Can you measure a floating voltage with a differential ADC

[Fibo]

Can you take a "fully differential" ADC, like the LTC2442, and measure a floating voltage reliably with it? Or are things going to drift and get unpredictable?

I have an LTC2442 Dev board, and in the datasheet it says, max and min voltages for any pin are 0.3V above Vcc and below ground.

But, I took a -2.5V floating voltage source, and it measured it just fine...

I was looking at this website, that states

"In the full differential mode, any voltage from the source ranging from +2.5 volts to - 2.5 volts can be applied to the ADC. There is no reference to ground at any point from the source to the input and this includes all components between including op amps. Nothing can be referenced to ground on the input side of the ADC in the full differential mode. The exception here is the use of a "fully differential" op amp used as a buffer between your source the final input to the ADC. But you must be meticulous with this buffer to be sure there is no ground reference after the buffer."

http://www.steveluce.com/24bits/Applications%20of%20the%20LTC%2024%20bit%20ADC.html


This seems weird to me though, is this an ok thing to do? I'm not sure how its working with the source just floating in space and not being tied back to the ADC's common point in any way.....

much thanks!

[bloggs.fred...@gmail.com]

Floating does not mean isolated. The differential voltage must be defined relative to ADC COM.

[Phil Allison]

bloggs.fred...@gmail.com wrote:

>
> Floating does not mean isolated.

** Really ?

When speaking of a signal source, the terms are normally synonymous.

If "floating" does not equal isolated then do enighten us as to what it does.

> The differential voltage must be defined relative to ADC COM.

** Just as ambiguous as your first unsupported assertion.


... Phil

[Phil Allison]

Fibo wrote:


> This seems weird to me though, is this an ok thing to do?
> I'm not sure how its working with the source just floating in space and not
> being tied back to the ADC's common point in any way.....

** A "floating source" has no conducting path to anything so references itself to whatever it is connected to.

Usually, the source is electrically shielded to prevent static charge build up or capacitive coupling of unwanted voltages that might harm the receiving amplifier's differential inputs or exceed their common mode range.

If need be, a leakage path to ground form each input can be created with a pair of high value resistors.

Similarly, a "floating measurement" is made when the test instrument has no conducting path to anything - like a battery operated DMM.



... Phil

[Jim Thompson]

On Tue, 11 Nov 2014 16:13:31 -0800 (PST), Fibo <panf...@gmail.com>
wrote:

It's probably a "flying-cap" circuit, so as long as the signal doesn't
go above/below rails, it's fine... and the ESD diodes provide the
leakage to ensure that condition with a floating source.

...Jim Thompson
--
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[miso]

When you state you have a -2.5V floating voltage source, you fail to
indicate the common mode of the signal.

What this "website" should write is "In the fully differential mode, any
differential voltage from +2.5V to -2.5V can be applied." Note that you
can't violate the common mode range of the inputs. That is each input can't
exceed the rail by 0.3V. In this case, I am not referring to a
differentially applied voltage, but rather the voltage on each pin.

If the part truly needs a dedicated website, well something must be lacking
in the data sheet. [I'm not saying that is the case since the manufacturer's
data sheet was clear regarding the limits of the analog inputs. There could
be other issues that the supplemental website clears up.] However this
supplemental website caused more confusion than it solved in this particular
instance.

[bloggs.fred...@gmail.com]

On Tuesday, November 11, 2014 9:08:40 PM UTC-5, Phil Allison wrote:
> bloggs.fred...@gmail.com wrote:
>
> >
> > Floating does not mean isolated.
>
>
> ** Really ?

Yeah, really, jackass. I can always tell a mental midget and their fixation on "terminology."Take your jackass ideas and go stuff them
If you read the data sheet, you would see the bullet:
"Differential Input and Differential Reference with GND to VCC Common Mode Range"
A search of the pdf datasheet for "floating" only brings up guidance on handling unused pins and NOTHING to do with any measurement signals.

[JW]

On Wed, 12 Nov 2014 05:15:38 -0800 (PST) bloggs.fred...@gmail.com
wrote in Message id:
<0306f9bc-d8aa-4d0b...@googlegroups.com>:

I just rang up your phone a few minutes ago. I loved the message.

"Hello, you have reached the assholes. All of our assholes are busy at the
moment. Please leave your name and number and an asshole will get back to
you shortly."

[whit3rd]

On Tuesday, November 11, 2014 4:13:36 PM UTC-8, Fibo wrote:
> Can you take a "fully differential" ADC, like the LTC2442, and measure a floating voltage reliably with it? Or are things going to drift and get unpredictable?

There are no floating ADC inputs, so the moment you connect the
floating voltage source, it's clamped (at least) to the ADC chip's rails.
It would be best (because your ADC might have some small common-mode
sensitivity) to terminate at the ADC input, to some middle-of-common-mode-range
potential. Things ARE unpredictable outside that range, and only get
predictable again outside the absolute-limit range (you won't like the prediction!).

It would also be prudent to protect the expensive parts against static discharge
from the originally-floating system, when it gets plugged in. This could be as simple
as a plug-operated shunt switch (as found in many audio jacks).