ADA4945-1 THD behavior

Hello,

I used the ADA4945-1 as ADC front-end (LTC2380-24) with supply rails centered
to ADC 2.5V Vref ( Vcc= +7.5 / Vee = -2.5V for 10V total supply).

The FDA is configured as fully differential filter, with 2 cutoff frequencies.
2 small relays allow to select proper filter part. Because of the size of relays itself,
the FDA input trace are not  short as they would be.

So now i have two different questions about the ADA4945-1 :
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First, it is indicated in datasheet that exposed pad of the IC has to
be connected to negative rail. (but it not seem to be directly connected to it).
I have notice that too late and I connected it to ground plane instead.
So anyway, i don't have seen wrong behavior (current/voltage rails are ok).

Q1 ) Does it is a problem if exposed pad is connected to ground instead of VEE ?
(my Vee= -2.5V).

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I have made several THD measurements with the couple ADA4945-1 and LTC23800-24
and results are very promising.
Anyway i noticed that when i use "full power mode" (for lowest THD), the THD (H2) rise
much (~20-30dB more).
The FDA output doesn't seem to oscillate (i see nothing in 500MHz bandwidth), but if
i add a small amount (~500pF) of capacitor between FDA +/- inputs the THD remain ok.
I tried to reproduce the behavior in LTspice, but without success.
I presume that Spice model doesn't take THD behavior in account.

Q2) Do you have some suggestions about this ?
What can explain THD degradation ?

I hope you can help me to solve the issue.
Best regards.

Olivier

Parents
  • Hello,

    About my first question on the exposed pad, i have noticed after some reading,
    that both evaluation modules for the ADA4945-1 seem to have made same as me...
    The exposed pad is connected to ground, not to -Vee as writed on ADA4945-1 datasheet !
    So, must be connected to Vee or GND ?.....

    About my second question, i don't have explanation and i would really like to get some idea.
    If required, i can post schematics/layout.
    Please, i need help :-/

    Regards.

    Olivier

  • 0
    •  Analog Employees 
    on Nov 26, 2019 6:51 AM 10 months ago in reply to olif

    Hi Olivier,

    The noise performance will be best with the epad connected to the lowest potential (-Vs) so that’s where the epad should be connected.  On the first round of eval boards the epad was mistakenly connected to ground so when operated with dual supplies the noise will be slightly higher, on a single positive supply it won’t matter since ground is the lowest potential.  Later revisions of eval board will have the epad connected to Vs-.

    For your THD problem, the schematic and layout would help. 

    Regards,
    Goz

Reply
  • 0
    •  Analog Employees 
    on Nov 26, 2019 6:51 AM 10 months ago in reply to olif

    Hi Olivier,

    The noise performance will be best with the epad connected to the lowest potential (-Vs) so that’s where the epad should be connected.  On the first round of eval boards the epad was mistakenly connected to ground so when operated with dual supplies the noise will be slightly higher, on a single positive supply it won’t matter since ground is the lowest potential.  Later revisions of eval board will have the epad connected to Vs-.

    For your THD problem, the schematic and layout would help. 

    Regards,
    Goz

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