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TIDA-01012: GND System

TI-CSC
Genius 12135 points
Part Number: TIDA-01012
Other Parts Discussed in Thread: OPA2312, ADS8885, TPS7A20, ADS127L21

Tool/software:

Hello E2E Experts,

Good day.

Im a bit lost on the grounding between signal ground and system ground.
 
If there is no ground connection between the opamp and the signal, where is the voltage difference right?
 
Is someone able to explain?
Regards,
TI-CSC
  • 0
    TI__Intellectual 1780 points

    Hello,

    In the TIDA-01012 design, input signal COM pin is connected to 1P35_VREF, i.e. mid point of the opa2312 rails, in the circuit. It is not floating with respect to measurement circuit. Hope this answers your questions. Let us know if you have any further questions.

    Thank you,

    Shaury

  • 0 in reply to Shaury Anand
    TI__Genius 12135 points

    Hello Shaury,

    Good day.

    Do you know that is the reasoning for having the signal be put in between the opamps 2 rails?

    Is this just for setting the common mode or am I missing something?

    Regards,

    TI-CSC

  • 0 in reply to TI-CSC
    TI__Intellectual 1780 points

    Hello,

    The input signal to the multimeter reference design is a differential signal like ±1V,±10V etc. If we connect the COM port to ADC GND, ADC circuit will see negative voltages. ADC VDD is 2.7V. To support negative input voltage, COM pin is connected to AVDD/2, i.e., 1.35V. This adds offset/common mode to the input signal.

    Regards,

    Shaury

  • 0 in reply to Shaury Anand
    TI__Genius 12135 points

    Hello Shaury,

    Good day.

    Is that also so you don’t need a negative voltage rail for the buffer?
     
    Won't the common mode shift the signal up anyway or does it need to be matched from the previous stage anyway?
    The voltage reference at the signal 0V input of the buffer will effectively shift the input + from the voltage divider to the common mode.
    Also, the common mode reference provides the ground through the opamp. Can I have some more info on that? I have seen this done before but have never implemented it for this application.
    Regards,
    TI-CSC
  • 0 in reply to TI-CSC
    TI__Intellectual 1780 points

    Hello,

    Yes, by connecting COM pin to VREF/2, we can avoid negative rail in the design.

    Sorry, I don't understand your comment about matching the common mode range. ADS8885 ADC requires common mode of VREF/2. The common mode to the input signal is added by using a full differential amplifier, U6. By connecting input COM pin to VREF/2, we are just level shifting IN_P and IN_N(COM) signals to VREF/2 to make sure neither IN_P nor IN_N(COM) input pins go to negative values with respect to the circuit GND. The multimeter reference circuit is designed for +/- 50 V, +/-5 V, +/-500 mV, and +/-50 mV. The input signal is first attenuated to using resistors. Let's say if COM pin is connected to GND net in the schematic, the multiplexers (U8 and U4), and amplifier U5A will see negative voltages.

    Below is the link of fully differential amplifier tutorial. I'm not sure if this is what you are looking for. Let us know if you have any questions.

    https://www.ti.com/video/series/precision-labs/ti-precision-labs-ti-pl-amps-fully-differential-amplifiers.html

    Regards,

    Shaury

  • 0 in reply to Shaury Anand
    TI__Genius 12135 points

    Hello Shaury,

    Good day.

    Are there any potential issues associated with copying this and creating a 2-channel device?

    Will there be any concerns regarding measuring voltages from the same and different sources?

    Regards,

    TI-CSC

  • 0 in reply to TI-CSC
    TI__Intellectual 1780 points

    Hello,

    There are no issues if the both channels are isolated/floating with each other. This means that we need isolated power supply for each channel. We have isolated power modules. You can check here. Let us know if you any questions.

    https://www.ti.com/power-management/dcdc-power-modules/isolated-power-modules/overview.html

    Regards

    Shaury

  • 0 in reply to Shaury Anand
    TI__Genius 12135 points

    Hello Shaury,

    Good day.

    Do you have any recommendations for a low noise circuit that will not affect the ADC (5V output @ 100mA).
    Will Voltage ripple be an issue?

    Regards,

    TI-CSC

  • 0 in reply to TI-CSC
    TI__Intellectual 1780 points

    Hello,

    TPS7A20 could be an option. Please check our Linear & low-dropout (LDO) regulators page for more details. 

    Also, we have a new ADCs that has better linearity. ADS127L21, for an example, has maximum INL spec 1.4ppm, i.e. much better than ADS8885. Linearity is a key consideration when designing digital multimeters. What is resolution you are targeting in this design?

    https://www.ti.com/power-management/linear-regulators-ldo/overview.html

    Regards,

    Shaury