Re: [PATCH v5 02/11] iio: adc: Add basic support for AD4170

[Marcelo Schmitt]

On 06/16, David Lechner wrote:
> On 6/10/25 3:31 PM, Marcelo Schmitt wrote:
> > From: Ana-Maria Cusco <ana-maria.cusco@xxxxxxxxxx>
> > 
> > The AD4170 is a multichannel, low noise, 24-bit precision sigma-delta
> > analog to digital converter. The AD4170 design offers a flexible data
> > acquisition solution with crosspoint multiplexed analog inputs,
> > configurable ADC voltage reference inputs, ultra-low noise integrated PGA,
> > digital filtering, wide range of configurable output data rates, internal
> > oscillator and temperature sensor, four GPIOs, and integrated features for
> > interfacing with load cell weigh scales, RTD, and thermocouple sensors.
> > 
> > Add basic support for the AD4170 ADC with the following features:
> > - Single-shot read.
> > - Analog front end PGA configuration.
> > - Differential and pseudo-differential input configuration.
> > 
> 
> ...
> 
> > +static int ad4170_fill_scale_tbl(struct iio_dev *indio_dev,
> > +				 struct iio_chan_spec const *chan)
> > +{
> > +	struct ad4170_state *st = iio_priv(indio_dev);
> > +	struct ad4170_chan_info *chan_info = &st->chan_infos[chan->address];
> > +	struct device *dev = &st->spi->dev;
> > +	int bipolar = chan->scan_type.sign == 's' ? 1 : 0;
> > +	int precision_bits = chan->scan_type.realbits;
> > +	int pga, ainm_voltage, ret;
> > +	unsigned long long offset;
> > +
> > +	ainm_voltage = 0;
> > +	ret = ad4170_get_ain_voltage_uv(st, chan->channel2, &ainm_voltage);
> > +	if (ret < 0)
> > +		return dev_err_probe(dev, ret, "Failed to fill scale table\n");
> > +
> > +	for (pga = 0; pga < AD4170_NUM_PGA_OPTIONS; pga++) {
> 
> From what I read in the datasheet, it sounds like if adi,reference-buffer is
> precharge, then the PGA is bypassed, so there would only be 1 option in that
> case (gain = 1).
> 
Although not explicit in the datasheet, looks like there are two precharge
buffers. One precharge buffer is used with the voltage reference inputs and is
associated with the adi,positive/negative-reference-buffer dt properties. The
configuration of that buffer is set through the REF_BUF_P/M fields of AFE
registers. The datasheet doesn't mention the PGA on the section dedicated to
describing reference and reference buffering features.

The other places a precharge buffer is mentioned refer to it as 'gain = 1 precharge'.
My understanding is that the PGA bypass precharge buffer is a different buffer
that is only used when PGA option 9 is set in the PGA_GAIN filed of AFE reg.
The PGA bypass precharge buffer would be used in the analog input path while
the reference buffers would be used in the reference input path.