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Original question:

TLV8544: PIR Motion Detection Refence Design Application Problem

TLV8544: Op-amp unwanted noise

Naig
Prodigy 250 points
Part Number: TLV8544
Other Parts Discussed in Thread: , TINA-TI

Hi TI experts,

I'm working on PIR sensor project and I'm trying to figure out where to lower the noise on the 2nd stage output of op-amp. Below is the waveform of my circuit design, the yellow is the 2nd stage output of op amp and purple for output after window comparator (DC_PIR_OUTPUT). How to filter the noise?

Here is my schematic. 

Additional detail: I'm using boost converter +3V3 TPS613221a for the Vin of this schematic. 

I have read the PIR TI reference designs and tried couple of workarounds but it didn't help. Please kindly help.

Thank you. 

  • 0
    TI__Expert 6625 points

    Hi Naig, 

    Please check to make sure that you've a clean GND and supply. It's very common to let noise couple in through supply pins. 

    Next, note that resistors are a good source of noise in your circuit. And you're using fairly big gain setting resistors such as: 1Mohm and 13kohm. The formula to calculate resistor noise is given as: sqrt(4*K*T*R*f). For more information on this please refer to TI precision labs videos on noise: https://training.ti.com/ti-precision-labs-op-amps-noise-1?context=1139747-1139745-14685-1138803-13232. 

    Next, please consider that the gain of your circuit is large. So any noise at the input is amplified by the large gain and seen at the output. What is the input to the op amp at the first stage? Is large gain actually needed?

    It is good that you've got a feedback capacitor, it helps limit the bandwidth and helps lower the cumulative noise across frequency. 

    In short, I would recommend trying to use lower resistor values and lower gain. You can also look into T-Feedback network. The advantage of this network is that it allows you to have a higher gain with small resistor values. 

    We've also got an EVM called BOOSTXL-TLV8544PIR that uses TLV8544 for PIR application. Please take a look at the user's guide if interested: https://www.ti.com/lit/ug/snou148a/snou148a.pdf. 

    I would also highly recommend simulating your circuit so you know what to expect at the output. We've a TINA-TI model available for TLV8544 at: https://www.ti.com/product/TLV8544?keyMatch=TLV8544&tisearch=Search-EN-everything&usecase=part-number#design-development. 

    Let us know if you've any questions. 

    Regards,

    Bala Ravi

  • 0 in reply to Bala Ravi
    TI__Expert 6625 points

    Hi Naig, 

    In addition to the above comments, one of my experienced colleague also suggested that you remove capacitors C19 and C18. Low-power op amps such as TLV8544 cannot drive a capacitor directly at the output. Adding large capacitors at the output adds an extra delay causing stability issues. 

    Regards,

    Bala Ravi

  • 0
    TI__Genius 15410 points

    Hi Naig,

    The issue you are seeing is likely a stability issue caused by the 10uF capacitor (C18) placed directly on the output of the op amp. Op amps cannot directly drive large capacitive loads because it degrades the phase margin of the amplifier. I recommend watching our TI Precision Lab Videos on Stability for more information. 

    I recommend either removing the 10uF capacitor or adding an isolation resistor between the output of the op amp and the cap load to stabilize the amplifier. 

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    Prodigy 250 points

    Hi Tim,

    Removing the 10uF capacitor (C18) solves the issue. Thank you for your help.  :)

  • 0 in reply to Naig
    TI__Genius 15410 points

    Hi Naig,

    I'm glad to hear your issue has been fixed. Let us know if you need anything else.

    Thank you,

    Tim Claycomb