Searching \ for '[PIC] Generic ADC conditioning circuits for PIC' 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/microchip/ios.htm?key=adc
Search entire site for: 'Generic ADC conditioning circuits for PIC'.

Exact match. Not showing close matches.
PICList Thread
'[PIC] Generic ADC conditioning circuits for PIC'
2007\10\22@051601 by Forrest W Christian

flavicon
face
I have an application where I need to provide several ADC inputs for
user to be able to connect various devices.   The two main issues I see
are that I would like to be able to have both positive and negative
voltages read, and that I really shouldn't make any assumptions about
whether or not the actual field "common" is at, below, or above Vss.

One more thing, I'm trying to keep costs *way* down on this project, so
I would prefer not to have to generate a negative rail, and/or have
exotic (expensive) components.   I'm not opposed to using something
external to the PIC, however (like a ADC better suited to this app -
assuming the ADC isn't too expensive).

Since I realize that it may not be possible to do what I am asking while
keeping costs low, I would be interested in hearing about solutions
which either permit reading above and below vss while requiring the
signal to be vss referenced, or allow vss independence but require the
signal to be positive.

-forrest

2007\10\22@053241 by Dario Greggio

face picon face
Forrest W Christian wrote:

> I have an application where I need to provide several ADC inputs for
> user to be able to connect various devices.   The two main issues I see
> are that I would like to be able to have both positive and negative
> voltages read, and that I really shouldn't make any assumptions about
> whether or not the actual field "common" is at, below, or above Vss.

I guess you can come out with 2 or 3 resistors at the ADC input,
shifting, say, -10/+10 into 0..5V

One such example was posted at microchip forum some days ago.

--
Ciao, Dario

2007\10\22@065345 by Michael Rigby-Jones

picon face


>-----Original Message-----
>From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu [.....piclist-bouncesKILLspamspam@spam@mit.edu]
>On Behalf Of Forrest W Christian
>Sent: 22 October 2007 10:18
>To: Microcontroller discussion list - Public.
>Subject: [PIC] Generic ADC conditioning circuits for PIC
>
>
>I have an application where I need to provide several ADC inputs for
>user to be able to connect various devices.   The two main
>issues I see
>are that I would like to be able to have both positive and negative
>voltages read, and that I really shouldn't make any assumptions about
>whether or not the actual field "common" is at, below, or above Vss.

In that case it sounds like you simply need a differential input for each ADC, i.e. you need to measure the difference between your signal ground and the signal, where the signal ground may or may not be the same as the PIC's ground.  Do you have any isolation requirements?

Regards

Mike

=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
person. Please contact us immediately to tell us that you have
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================

2007\10\22@071106 by Forrest Christian

flavicon
face
Dario Greggio wrote:
> I guess you can come out with 2 or 3 resistors at the ADC input,
> shifting, say, -10/+10 into 0..5V
>
> One such example was posted at microchip forum some days ago.
>  
I just went digging in the microchip forums and found an elegant
solution to the +- voltage issue, which I believe might have been the
posting you were talking about...

The solution I found involves 3 resistors - one from Vref to the ADC
input, One from the ADC input to ground, and one from the voltage to be
measured to the ADC input.  Looks like an elegant solution... will need
to play with it for a bit.   However, it does assume that the measured
input is ground referenced.  Is there an inexpensive way to remove the
ground reference requirement?

-forrest

2007\10\22@075657 by Russell McMahon

face
flavicon
face
LM358 or LM324 are about as cheap as it gets.
Suitability subject to certain signal quality issues.


{Quote hidden}

2007\10\22@082613 by Dario Greggio

face picon face
Forrest Christian wrote:

> I just went digging in the microchip forums and found an elegant
> solution to the +- voltage issue, which I believe might have been the
> posting you were talking about...

ok,

> The solution I found involves 3 resistors - one from Vref to the ADC
> input, One from the ADC input to ground, and one from the voltage to be
> measured to the ADC input.  Looks like an elegant solution... will need
> to play with it for a bit.  

good.

> However, it does assume that the measured
> input is ground referenced.  Is there an inexpensive way to remove the
> ground reference requirement?

hmmm, and to what is your voltage referred?
I'd say that you have to connect some 2 common points of your source and
PIC boards... so, you can probably adjust the GND of the resistive
partitor...

--
Ciao, Dario

2007\10\23@063228 by Forrest Christian

flavicon
face
Michael Rigby-Jones wrote:
> In that case it sounds like you simply need a differential input for each ADC, i.e. you need to measure the difference between your signal ground and the signal, where the signal ground may or may not be the same as the PIC's ground.  Do you have any isolation requirements?
>  
Not really...  As long as there is reasonable impedance between the
inputs.  (When I think isolation, I think optical, aka infinite
impedance between the inputs).

What I would really like to be able to provide are some terminal pairs
which operate for the most part like you would expect 4-5 independent
voltmeters to operate.  I.E. you don't have to think about what is
interconnected with what and what is at what potential in relation to
each other.   These are basically going to be additional remote
telemetry inputs at remote sites which may be hooked to anything from
the battery voltage, or across a battery shunt, or across some other DC
voltage they need to monitor.   The more constraints I put on these
inputs, the more likely they are to be blown apart by someone hooking
something up outside of the Absolute Maximum ratings.

I'm kinda just trying to find the balance between the perfect (very
expensive) solution and some not-so-perfect, but will work in most cases
and is cheap to implement solution.

-forrest

2007\10\23@063438 by Forrest Christian

flavicon
face
Dario Greggio wrote:
> hmmm, and to what is your voltage referred?
> I'd say that you have to connect some 2 common points of your source and
> PIC boards... so, you can probably adjust the GND of the resistive
> partitor...
>  
The voltage will be referenced (in an ideal world) to an accompanying
reference connection, which may be at, above, below, or isolated from
the GND/Vss on the PIC board.  

I.E.  think 3-4 separate voltmeters which you can tie to any device.

-forrest

2007\10\23@070356 by Forrest Christian

flavicon
face
Russell McMahon wrote:
> LM358 or LM324 are about as cheap as it gets.
> Suitability subject to certain signal quality issues.
You know, that might just work....

4 resistors and 1/4 of a LM324 (or one of the LF's) along with some
protection diodes, probably on the inputs.

I'll have to dig a bit to figure out what the maximum impedance I can
use is.

-forrest

More... (looser matching)
- Last day of these posts
- In 2007 , 2008 only
- Today
- New search...