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Cycling the ADS1256 Input Multiplexer

Philippe47787
Prodigy 50 points
Other Parts Discussed in Thread: ADS1256, ADS1158, ADS1258, ADS1255, ADS1274, ADS1174

Dear all,

I am working on a project with the TI ADS1256. In the datasheet there is a step-by-step tutorial [1] how to cycle channels and read data. My question is, would this also work with the "Read Data Continuously" (RDATAC) command or in other words can I write to the MUX register when the ADS1256 is in continuous mode?

Furthermore Table 14 [1] is confusing to me: Data Rate and Cycling Throughput are only matiching from 2.5 to 50 SPS afterwards the Cycling Throughput Frequency is lower than the Data Rate. Therefore I have to ask would it be possible to set the ADC to 7500SPS and mulitplex between 3 inputs to have 3 channels with 2500SPS?

Best regards and thanks for the help.
Philippe

[1] SBAS288J Page 21

  • 0
    TI__Mastermind 43915 points

    Philippe, 

    When you are in the "Read Data Continuously" the DIN is constantly monitoring and is able to decode op codes that are written to it. When you change the mux, you need to give the converter some time for the digital filter and modulator to settle before you can read valid data. So, you may not be saving any time by using this method compared to the recommended method in Figure 19. For me, I would rather use the recommended procedure on how to cycle the input mulitplexor in the data sheet. The step by step process on page 21 validates that you give the converter enough time for the internal components to settle so you achieve valid data. In addition, it recommends you send a SYNC command to make sure everything is in sync timing wise for valid and accurate data.

    As for your Table 14 concern: The reason that the cycling throughput does not match the data rate after 50 SPS is because the modulator and digital filter needs additional time to settle once it changes from channel to channel. For that reason, the cycling throughput cannot be the data rate. So to answer your last question; The answer is No. You cannot set the ADC to 7500 SPS and multiplex between 3 inputs to have an effective 2500 SPS per channel rate. If you set your Data rate to 7500 SPS, your throughput will be 3043 Hz. 

    Regards,

    Tony Calabria 

  • 0 in reply to Tony Calabria
    Prodigy 50 points

    Hi Tony

    Thanks for the help! We managed to make the system run by setting it to 7500 SPS, cycling the MUX between 3 different input-pairs like descirbed in the procedure. This gives us 3043 Hz in total and for every channel ~1014Hz. We managed to time the SYNC command so that we actually have ~1000Hz. Our biggest problem is that this ~1000Hz are depending heavily on the timing of the MCU. In other words sending the SYNC command has to be repeated exactly every 333 us othewise the sampling is not accurate. Is there another way to control the sampling rate except sending SYNC commands every 333 us?

    Furthermore is there a similar ADC for which the MUX could be set to SCAN-Mode? Meaning that one would program the MUX to cycle between different inputs but the digital logic of the chip would take care about the actual cycling and timing and not the firmware of the MCU?

    Best regards,

    Philippe

  • 0 in reply to Philippe47787
    TI__Mastermind 43915 points

    Philippe,

     Unfortunately, the ADS1256 depends heavily on the MCU for cycling the multiplexer for converting multiple channels. When you change the multiplexer channel you need to restart the conversion process using the SYNC and RESET commands. 

    I am not sure about all of the requirements of your system, but the ADS1158/ADS1258 has a built in auto-scan mode that is great for exactly what you are doing. If the ADS1158/ADS1258 does not meet your requirements, give me some insight on what you need the converter to do and I will find the right one for you. 

    Regards,

    Tony

  • 0 in reply to Tony Calabria
    Prodigy 50 points

    Hi Tony

    Thanks a lot for your valuable reply. We already have some experience with the ADS1255 and therefore wanted to use the ADS1256 in another design. The ADS1158/ADS1258 sounds promising but needs some further investigation from our side.

    Therefore I wanted to ask if there is: i) a pin compatible chip to the ADS1256 with auto-scan mode and ii) a pin compatible chip to the ADS1255 which is not as expensive.

    Thanks a lot for your help!

    Best regards,
    Philippe

  • 0 in reply to Philippe47787
    TI__Mastermind 43915 points

    Philippe, 

    The ADS1158/1258 are the only delta sigma parts that we have that support an auto-scan mode. We have other multichannel parts, such as the ADS1255/1256 and the ADS117x/127x parts but they do not have an autoscan mode. The ADS1174/8 and ADS1274/8 are simultaneous sampling parts (think of it as having 4 or 8 individual ADCs inside the chip). This may be an option for the customer. In my opinion the ADS1158 is the best choice for them. It is a 16 bit part that has the autoscan mode that they would need. 

    From a price standpoint, the ADS1255/1256, ADS1158/1258, and ADS117x/127x parts are the three delta sigma families that would meet the customer's requirements from my understanding. If they are willing to use a SAR converter, I am sure their are other families that would work for them. The ADS1158 is a $6 part and the ADS1256 is an $8 part so that is an improvement right there.

    Regards,

    Tony Calabria