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INA826: Need support to run differential and common mode noise simulation in TINA and floating input detection feature implementation.

Swaminath Katte
Prodigy 60 points
Part Number: INA826
Other Parts Discussed in Thread: ADS8688,

Tool/software:

Hello Team,

I am exploring to use INA826 in unity gain configuration for industrial +/10V input stage design with ADS8688 bipolar input ADC.  I checked one of the reference design https://www.ti.com/lit/ug/tidu491/tidu491.pdf?ts=1739693128204  in which INA826 is used in unity gain configuration(fig.3). I would like to run a differential AC (50/60Hz) and common mode AC noise rejection for this INA stage. Please do suggest steps to be followed in the TINA simulation software. Additionally I would like to add floating input detection feature in this circuit using simple pullup resistor and pulldown resistor to make INA stage output to 10.24V when input is disconnected. Is it write method  to use voltage divider circuit on IN+ pin of INA826?

Best Regards,

Swaminath Katte

  • 0
    TI__Genius 14030 points

    Hi Swaminath, 

    For the differential and common-mode noise rejection, you can implement it via adding differential and common mode capacitors or using the existing X2Y capacitor on the reference design. Here are some reference material on the topic: 

    (+) INA828: How to design the input filtering circuit for the IC INA828ID (common mode filter and differential mode filter) and Phase shift of the circuit attached? - Amplifiers forum - Amplifiers - TI E2E support forums 

    common mode and differential filter.pdf

    What type of sensor are you planning on connecting to the input? The pullup and pulldown resistor could impact the sensor impedance, so I would caution using this method if the sensor has a low impedance. For the INA826, this method would be fine. 

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 60 points

    Thank You for valuable your inputs.

    The floating input detection is working fine with circuit configuration as shown below.

    "common mode and differential filter.pdf " - I think any additional link is missing for this?

    I have further queries on this with respect to end use case.

     

    1. The end use case is illustrated below.  The external transducer will be powered by an internally generated isolated 24V DC supply. The sensor’s output is single-ended relative to the common AGND ground. The analog input circuit is isolated from the MCU circuit using an isolated power supply and digital isolators. The MCU side ground is connected to the chassis earth ground.

    Queries-

    1. Is it worthwhile to use an INA for a single-ended input signal, or can we use a simple voltage follower configuration as suggested in another design note - https://www.ti.com/lit/an/sbaa507/sbaa507.pdf?ts=1739871749536&ref_url=https%253A%252F%252Fwww.google.com%252F
    2. if we use INA stage with input RC filter , which ground shall we use for the common mode filter capacitor or X2Y capacitor AGND or earth ground?   
    3. In single-ended modes, more electrostatic and magnetic noise can couple into the signal connections compared to differential configurations.      However, I’m not certain if this coupling results in differential mode noise or common mode noise relative to earth ground considering 10 meters of length of sensor cable. In addition to this the power frequency of 50/60Hz coupling also need to be filtered out from analog dc signal. Which input stage will be more immune to EMI noise? INA or simple voltage follower with low pass filter?                                                                                         
    4. How can I introduce common mode noise in TINA with respect to earth ground in the circuit below to simulate attenuation over a frequency range? I’m unsure how to distinguish between signal ground and earth ground.

     

    Best Regards,

    Swaminath Katte

  • +1 in reply to Swaminath Katte
    TI__Genius 14030 points

    Hi Swaminath, 

    Thank you for the additional information. Here is the link again - apologies for the broken link: Link

    For your queries: 

    1. You can use an op amp configuration, but the advantage of the INA solution is that it is able to measure the difference in signals rather than the op amp will need to be built out discretely to do the same function. How you have set up the circuit with the pullup resistor for input detection and the INA should be sufficient. 

    2. It's recommended to use the AGND as the analog input circuitry should all be connected to the same ground to prevent ground issues. It appears that earth ground is located on the digital side and should be kept separated. 

    3. The INA has inherent EMI protection via the input stage and is comparable to op amp performance. Please see this app note for more information on EMIRR in general: EMI Rejection Ratio of Operational Amplifiers (With OPA333 and OPA333-Q1 as an Example) (Rev. A) This talks about op amps but is still relevant for INAs. 

    4. You can simulate multiple types of grounds in TINA but there is not a way to simulate how noise would propagate through grounds. How ground bouncing and signals propagate and couple can only be seen via board testing and it is difficult to simulate this. 

    Please let me know if you have further questions.
    Thank you!

  • 0 in reply to Ashley Kang
    Prodigy 60 points

    Thank you for your detailed explanation on the queries I raised.