National Semiconductor ADS54J69, User Manual

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Optimizing Evaluation Results
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10 SLAU662B – January 2016 – Revised May 2016
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Copyright © 2016, Texas Instruments Incorporated
ADS54J69EVM
3 Optimizing Evaluation Results
This section assists the user in optimizing the performance during evaluation of the product.
3.1 Clocking Optimization
The sampling clock provided to the ADC needs to 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, the LMK04828 can be bypassed 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. The LMK04828 can be used as a clock distributor by using an external clock as the input to the
LMK04828. Filters should still be used 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 needs to 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 and 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 .