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Original question:

ISO224: About ISO224 output noise peak

ISO224: ~550kHz noise

Erik Anderson41
Intellectual 915 points
Part Number: ISO224

Hi,

Attaching my schematic.  I am noticing a 300mVpp 550kHz (with distortion) at the output of the ISO224.  I suspect my layout is not good enough and that I'm just noticing a beat frequency of the internal modulator frequency (500MHz), but not sure.  Have you seen this before?

Thanks,

Erik

main.pdf

  • 0
    Guru 140200 points

    Hi Erik,

    some high frequency noise at the output of ISO224 is entirely normal. That's why figure 48 of datasheet recommends low pass filtering at the output of ISO224. Also the ISO224EVM contains a low pass filter:

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 47625 points

    Hi Erik,

    This is a known behavior of the IC and is considered normal. As Kai said, filtering of the output should remove the oscillation. 

    Thank you Kai! 

  • 0 in reply to Alexander Smith
    Intellectual 915 points

    Okay, I didn't really see anything about this in the datasheet.  It says the carrier frequency is 480MHz, so I was sort of assuming that bandwidth of the amplifiers would filter this high frequency out.  Not sure why there would be 300mVpp at 550kHz. 

    The datasheet specifies the noise at 360uV-RMS with a 100kHz bandwidth.  If I put a pole at 100kHz, I'm not sure the 300mVpp (~100mV-RMS) will reduce to 360uV-RMS, right?  Even if I assume that 550kHz is a decade higher than 100kHz, a 20dB (10x) noise reduction would only get me to 10mV-RMS, which is 27x higher than what the datasheet specifies.

    So, I'm still not sure what is going on here, seems like if this noise is expected, it should be a lot smaller.  Unless there is a nonlinear relationship going on here where a 100kHz pole will drastically reduce the noise beyond a simple single pole system.

  • 0 in reply to Erik Anderson41
    Guru 140200 points

    Hi Erik,

    the EVM provides a corner frequency of low pass filtering of about 50kHz. Simply give it a try and see what happens :-)

    Kai

  • 0 in reply to Erik Anderson41
    TI__Mastermind 47625 points

    Hi Erik,

    Since the origin of this frequency is due to internal device structure that we cannot share publicly, it is not mentioned in the datasheet. Furthermore, it is beyond the bandwidth of the amplifier itself and we do recommend filtering of the output in the datasheet, which often solves this issue. The magnitude of 300mVpp is indeed quite high, perhaps it is a layout issue as you mentioned in the OP.

    What is the frequency of the input signal you are measuring?

    Can you try replacing the 0ohm resistor to filter the single-ended output?

    Can you add capacitors to filter the differential to single-ended conversion? 

    Can you share your layout for review?