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I'm need an advice to select a data converter

Cam Ban Duong
Prodigy 40 points
Other Parts Discussed in Thread: ADS124S06, ADS124S08

Hi,

I'm looking for an ADC that fits my project.

A signal (+-4V) will be measured with a sensor and converted to a digital signal. The ADC resolution should be greater than 20Bit. The signal will lead to a microcontroller
over SPI. It process the signal and send it to a digital and an analog display. The digital display will display the actual value and has a refresh rate of 5Hz and the analog display, which contains a row of  LEDs, has a refresh rate of 20Hz. Frequencies like 16 2/3Hz, 50Hz and 60Hz should be filtered out particularly by the ADC. A keypad is available to interact, like setting limits, ranges, etc.. A Speaker is integrated. The system will be powered by batteries and the power consumption should be as low as possible.

Hope you can help me out. Thx a lot

  • 0
    TI__Mastermind 20835 points
    Hi Cam Ban Duong,

    welcome to our forum and thank you very much for your interest in our ADCs.

    When looking at your requirements, the ADS124S06 might come very close to your needs.
    We don't have an ADC that offers rejection of 16 2/3, 50 and 60Hz all at the same time. Our ADCs usually offer simultaneous rejection of 50 and 60Hz or of 16 2/3Hz and 50Hz, depending on the selected data rate.

    In order to measure a +/-4V signal you will have to use an external voltage reference with a voltage >=4V.
    When using the ADS124S06 internal 2.5V reference you could only measure signals as large as +/-2.5V.

    I might need some more clarification from you on what you exactly mean by '+/-4V signal'. Is this a fully differential signal where for example the positive input swings between 0V and 4V and the negative input is 180° out of phase and swings between 4V and 0V?

    Regards,
  • 0 in reply to Joachim Wuerker
    Prodigy 40 points

    Hi Joachim Wurker,

    thanks for the quick response!

    My fault, I didn't mean to reject all frequencies at the same time. Question: The frequency rejections depend of the datarate? If I want to filter a 16 2/3 Hz I have to select a specific datarate to do that?

    I have a picture attached to describe how the Input has to be. The signal will be +/-2.5 V at the input of the ADC.

    Is the ADS124S06 a new model? Is the only difference between ADS124S06 and ADS124S08 the amount of Input channels?

    Best regards

  • 0 in reply to Cam Ban Duong
    TI__Mastermind 20835 points

    Hi Cam Ban Duong,

    yes, you are correct. The frequency rejection depends on the selected output data rate. To get good 16 2/3Hz rejection you would select the SINC3 filter option in ADS124S06 and a data rate of 16.6SPS.
    We have some details for 50Hz and 60Hz rejection in the datasheet at the different data rates, but unfortunately not for 16 2/3Hz.
    However, you might want to download the 'ADS124S08 Design Calculator' (www.ti.com/.../ads124s08-excel-calc-tool). We have a tab in there which shows you the digital filter response at the different filter and data rate options. You can also calculate the rejection at certain frequencies there.

    Thanks a lot for sharing your input circuitry. If you wanted to, you could operate the ADS124S06 even with a bipolar +/-2.5V analog supply. That way you would not have to level shift your signal.

    The ADS124S06 and ADS124S08 released at the same time last year. The only difference between the two is the number of analog input channels.

    Regards,

  • 0 in reply to Joachim Wuerker
    Prodigy 40 points

    Hi Joachim Wurker,

    is it possible to use two ADS124S06 at the same time on a SPI-Interface? (Daisy Chain)

    If yes, can you give me an example of how to do it?

    Thanks and I'm looking forward to your suggestion.

    Regards,

    Cam Ban Duong

  • 0 in reply to Cam Ban Duong
    TI__Mastermind 20835 points
    Hi Cam Ban Duong,

    you can use multiple ADS124S06 on the same SPI bus. However the device does not offer daisy chain capability.
    In order to use multiple ADS124S06 on the same SPI bus, you need to use a dedicated /CS signal for each device in order to select which ADC you are interfacing to. The SCLK, DIN, DOUT lines can all be shared between the devices.

    Regards,