linux-kernel - Re: [PATCH 1/5] Documentation: ABI: IIO: add calibphase

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Message-ID: <jvhwdzmruov3je7qvsncn4naxg2cbbset27vr6tfjl3fumw7es@v3ho7m6iwaqp>
Date: Thu, 1 May 2025 14:33:42 +0200
From: Angelo Dureghello <adureghello@...libre.com>
To: David Lechner <dlechner@...libre.com>
Cc: Andy Shevchenko <andy@...nel.org>, 
	Nuno Sá <noname.nuno@...il.com>, Jonathan Cameron <jic23@...nel.org>, 
	Nuno Sá <nuno.sa@...log.com>, Lars-Peter Clausen <lars@...afoo.de>, 
	Michael Hennerich <Michael.Hennerich@...log.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, linux-iio@...r.kernel.org, 
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 1/5] Documentation: ABI: IIO: add calibphase_delay
 documentation

On 30.04.2025 10:04, David Lechner wrote:
> On 4/30/25 9:56 AM, David Lechner wrote:
> > On 4/30/25 9:45 AM, Andy Shevchenko wrote:
> >> On Wed, Apr 30, 2025 at 09:21:28AM -0500, David Lechner wrote:
> >>> On 4/30/25 12:40 AM, Nuno Sá wrote:
> >>>> On Tue, 2025-04-29 at 15:06 +0200, Angelo Dureghello wrote:
> >>>>> From: Angelo Dureghello <adureghello@...libre.com>
> >>>>>
> >>>>> Add new IIO calibphase_delay documentation.
> >>>>>
> >>>>> The delay suffix is added to specify that the phase, generally in
> >>>>> radiants, is for this case (needed from ad7606) in nanoseconds.
> >>
> >> ...
> >>
> >>>>> +What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_calibphase_delay
> >>>>
> >>>> Not sure if I'm too convinced on the _delay suffix
> >>>>
> >>> Phase is measured in radians, not seconds, so it seems wrong to use it here.
> >>>
> >>> https://en.wikipedia.org/wiki/Phase_(waves)
> >>>
> >>> And the delay here is with respect to individual samples in a simultaneous
> >>> conversion without regard for a sampling frequency, so I don't see how we could
> >>> convert the time to radians in any meaningful way.
> >>
> >> And how this delay is aplicable to the phase in the hardware? Sounds to me that
> >> HW has some meaningful way of such a conversion?
> >>
> > 
> > It is a calibration to account for a phase difference between two input signals.
> > This is a simultaneous sampling ADC, so all channels normally sample at exactly
> > the same time. This phase delay calibration factor can introduce a small delay
> > on an individual channel so that it starts it's conversion some microseconds
> > after the others.
> > 
> > There is a nice diagram here:
> > 
> > https://www.analog.com/media/en/technical-documentation/data-sheets/ad7606c-18.pdf#%5B%7B%22num%22%3A113%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C34%2C594%2C0%5D
> > 
> > To convert the phase delay to a phase angle and back would require also knowing
> > the frequency of the input voltage signals.
> 
> Maybe calling it "conversion delay" would make more sense? Since the phase part
> of it is really referring to the application rather than to what we are actually
> adjusting.

Are there examples of a phase calibration in iio ? Becouse apply a radians 
calibration seems complicated and maybe non approrpiate for non-periodic 
signals as often used in real world applications.

So another viable idea could be to use a IIO_CHAN_INFO_CALIBDELAY instead.

Regards,
angelo