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