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Filters used on Differential Transceivers

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zktec
zktec Contributor Level 1
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Referring to the Controller schematic for the CRD300DA12E_XM3 reference design, I noticed the filters used on the AM26LV32 receivers are an LCL.

image-b87482bd62977-44c7.png

If I simulate this in SPICE, these inductors seem to actually cause transients at the reciever input just from the normal switching of the transmitter. I'm sure the designer that chose 1uH and 27pF had good reason, just would like to know what those reasons were. Appreciate any insight on things like…

what type of noise are you trying to reject? inductive coupling? capacitive coupling?

what noise frequencies of interest?

were there concerns about crosstalk from other signals in the ribbon cable?

were there concerns about differential-mode noise? or mainly common-mode?

I'm not experienced with SiC designs at all, but my first thought is that the common-mode noise caused by the SiC MOSFET would be the biggest noise contributor on these differential lines. Why wasn't a traditional common-mode signal choke used instead, like a 0603USB-222MLC for example? Appreciate any insight you may provide.

Thank you,

Zackary Long

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  • Forum_Moderator
    Forum_Moderator Wolfspeed Employee - Contributor Level 5
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    Thank you for your post, it has been approved and we will respond as soon as possible.

  • ChrisN
    ChrisN Wolfspeed Employee - Contributor Level 4
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    Hello Zackary,

    This differential filtering implementation is based on the Texas Instruments® "Design of Differential Filters for High-Speed Signal Chains" Application Report (SLWA053B) (citation below). By design (assuming balanced signal paths), adopting differential signaling achieves a high common-mode (CM) rejection since noise influences both signals equally (see the figure below from PRD-06992). The filtering on the CRD300DA12E-XM3 design is intended to be a differential low-pass filter. For long connections, we recommend using twisted ribbon cables for these differential signals for better signal balance (example: 3M® 1700/16 100SF; see Section 3.1 of PRD-06992 and Section 7.1 of PRD-09301 for more information). With this cabling and smart cable routing, we don't observe crosstalk issues in the cable bundle. Overall, we have used this filtering approach on several of our reference designs (e.g. CRD300DA12E-XM3 and CRD200DA23N-GMA) and gate drivers (e.g. CGD1700HB2M-UNA and CGD1700HB2P-XM3) with high reliability in significant field hours.

    image.png

    Citation: K. Chan, "Design of Differential Filters for High-Speed Signal Chains", Application Report SLWA053B, Texas Instruments, Nov. 2007 [Revised Apr. 2010].

    Thanks,

    Chris N.

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