Re: [PATCH RFC v4 09/10] Documentation: ABI: testing: add docs for ad9910 sysfs entries

[Rodrigo Alencar]

On 26/05/17 03:58PM, Jonathan Cameron wrote:
> On Fri, 08 May 2026 18:00:25 +0100
> Rodrigo Alencar via B4 Relay <devnull+rodrigo.alencar.analog.com@xxxxxxxxxx> wrote:
> 
> > From: Rodrigo Alencar <rodrigo.alencar@xxxxxxxxxx>
> > 
> > Add custom ABI documentation file for the DDS AD9910 with sysfs entries to
> > control Parallel Port, Digital Ramp Generator and OSK parameters.
> > 
> > Signed-off-by: Rodrigo Alencar <rodrigo.alencar@xxxxxxxxxx>
> I'm fine with phase and frequency as defined, but for the scaling it made me wonder.
> For outvoltage0 channels the assumption the value is the peak voltage so if
> we know what input to be modulated by the ramp generator can we express them
> in volts (well milivolts) rather than as a scaling multiplier?

The DAC output is current-based and differential. Voltage conversion would happen
outside the device... using a resistor load or an op-amp transimpedance stage,
and I am no expert on that, but that often requires impedance matching so voltage
levels may depend on the frequency. Then, I suppose that voltage is not the right
unit to use.

The scale here controls the amplitude of the varying signal. Assuming the peak voltage
(amplitude) is constant means we have a constant envelope, but that should not mean
we can't control it or it should not mean that the hardware can have other ways to
control it. That said, scale behaves as a "gain multiplier".

> 
> That seems to me like it fits better with the overall ABI.
> 
> > +What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_scale_offset
> > +KernelVersion:
> > +Contact:	linux-iio@xxxxxxxxxxxxxxx
> > +Description:
> > +		For a channel that allows amplitude control through buffers, this
> > +		represents the value for a base amplitude scale. The actual output
> > +		amplitude scale is a result with the sum of this value.
> > +
> 
> > +
> > +What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_scale_roc
> 
> Silly question perhaps but can work out how this related to millivolts/sec
> That might make a more intuitive interface than scaling multiplier per sec
> Perhaps the combination with offset makes this impossible though maybe that
> could be a expressed as a voltage offset?  Afterall if the amplitude being
> scaled is 5V then 5 * (offset + scale) = 5 * offset + 5 * scale
>  
> > +KernelVersion:
> > +Contact:	linux-iio@xxxxxxxxxxxxxxx
> > +Description:
> > +		Amplitude scale rate of change in 1/s for channels that ramp
> > +		amplitude. This value may be influenced by the channel's
> > +		sampling_frequency setting.
> 
> 

-- 
Kind regards,

Rodrigo Alencar