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Date: Wed, 20 Dec 2023 13:59:13 +0000
From: Jonathan Cameron <jic23@kernel.org>
To: Mike Looijmans <mike.looijmans@topic.nl>
Cc: Jonathan Cameron <Jonathan.Cameron@Huawei.com>, Conor Dooley
 <conor@kernel.org>, devicetree@vger.kernel.org, linux-iio@vger.kernel.org,
 Conor Dooley <conor+dt@kernel.org>, Krzysztof Kozlowski
 <krzysztof.kozlowski+dt@linaro.org>, Lars-Peter Clausen <lars@metafoo.de>,
 Rob Herring <robh+dt@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: iio: adc: ti-ads1298: Add driver
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> >>> +  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