AVR-Chat

Hi!

I'm looking at the ATmega128 datasheet and table 98 'input channel and 
gain selections'. There are a few differential settings with the same 
ADC channel in both positive and negative differential input columns. 
Are those to be considered single ended inputs with gain or how should I 
interpret the table?

At 09:42 AM 2/24/2005, Magnus Johansson wrote:

>Hi!
>
>I'm looking at the ATmega128 datasheet and table 98 'input channel and
>gain selections'. There are a few differential settings with the same
>ADC channel in both positive and negative differential input columns.
>Are those to be considered single ended inputs with gain or how should I
>interpret the table?


Differential inputs measure the difference between the two inputs.
If the - is at 3V, and the + at 4V, then you'll see 1V.
They are commonly used for measuring current, with a sense resistor.

You probably CAN do it if you bias the second input
correctly. This might mean, for example, biasing one input
at Vref/2. Or, you might bias both inputs from a common
voltage divider and AC-couple the signal into the active
input. But you will need to watch out for impedance errors
(impedance can't be too high or there will be errors due to
current flowing into the S/H when it pulses).

Jim

On Thu, 24 Feb 2005 17:35:08 +0100
 Magnus Johansson <rnd-avrchat@rnd.se> wrote:
> 
> 
> 
> 
> 
> 
> > Differential inputs measure the difference between
> the two inputs.
> > If the - is at 3V, and the + at 4V, then you'll see
> 1V.
> > They are commonly used for measuring current, with a
> sense resistor.
> 
> Yes, that part I got, but in the table there's
> differential inputs with
> (one example) ADC0 in both positive and negative column.
> I was sort of
> hoping I could use it as I would a single ended input but
> with the gain
> available. I've now tried and can't get it to work so I'm
> mostly curious
> why those combinations are in the table in the first
> place, and not
> marked out as unusable or something...
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
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> Differential inputs measure the difference between the two inputs.
> If the - is at 3V, and the + at 4V, then you'll see 1V.
> They are commonly used for measuring current, with a sense resistor.

Yes, that part I got, but in the table there's differential inputs with
(one example) ADC0 in both positive and negative column. I was sort of
hoping I could use it as I would a single ended input but with the gain
available. I've now tried and can't get it to work so I'm mostly curious
why those combinations are in the table in the first place, and not
marked out as unusable or something...

On Thu, 24 Feb 2005 17:35:08 +0100 you wrote:

>
>Yes, that part I got, but in the table there's differential inputs with
>(one example) ADC0 in both positive and negative column. I was sort of
>hoping I could use it as I would a single ended input but with the gain
>available. I've now tried and can't get it to work so I'm mostly curious
>why those combinations are in the table in the first place, and not
>marked out as unusable or something...

I have wondered about that a bit myself. I suspect that it could provide a high
speed amplified indication of changes in the input but haven't tested it.
For an amplified single ended measurement I just use 2 channels and ground the
negative input but it could go to a reference.
--
Ralph Hilton
http://www.ralphhilton.org
C-Meter: http://www.cmeter.org
FZAOINT http://www.fzaoint.net