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TAS3204 radiated noise at 135mhz (clock)

David Alexandrou
Intellectual 410 points
Other Parts Discussed in Thread: TAS3204

Hey there, 

We have a design that uses two TAS3204 chips, and it's having trouble passing C.E. in the emitted RFI test, specifically with an RCA attached, at 135mhz we're between 5 and 10 dB over the acceptable limit.

The board is designed well, we have ferrites isolating all power supply lines, also between analog and digital grounds. There aren't any ground loops.

Shielded case has little effect, the closest we can come is by putting a ferrite choke over the power supply cable, and also the RCA cables. It gets the noise down to 4db over the limit. We tried ferrite pads on top of the chip and under the board where the chip is, it had almost no effect.

The unit has potentially 10 rca cables connected at a time, so putting ferrite chokes on all of these cables is not feasible in a consumer level product.

Any thoughts / tips / things we should look for? The circuit design itself was based around the EVM schematics, though I don't know if the EVM would pass CE either.

Any tips or recommendations, or "gotcha" mistakes to look out for would be most appreciated.

~David

  • 0
    TI__Guru**** 189399 points

    Hi, David,

    I've highlighted your post to Susan, although I'm not sure she can help you; she's more of an expert on s/w. Hopefully, she knows someone who can help you on the hardware side.

    -d2

  • 0
    TI__Expert 3215 points

    David,

    I am trying to find some HW expert to help on this.  I will keep you updated.

    Susan

  • 0 in reply to David Alexandrou
    TI__Expert 3215 points

    David,

    This is what I got from our HW expert:

    What we usually do:

    -50Ohm resistors on MCLK, SCLK, LRCLK in series

    -10pF to ground on MCLK, SCLK, LRCLK

    -Ground crystal case

    -In layout – create an Analog ground island for xtal, PLL external filters, then connect this ground to main ground with FB or 1-3 Ohm resistor

    Hope this helps.

    Susan

  • 0 in reply to susan xu
    Intellectual 410 points

    Hi Susan, 

    Thanks for the reply, I'll look at the board w.r.t. your suggestions.

    One thing - we're running the two chips both in Master mode, neither is slave, so the MCLK, SCLK and LRCLK aren't actually being used. In this case, should these pins just be left floating with no connection?

    Incidentally GPIO1, GPIO2, GPIO3, GPIO4, SDO1, SDO2, MCLK_OUT1, MCLK_OUT2, MCLK_OUT3 are not being used at all - currently they are just floating  / not connected. I couldn't find anything in the datasheet about how to treat these pins if they're not connected.

    Thanks again

    ~David., 

  • 0 in reply to David Alexandrou
    TI__Expert 3215 points

    David,

    This is the guildline:

    The digital input pins that are not used should be grounded preferably through a 0Ohm resistor.

    The digital output pins can be left open.

    The analog input pins that are not used should be grounded through a ~1nF cap.

    The analog output pins can be left open.

    Hope this helps.

    Susan

  • 0 in reply to susan xu
    Intellectual 410 points

    Heya Susan , 

    That was what I assumed, I just wasn't sure if maybe the unused digital output pins could be radiating much if any energy.

    Cheers

    ~David

  • 0 in reply to susan xu
    Intellectual 410 points

    Susan,

    Something that strikes me - your response says to create an analog ground island for Xtal and PLL filters.

    In the application circuit and EVM however, the xtal is tied to digital ground.

    ~David

  • 0 in reply to David Alexandrou
    TI__Expert 3215 points

    David,

    Let me consult our HW expert on this.

    Susan

  • 0 in reply to susan xu
    Intellectual 410 points

    Hi Susan, did you talk to the HW expert about it?

    Cheers
    ~David

  • 0 in reply to David Alexandrou
    TI__Expert 3215 points

    David,

    Yes, I did. But I haven't got his response yet.  He is very busy and TAS3x was just one of his old projects, since it's NRND, no one on the HW side is currently working on it.

    Have you got a chance to try any of his previous suggestions yet? anything makes any difference?

    Regards,

    Susan

  • 0 in reply to susan xu
    Intellectual 410 points

    Hi Susan, 

    Apart from grounding the xtal case (which had no effect) - no. The clock suggestions didn't apply as we aren't using those lines - the other suggestions required re-working the board, which is what I'm currently doing, but we don't want to do multiple iterations, I'd rather incorporate all of the suggestions into one update since we're on a tight budget and schedule.

  • 0 in reply to David Alexandrou
    Intellectual 410 points

    Hello again Susan, 

    I understand that the person in charge of that hardware has moved on, but we did start using this product before it became NRND, and it is still being sold. It is also the core of our product, I really would have hoped to get some sort of reply in the last two months.

    ~David

  • 0 in reply to David Alexandrou
    TI__Expert 3215 points

    David,

    I am very sorry that we couldn't help on this issue.  The HW person that's originally in charge of TAS3x is not with us anymore, so all the HW people I asked around are pretty much at the similar postion as you, if not the same, so all the suggestions they could give are general recommandation, they can't give anything very specific without knowing your HW design.  I have also other information from other HW people, but I am not sure how helpful it could be for you, but I will put it here for  your reference.

    • You mentioned 2 TAS3204 used in the system, what happens if you disable one of the TAS3204.  Can you pass the C.E. with only one TAS3204 running?
    • What happens if you ground all the inputs?  Do you still see peaks at 135MHz?  Asking because you mentioned there are 10 RCA cables connected, not sure if it causes the issue, may try to remove all the cables and probably also ground the inputs?  Do you use Op-Am’s following each RCA input jack?  If not, please refer to the EVM schematics.
    • Is there other clocks in the system, say crystal for the system MCU?  If so, what is the clock speed here, is 135MHz a harmonics of the MCU clock (say MCU CLK at 13.5MHz or 27MHz, etc.)?

    I hope you could resolve this issue soon.

    Regards,

    Susan