Part Number: ADS54J60
Other Parts Discussed in Thread: LMH5401
In this forum question we asked about missing input test setup for ADS54J60 ADC. We are trying to get signals from 100MHz-1GHz bandwidth, however for >500 MHz we get much lower ENOB than in datasheet. I have actually managed to pinpoint one of the main issues of the design and it appears to be phase imbalance from balun.
To repeat the previous post, this was our ADC input circuit:
And this was our resulting FFT for 770 MHz input signal:
8.8 bits of ENOB vs >10 bits stated in datasheet.
Turns out the strong spur at 460 MHz is 2nd harmonic from 1540 MHz reverting back and it almost certainly is linked to phase imbalance of our balun.
Following this article we have tried following circuit, a cascaded transformer setup:
And sure enough, we got 9.4 bits of ENOB, a significant gain. I do not have the FFT at hand, but the spur resulting from 2nd harmonic is still present and stronger than all the other spurs. So there surely is still room for improvement.
So my questions are:
- How can we further ensure phase balance as it appears to be critical for our ENOB requirement? Alternatively, we have a design with LMH5401 opamp with SE->DIFF setup instead of baluns in development as well.
- The cascaded balun, TC-1-13MX+ used has phase imbalance of 0.81->1.59 deg in 500->1000 MHz range, but the original balun, which is precompensated (alternate solution from previously linked article), TCM2-33WX+ has 0.82->0.62 in 400->1000 MHz range (best in class from this vendor), which by simple intrapolation should yield better performance at 770 MHz, but cascaded baluns beat it. What is the actual reduction of phase imbalance when baluns are cascaded? Can I cascade precompensated baluns for even better performance given these components? (Keeping in mind that the precompensated baluns is 1:2)
