ADC3669EVM: Evaluating ADC3569/ADC3568 with lower Samplingfrequencies not possible

Tool/software:

Hello Everyone!

I'm currently evaluating the ADC3669 as device, variant ADC3569 and ADC3568 with TSWDC155EVM. With 500MHz sampling frequency, the ADC3569 works fine, and the critical parameters like SNR and SFDR are nominal. When lowering the sampling frequency to 250MHz, no Fundamental Frequency is shown in the FFT of HSDC Pro, as shown in the screenshot. But when enabling the DDC, the fundamental is shown just fine. Device variant ADC3568 behaves the same.
Is there a way to evaluate the 250MSPS ADC without the DDC? Or is this behaviour expected?

HW Setup:

R&S SMIQ 03E Signal Generator with 25MHz RF (no Modulation)  -->  110MHz R&S passive LPF  -->  AINp
Marconi Instruments 2040 Signal Generator 250MHz RF  -->  250MHz passive BPF --> CLK

SW:

ADC3669EVM GUI v1.0.0
MSPS FPGS Server 1.0.0
HSDC Pro v5.31 64bit

Thanks for the help