AVR-Chat

Ah, got it.  So it sounds like I'm just fine merely doubling it to 20 Kohms
and only sampling once a second.   I assume you intended to say "100k
equivalent input"?   Steve

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Jeff Blaine AC0C
Sent: Friday, February 05, 2010 12:48 PM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] ADC source impedance

 

  

What Tim is talking about is the equivalent of an RC time constant. The
input of the ADC is a capacitor based sample/hold type. Which means, as Tim
points out, the longer the signal is at that pin, at a fixed level, the less
current will be required for the sample cap to charge up. The 10K equivalent
number in the datasheet allows you to run the ADC at more or less full
speed. 

I believe the RC time should be linear. So if the samples are running at
1/10 max (around 25Khz?) then a 100khz equivilant input would be ok. And so
on. 

Regards/Jeff

From: tim gilbert 
Sent: Friday, February 05, 2010 2:41 PM
To: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com>  
Subject: Re: [AVR-Chat] ADC source impedance

Steve,
How fast is the input signal varying and how often are you sampling? If it's
a slow signal you'll probably get away with it.

Tim Gilbert
JEM Innovation Inc.
303-926-9053 (office)
303-437-4342 (cell)
720-890-8582 (fax)
www.jeminnovation.com
www.pdksolutions.com

----- Original Message ----- 
From: Steve Hodge 
To: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com>  
Sent: Friday, February 05, 2010 1:35 PM
Subject: RE: [AVR-Chat] ADC source impedance

I didn't give enough information. The particular application does not need
the full 10 bits of resolution. 9 bits or even 8 bits would be ok. I'm
trying to keep the board to minimal size so I'd prefer not to have to cram
in an opamp. In addition, the application will allow things to be
calibrated, so a correction could be applied in software. Anyone have a gut
feeling given all that? That's all I'm looking for. Steve

From: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com>
[mailto:AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> ] On
Behalf
Of David VanHorn
Sent: Friday, February 05, 2010 12:22 PM
To: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> 
Subject: Re: [AVR-Chat] ADC source impedance






[Non-text portions of this message have been removed]