Hi, Just wondering if you have any ideas to improve the ADCJ4000EVM at low frequencies, would the balun on the board be causing this at all?
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As noted in the Introduction section of the ADC12J4000EVM User Guide, the balun transformer coupled input is rated for 400 MHz to 3 GHz.The ADC12J4000 device itself has excellent performance all the way down to DC input signals.
If you want to operate the EVM with lower frequency input signals you can reconfigure the board for differential AC-coupled or DC-coupled input. In that configuration you will need to apply a differential signal, so may need to use an external balun or amplifier EVM to perform single-ended to differential conversion.
The changes needed to enable differential input signals have been addressed in this previous E2E post here:
There are a variety of baluns available at different quality and price points. If the frequency range is not too big it is possible to get baluns with relatively low (1 degree typical) phase mismatch on the differential outputs. If the frequency range of your signals is relatively low, the relative phase mismatch between Fmin and Fmax will also be relatively low.
Depending on your signal frequencies you could consider a differential amplifier instead of a balun. The LMH3401, LMH5401 and LMH6401 would be good devices to consider.
I hope this is helpful.