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.
