Hi 
I discovered the problem when I was in the process of connecting an external
reference it turns out the "something else" in this instance was to connect
the pin.
The part has separate pins that are AVDD and AVSS as opposed to VDD VSS.
When my partner did the schematic, he left the AVDD pin off the power net
thinking it was something else so when I routed the board they it was not
connected.  (He did manage to get the AVSS pin to GND)
Apparently, if the reference pin is unconnected it floats to near 2.5 volts
so all my measurement values from the AD were higher than I expected.
All's well that ends well.
 
 
 
 
Phillip
Things should be as simple as possible but no simpler
 
 
 
Phillip Coiner
CTO, GPS Source, Inc.
 
 
Your source for quality GNSS Networking Solutions and Design Services, Now!
-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Phillip
Sent: Tuesday, September 05, 2006 6:10 PM
To: 'Microcontroller discussion list - Public.'
Subject: [PIC]18F6520 10 bit AD converter internal reference

Hi All

I am using the AD converter of an 18F6520 whilst  VCFG1 & VCFG0 bits are set
to zero.
The data sheet says this connects AVDD and AVSS as the reference.  (I'm
hoping maybe they mean "connects to some kind of band gap reference")

My VDD is 3.3V and VSS is GND.
I'm getting numbers as though the reference is actually 2.5 volts instead of
3.3V.  (see the measurements below)

I've been all over the data sheet and errata I don't see a mention of a 2.5
volt internal reference anywhere.
Am I losing my mind or is there a 2.5 volt internal reference on this part
when VCFG1 & VCFG0 bits are set to zero?

Has anyone else seen this type of behavior on an AD converter due to
something else?

 

I plan tomorrow to connect a voltage to the external reference pins to be
sure...........though it is/will be a pain in butt because I was not
planning on using an external reference.

 

Here are the measurements that make me think this.

 

Input voltage   Measurement            Theoretical count VDD =3.3V ref
Theoretical count 2.5V ref

2.362              974                             733
967

1.987              817                             617
814

1.516              624                             470
621

0.999              412                             310
409

0.519              212                             161
213

0.322              131                             100
132

0.124              49                                38
51

 

Note : I've got about 40mV of noise on the power supply.

Thanks in advance.

 

Phillip

Things should be as simple as possible but no simpler

 

 

 

Phillip Coiner

CTO, GPS Source, Inc.

 

 

Your source for quality GNSS Networking Solutions and Design Services, Now!

 


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