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THS4551: RF testing

Brian Wang0928
Genius 15220 points

Part Number: THS4551

Hello team,

Customer is using THS4551 to amplifier (20x) the current measured signals to AMC1303(ISO Modulator) device. 

However, during device is in operation and the then mobile phone gets closer to EVM board (as picture below). There are some ripple noise found of up to 65mVpeak on the input rail of the THS4551.

As a result, it could impact the current measurement accuracy. 

 

Mobile phone gets close to the board of the THS4551.

Schematic:

Test results:

  • 0
    TI__Mastermind 22650 points

    Hi Brian,

    This is a very interesting phenomenon. We sometimes expect some noise due to external factors but 63mV seems high. One thing I am curious about is the supplies, from the way the unit is laid out it seems like the VS- pin would be at the top where +5V is being supplied. It is hard to tell but is there a way you could confirm the device is biased appropriately. Is the Iq when the device is in stand-by around the value highlighted in the datasheet?

    Best Regards,

    Ignacio

  • 0
    Prodigy 30 points

    Hi Ignacio,

    I measure  Iq = 2.4mA  when THS4551 is in stand-by. When I switch phone to airplane mode, it has no noise. So i think it should be RF noise. 

    Also I test TI CSA INA241A2 EVB, has a fixed gain 20 same as THS4551, and I using same Delta-Sigma AMC1303 to sense.  It has no noise phenomenon.

    The noise seems to EMIRR issue and caused a offset during 1ms intervals.

    Best Regards,

    Mario

  • 0 in reply to Mario Lin
    TI__Mastermind 22650 points

    Hi Mario,

    It does seem to be some kind of RF noise being picked up from the device. However, I was hoping you could confirm the device is powered up appropriately. Is pin 3 biased to 5V, and pin 6 GND? It is hard to tell but I want to make sure the device is configured appropriately. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 30 points

    After twist Vcc & GND derectly connected to bypass MLCC of TH4551, the noise is decreased to 1.5mV / 0.5mV which phone is 5mm / 20mm distance near the TH4551. The Vcc wires are about 50mm. I think it's not a issue again on design-in on product PCB next. Thanks for your comments!

  • 0 in reply to Mario Lin
    TI__Mastermind 22650 points

    Hi Mario,

    Just to confirm, was switching the supplies fixing the issue you were seeing?

    Best Regards,

    Ignacio 

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 30 points

    I had test 2 setups, one with a power supply and the other one with bootstrap power of a half-bridge topology, and both had same noise phenomenon. After twist the Vcc & GND wires, noise greatly derease in both setups.