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ADS1248 questions

Tomas Larsson
Prodigy 20 points
Other Parts Discussed in Thread: ADS1248

I'm looking into to use the ADS 1248 converter in one of my designs.

Have been reading the datasheet both forward and backwards, and there is a few things that I find unclear.

First of all, in the data sheet it says that that the number of singleended inputs is limited to 7, leving one input un-used., however can I use 2 say 1 differential and 6 single-ended inputs, totaling the number of inputs to 7.

Is the same limit applied if I use some of the inputs as GPIO's?

In my design I can't iuse the DRDY signal nor the START signal, Should I tie the START-input to DVDD and use the RDATA-command?

If I understand it correctly, when START is tied to DVDD it converts continiously, if so, how do I stop it in order to change channels and PGA settings etc.

Or is there another way to start the conversion, independantly of the START-pin? SYNC-command??

  • 0
    TI__Guru* 97775 points

    Tomas,

    The ADS1248 has 8 multiplexed inputs. When we describe a single ended input, we use one input as a common line, and use the remaining 7 inputs measured against common. In this way we decribe this each as a single ended input. That's how we get 7 single ended inputs.

    If you use an input line as a GPIO, then you reduce the number of potential inputs. Since the inputs are individually selectable, you could have 4 fully differential inputs, or 1 fully differential input, and 1 common line measuring 5 differential channels.

    The START signal isn't really necessary. If you choose not to use it, then you can tie it high and start conversions with a SYNC command. If you want to stop the conversions and change settings, you can use the SDATAC command to stop the conversions to make a setting change with a WREG command to the configuration register or you can just make the setting change. DIN is still being read as DOUT is being clocked out.

    However, if you don't use the DRDYn pin, make sure you get the data ready indication from the DOUT/DRDYn pin. It indicates that the conversion data is ready for a read. If you have good control of timing, then you can extract the data when you know that the conversion is completed. However, you can't just read the data at any time. If you start to read a conversion data at a random time, and then it is interrupted by a DRDYn indication, the new conversion data is still put onto the output register. In that case, the output read out will be half of the previous conversion and half of the new conversion.

    You can use the combo DOUT/DRDYn to make it easier to find DRDYn if you force it high after reading a conversion as shown in Figure 51 of the datasheet.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    THanks for the clarification, the datasheet is very unclear about this.

    So, if I understand you right, I have to tie one of the inputs to AGND (single supply) in order to use single ended inputs.

     

    When reading the datasheet is says that issuing the command "RDATA" will load the most recent ADC-result into the shiftregister and I can read it directly with 24 SCLK-cycles, but, do I have to wait for DRDY before I issue the RDATA command? Or is the latest conversion buffered until the next one is completed.

  • 0 in reply to Tomas Larsson
    TI__Guru* 97775 points

    One of the input should be tied to a known point, but it should not go to the same voltage as AVSS. Note that the input range of the ADC only goes to AVSS+0.1V. If you are running on 0 to 5V supplies, then the input should be at least 0.1V. This is because the input gain amplifier is limited in its common-mode range to 0.1V to either supply.

    When it comes to reading the data, you don't necessarily need to wait for the next DRDY pulse, but it is safer. If you send the RDATA command, and your 24 SCLKs are interrupted by a DRDY pulse, your data read is corrupted by the next data. The conversion is not buffered through the read process.

    Joseph Wu

     

  • 0 in reply to Joseph Wu
    TI__Guru* 97775 points

    I have to correct myself on this one.

    In the case where the SDATAC is first given, so that the part is no longer in continuous read, and the RDATA is sent to read the data, the data is buffered. You should be able to read it out at your convenience.

    In the case of an RDATAC, the data needs to be read out before the next DRDY indicator.

    Joseph Wu