Texas Instruments ADS54J42, User Manual

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Optimizing Evaluation Results
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10 SLAU674 – February 2016
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ADS54J42EVM
3 Optimizing Evaluation Results
This section provides assistance for optimizing the performance during evaluation of the product.
3.1 Clocking Optimization
The sampling clock provided to the ADC must have very low phase noise to achieve optimal results. The
default EVM configuration uses the LMK04828 clocking device to generate the sampling clock. There are
two options to improve the clock noise performance:
1. To achieve the best performance, bypass the LMK04828 in favor of an externally-provided clock that is
transformer-coupled to the ADC. The clock must have very low noise and must use an external narrow
pass-band filter to achieve optimal noise performance. The clock amplitude must be within the
datasheet limits. See
Section 5 for more information regarding this setup.
2. Use the LMK04828 as a clock distributor by using an external clock as the input to the LMK04828. Use
filters on the clock to optimize the noise performance. See Section 5.2 for more information regarding
this setup.
3.2 Coherent Input Source
A Rectangular window function can be applied to the captured data when the sample rate and the input
frequency are set precisely to capture an integer number of cycles of the input frequency (sometimes
called coherent frequency). This may yield better SNR results. The clock and analog inputs must be
frequency-locked (such as through 10-MHz references) in order to achieve coherency.
3.3 HSDC Pro Settings
HSDC Pro has some settings that can help improve the performance measurements. These are
highlighted in
Table 3 .
Table 3. HSDC Pro Settings to Optimize Results
HSDC Pro Feature Description
Analysis Window (samples) Selects the number of samples to include in the selected test analysis. Collect
more data to improve frequency resolution of Fast-Fourier Transform (FFT)
analysis. If more than 65,536 samples are required, the setting in the
Data
Capture Options must be increased to match this value.
Data Windowing Function Select the desired windowing function applied to the data for FFT analysis.
Select
Blackman when sampling a non-coherent input signal or Rectangle
when sampling a coherent input signal.
Test Options → Notch Frequency Bins Select bins to be removed from the spectrum and back-filled with the average
noise level. May also customize which Harmonics/Spurs are considered in
SNR and THD calculations and select the method for calculating spur power.
Test Options → Bandwidth Integration Markers Enable markers to narrow the Single-Tone FFT test analysis to a specific
bandwidth.
Data Capture Options → Capture Options Configure the number of contiguous samples per capture (capture depth). May
also enable
Continuous Capture and FFT Averaging .