Re: [PATCH v2 11/17] iio: core: Introduce _inputoffset for differential channels

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 20 Jun 2022 20:35:57 +0200
Andy Shevchenko <andy.shevchenko@gmail.com> wrote:

> On Mon, Jun 20, 2022 at 8:13 PM Jonathan Cameron <jic23@kernel.org> wrote:
> > On Mon, 20 Jun 2022 01:20:08 +0200
> > Andy Shevchenko <andy.shevchenko@gmail.com> wrote:  
> 
> ...
> 
> > So to try and explain what this is doing in more depth.
> >
> > This is basically applying a negative offset X to both the P(ostitive) and
> > N(egative) lines. Hence
> >
> > measured capacitance = (P - X) - (N - X)
> >                      = P - N
> >
> > the aim of X being to keep the signal hitting some internal point on
> > the device within a range that is measurable.
> >
> > The relevant text on the datasheet is:
> >
> > "The CAPDAC can be understood as a negative capacitance
> > connected internally to the CIN pin."
> >
> > "Each of the two input capacitances CX and CY between the EXC
> > and CIN pins must be less than 4 pF (without using the
> > CAPDACs) or must be less than 21 pF and balanced by the
> > CAPDACs. Balancing by the CAPDACs means that both
> > CX–CAPDAC(+) and CY–CAPDAC(–) are less than 4 pF."
> >
> > So basically if you have both sides of the differential pair
> > that are too large (>4 pF) then you can drag them together
> > down to that range by adding negative capacitance.  
> 
> Ah, in a long wording it's "measurement window offset". Dunno how to
> make it shorter.
> Also, 0-point on the axis in math.
> 
Hmm. Both good descriptions so I'll add something along these lines
to the description.

Characters are cheap aren't then? :)

in_capacitanceY-capacitanceZ_zero_point
isn't too bad - will think more.  No connection to offset which is
what it would be for a single ended channel though...

Jonathan