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Report SBAA113 "PCB Layout..."

mark kovach
Intellectual 300 points

Other Parts Discussed in Thread: THS3061

Has anyone read this?  I can't make heads or tails of the PCB layouts.  Is there a gerber file with individual layers that I can import into my PCB layout software?... Yes, I am looking at the color version!

on pg.5:

. In addition to these capacitors, a third
smaller capacitor (10nF) can also be added between the supplies. This extra capacitor usually
helps reduce 2nd-harmonic distortion.
Figure 5 shows specific capacitor locations. Larger capacitors are shown as C93, C94, C95 and
C96. High-frequency capacitors are labeled as C18, C97, C98 and C100. C101 and C102 are
the smaller capacitors between the supplies.

What is meant by "Between the supplies?"

  • 0
    TI__Mastermind 22825 points

    Hello Mark,

    I don't have any gerber files to share, but I want to chime in on what "between the supplies" means in the Application Report you have noted:

     ADS5500, OPA695: PC Board Layout for Low Distortion High-Speed ADC Drivers

     It means the two capacitors leads (C101 and C102 each in this case shown in Figure 5) are tied to V+ and V- respectively (as opposed to most decoupling capacitors placed between either supply and ground). That's what "between supplies" means.

     

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Intellectual 300 points

    Hello Hooman

    I thought so.  Thanks for verifying.  Does that imply one cap per each op amp?  

    This is the first I've heard of this design addition, so does 2nd harmonic distortion apply to all frequencies of the operating range or to only higher ones like in the mega and gigahertz range?  I ask because I use the THS3061 operating flat from 5Hz to 100kHz, and would certainly like to remove any and all 2nd harmonic distortion up to 40kHz.

    Thanks

    Mark

  • 0 in reply to mark kovach
    TI__Mastermind 22825 points

    Hello Mark,

    To answer your questions:

    1.  Does that imply one cap per each op amp?

    Ans: One capacitor between V+ and V- (but having more won't hurt anything). The Application Report uses two capacitors (C101 and C102).

    http://electronicsproject.org/frequency-component-of-half-wave-rectifier-voltage-and-current/

     

    2. Does 2nd harmonic distortion apply to all frequencies of the operating range or to only higher ones like in the mega and gigahertz range?


    Ans: This applies to all frequencies. The source of the 2nd order distortion is related to the half wave rectified current that flows to either supply (V+ and V-). Cross-supply decoupling caps mitigate the effect of these currents which are rich in 2nd harmonic (especially if your layout is less than optimal) by reducing their impact.

     

    Regards,

    Hooman