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ADC121S021: ADC121S021 with Op-Amp

Part Number: ADC121S021
Other Parts Discussed in Thread: ADS8661, ADS8665, OPA328, OPA206

Tool/software:

Hi, I would like to use the ADC121S021 with the STM32F446 microcontroller as an external ADC.

The signal is coming from a vibration sensor (PCB 353B03) via a signal conditioner (PCB 480C02). The signal conditioner outputs a ±5 VAC signal, which cannot be fed directly to the ADC. Therefore, I need to convert this bipolar signal into a unipolar signal. I am using ±12 VDC rail-to-rail supplies.

While researching bipolar-to-unipolar conversion, I came across a TI application note:
https://www.ti.com/lit/an/sbaa604/sbaa604.pdf

Could you please confirm whether I can follow the approach outlined in this document for use with my vibration sensor? Also, is the ADC121S021 a suitable choice for this application, or would you recommend a different ADC?

Thank you,

Fahad

  • Hi Fahad,

    Yes, that circuit serves as good guidance to scaling down your input signal and shifting it up to a unipolar input range. Alternatively, we offer an ADS8661 which can directly support up to a +/-12.288V input range on a single +5V supply, with programmable input ranges in between.

    Do you have any other speed, resolution, or power requirements? The ADC121S021 is still a good device if it meets your needs, but maybe there are other alternatives you might prefer.

    Regards,
    Joel

  • Hi,

    I appreciate your response. Please check the attached schematic for your review.

    i) Using ADS8665 to directly interface the sensor with the microcontroller.

    ii) Using ADC121S021 with an op-amp circuit.


    Kindly review and confirm the connections. Please let me know if any improvements are needed to ensure accurate output. I would greatly appreciate it if you could also provide some PSpice simulations for better understanding for op-amp circuit.
    Thank you!

  • Hi, 

    Kindly provide the update on this so I can proceed with my schematics design. Thank you

  • Hi Fahad,

    Apologies for the delayed response. The schematics for the ADS8665 and the ADC121S021 look good, but I would recommend putting series resistors on the digital lines, especially SCLK, if you are also placing them in series with the SDO line. They might already be there on the controller side though. For the ADC121S021, I would also place a small RC filter to help drive the input of the ADC. A 22 ohm, 1nF filter should be good. This is not necessary for the ADS8665 since it has an integrated front end.

    I haven't been able to verify the performance in SPICE, but maybe it would help to crosspost this schematic to the amplifiers forum, so they can give more specific guidance to the operation of the OPA206 and OPA328, and whether any limitations on the input or output exist that might affect your input to the ADC. I suspect they are mostly covered in the example circuit you shared though. 

    Regards,
    Joel