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Message-ID: <20231220135913.50edaa7f@jic23-huawei>
Date: Wed, 20 Dec 2023 13:59:13 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: Mike Looijmans <mike.looijmans@...ic.nl>
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>, Conor Dooley
<conor@...nel.org>, devicetree@...r.kernel.org, linux-iio@...r.kernel.org,
Conor Dooley <conor+dt@...nel.org>, Krzysztof Kozlowski
<krzysztof.kozlowski+dt@...aro.org>, Lars-Peter Clausen <lars@...afoo.de>,
Rob Herring <robh+dt@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: iio: adc: ti-ads1298: Add driver
> >>> + avdd-supply:
> >>> + description:
> >>> + Analog power supply, voltage between AVDD and AVSS. When providing a
> >>> + symmetric +/- 2.5V, the regulator should report 5V.
> >
> > Any precedence in tree for doing this? I thought we had bindings that required negative
> > supplies to be specified separately if present - so this would need to be 2
> > supplies. e.g.
> > https://elixir.bootlin.com/linux/v6.7-rc5/source/Documentation/devicetree/bindings/iio/adc/ti,adc12138.yaml#L37
> >
>
> Given that some serious thought...
>
> Splitting into positive and negative supplies would make sense if the chip
> would have terminals for positive, negative and ground. Which it does not
> have, there's only positive and negative (which the datasheet misleadingly
> calls "analog ground").
>
> The analog voltage supplied to the chip has no effect on its outputs, the
> analog supply must be connected between the AVDD and AVSS pins. Its relation
> to analog ground is not relevant, so whether the voltages are +5/0 or
> +2.5/-2.5 or +4/-1 or whatever does not affect the output of the ADC, which
> only reports the difference between its "p" and "n" input signals. It only
> affects the range of the inputs, as it cannot measure (p or n) outside the
> analog supply.
>
Ah. Not having seen any isolators, I assumed this device was running
against a common ground with the digital side of things.
AVSS to DGND is between -3 and 0.2 so I think there is a ground even if it's
not directly connected.
Given this only operates in differential mode the fact both signals are
referenced to +-vref which is from VASS doesn't matter in the end and any
offset goes away.
So yes I think I agree. This should be a single supply.
Jonathan
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