ADC in AVR AT90S8535.
>I remeber the strong requirements for signal source impedance in PICs,
>but the only info in 8535 datasheet was "RAIN Analog Input Resistance = 100
>MOhm (typ.)". Does it mean that the S/H in 8535 features the internal
>voltage follower or something like this?
>I'd appreciate any pointers.

Basically yes.  It's input impedance is high enough that in most
applications you can consider it infinite.

--
Dave's Engineering Page: http://www.dvanhorn.org

Got a need to read Bar codes?  http://www.barcodechip.com
Bi-directional read of UPC-A, UPC-E, EAN-8, EAN-13, JAN, and Bookland, with
two or five digit supplemental codes, in an 8 pin chip, with NO external parts.

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestspamKILLspammitvma.mit.edu


2001\11\02@170530 by Olin Lathrop

face picon face
> I have to design a simple circuit using the ADC in AVR AT90S8535.
> I remeber the strong requirements for signal source impedance in PICs,
> but the only info in 8535 datasheet was "RAIN Analog Input Resistance =
100
> MOhm (typ.)". Does it mean that the S/H in 8535 features the internal
> voltage follower or something like this?

This certainly sounds like there is an internal buffer amp with FET input.
That means source impedance is much less of an issue than with PICs, but now
you have to watch the offset voltage and bias current carefully to make sure
your accuracy requirements are met.


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

--
http://www.piclist.com hint: To leave the PICList
EraseMEpiclist-unsubscribe-requestspam_OUTspamTakeThisOuTmitvma.mit.edu


2001\11\02@170538 by Olin Lathrop

face picon face
> >I have to design a simple circuit using the ADC in AVR AT90S8535.
> >I remeber the strong requirements for signal source impedance in PICs,
> >but the only info in 8535 datasheet was "RAIN Analog Input Resistance =
100
> >MOhm (typ.)". Does it mean that the S/H in 8535 features the internal
> >voltage follower or something like this?
> >I'd appreciate any pointers.
>
> Basically yes.  It's input impedance is high enough that in most
> applications you can consider it infinite.

Don't be so quick to say that.  If this is a 10 bit A/D for example, then
100K source impedance could result in 1 LSB error at the typical input
impedance.  Typical specs are pretty useless, so you either have to derate
them a lot or use other parameters that are better specified.  Based on the
above spec alone, 100K is obviously too high.  50K is only a factor of 2.
20K sounds a lot safer.  Note that now you're close to the PIC's 10K but
you've still got amplifier offset error to deal with which the PIC doesn't.
In reality you'd be using the bias current spec to decide on the maximum
source impedance, not the input impedance.  Note that bias current can go up
sharply with temperature, so in the end you may actually be better off with
the PICs unbuffered input and 10K maximum source impedance.

When you care about 1 part in 1000, you have to think about and account for
all the sources of error.  These are often different than what they appear
at first glance.


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

--
http://www.piclist.com hint: To leave the PICList
@spam@piclist-unsubscribe-requestKILLspamspammitvma.mit.edu


More... (looser matching)
- Last day of these posts
- In 2001 , 2002 only
- Today
- New search...