On Thu, 18 Jan 2024 10:10:46 +0200 Ceclan Dumitru <mitrutzceclan@xxxxxxxxx> wrote: > On 1/17/24 18:37, Jonathan Cameron wrote: > > On Wed, 17 Jan 2024 14:43:21 +0200 > > Ceclan Dumitru <mitrutzceclan@xxxxxxxxx> wrote: > >> On 1/16/24 18:30, Jonathan Cameron wrote: > >>> On Mon, 15 Jan 2024 15:53:39 -0600 > >>> David Lechner <dlechner@xxxxxxxxxxxx> wrote: > >>>> On Wed, Dec 20, 2023 at 4:48 AM Dumitru Ceclan <mitrutzceclan@xxxxxxxxx> wrote: > > ... > > >>>>> + > >>>>> + refin-supply: > >>>>> + description: external reference supply, can be used as reference for conversion. > >>>> If I'm understanding correctly, this represents both voltage inputs > >>>> REF+ and REF-, correct? The datasheet says "Reference Input Negative > >>>> Terminal. REF− can span from AVSS to AVDD1 − 1 V". It seems like they > >>>> should be separate supplies in case REF- is non-zero. Otherwise, how > >>>> can we know what voltage it is? (same comment applies to refin2.) > >>> Agreed, in this case these are directly used as references (we recently > >>> had another driver that could take a wide range of negative and positive > >>> inputs but in that case an internal reference was generated that didn't > >>> made it not matter exactly what was being supplied. Not true here though! > >>> > >> Wouldn't it be alright to specify that the voltage specified here should > >> be the actual difference (REF+)-(REF-)? > > How do you establish the offset to apply to single ended channels if you don't > > know the value of REF- (relative to local ground)? > > > > So no - as the device supports single ended channels the difference isn't > > enough information. It would probably be fine to do as you say if it > > were a device with only differential channels where all that matters is > > the scaling. > > I suppose that you are referring to the first page presentation: "Cross > point multiplexer; 8 full differential or 16 single-ended channels". I > consider this to be a bit misleading as all channels are actually fully > differential (must select positive and negative source, AVSS is not one > of them). > > > Even more, the datasheet specifies that when using "single-ended" > inputs you need to select which of the pins is the common one and > connect it to the desired GND (be it AVSS, REF-): > > "Because there is a cross point mux, the user can set any of the analog > inputs as the common pin. An example of such a scenario is to connect > the AIN16 pin to AVSS or to the REFOUT voltage (that is, AVSS + > 2.5 V)" (ad7173-8 page 27) > > For me this is 100% the case that this is a fully differential ADC in > which the datasheet presents a way to use it single-ended. Let's say > that we are using EXT_REF, and REF- is non zero. If someone connects > AVSS to the desired common pin, the ADC will still measure correctly the > difference of voltage between AIN_POS and AIN_NEG and compare it to the > EXT_REF. Thanks for the explanation. I indeed was mislead by the introduction! Seems that the difference in the reference inputs is sufficient for this device. Jonathan
