linux-kernel - Re: [PATCH v2 1/1] Documentation: iio: Add ADC documentation

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Message-ID: <f3c1f7d2-3cc7-43b0-a166-e2053b2dabfa@baylibre.com>
Date: Sat, 18 Jan 2025 11:03:26 -0600
From: David Lechner <dlechner@...libre.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Marcelo Schmitt <marcelo.schmitt@...log.com>, linux-iio@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, lars@...afoo.de,
 corbet@....net, marcelo.schmitt1@...il.com
Subject: Re: [PATCH v2 1/1] Documentation: iio: Add ADC documentation

On 1/18/25 9:51 AM, Jonathan Cameron wrote:
> On Wed, 15 Jan 2025 11:23:24 -0600
> David Lechner <dlechner@...libre.com> wrote:
> 
>> On 1/14/25 7:53 AM, Marcelo Schmitt wrote:

...

>>> +1.2.2 Differential Unipolar Channels
>>> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>> +
>>> +For **differential unipolar** channels, the analog voltage at the positive input
>>> +must also be higher than the voltage at the negative input. Thus, the actual
>>> +input range allowed to a differential unipolar channel is IN- to +VREF. Because
>>> +IN+ is allowed to swing with the measured analog signal and the input setup must
>>> +guarantee IN+ will not go below IN- (nor IN- will raise above IN+), most
>>> +differential unipolar channel setups have IN- fixed to a known voltage that does
>>> +not fall within the voltage range expected for the measured signal. This leads
>>> +to a setup that is equivalent to a pseudo-differential channel. Thus,
>>> +differential unipolar channels are actually pseudo-differential unipolar
>>> +channels.  
>>
>> I don't think this is equivalent to pseudo-differential unipolar. That one has
>> a common mode voltage supply on the negative input. This one has a full range
>> signal on the negative input. This is the diagram I was expecting here.
>>
>> ::
>>
>>   -------- VREF -------
>>     ´ `       ´ `               +-------------------+
>>   /     \   /     \   /        /                    |
>>          `-´       `-´    --- <  IN+                |
>>   ------ GND (0V) -----        |                    |
>>                                |            ADC     |
>>   -------- VREF -------        |                    |
>>         ´ `       ´ `     --- <  IN-                |
>>   \   /     \   /     \        \       VREF         |
>>    `-´       `-´                +-------------------+
>>   ------ GND (0V) -----                  ^        
>>                                          |       
>>                                   External VREF
> 
> If it's unipolar, output must be positive which isn't true here.
> Do we actually see differential unipolar except for the pseudo case with
> common mode voltage?   Seems like a weird device.

OK, it sounds like you and Marcelo are considering bipolar to mean that the
difference is bipolar rather than the inputs. In that case, it doesn't seem like
there would ever be such a thing as unipolar (true) differential.

I was looking at this from the point of view of only the inputs and not the
difference. I'm seeing that the input voltage can only be positive, so to me
that would be unipolar.

So at the very beginning, when we first mention unipolar and bipolar, it would
be helpful to add a bit making it clear exactly which point in the system we
are talking about, the input or the output.