SNAU127
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2.6 Data Capturing
The board is now ready for a data capture. Before proceeding, perform a manual calibration of the ADC. Even
though the ADC performs a self-calibration at the time of power-up, it is recommended that the user perform
another calibration after sufficient time has passed for the system (primarily temperature) to stabilize. Manual
calibration is performed by clicking the
Calibrate ADC
feature in the
Register
control panel,
Settings
sub-tab.
2.6.1 Configure Display Settings
Open the FFT Control left panel. Confirm that the
dBFS
unit is selected. Also confirm that the correct clock
frequency is being measured by the software by checking in the
Signal Source
right panel. The default frequency
of the on-board clock source is shown in the board performance data shipped with your board.
2.6.2 Check Input Amplitude
The ADC's inputs can be damaged if the Operating Maximums are exceeded (see Operating Ratings section in the
datasheet).
IMPORTANT: Since the ADC signal and clock inputs are not provided with additional protection circuitry on
this board, the burden is on the user to not overdrive the inputs to the extent of damaging them. An "Over-
range" LED is provided for each channel to indicate that the signal amplitude is beyond the ADC full-scale
range, i.e. 0dBFS. Keep the signal amplitude within the operating ratings as specified in the datasheet.
2.6.3 Acquire and Display Data
Perform a data acquisition by clicking the Acquire Data button (Item #5 in Figure 6).
The acquired data will now appear in the (default) time domain plot window. Switch to the frequency-domain
window (FFT) using the WaveVision 5 controls. Type Ctrl-r to obtain the summary of the acquisition. Place the
software in continuous mode (Item #6 of Figure 6) and then acquire again.
At this point, dynamic performance metrics similar to those shown on the reference data shipped with the board
may be obtained. One of the basic variables that you may experiment with at this point is to change the input signal
magnitude and frequency. Please note that to achieve the reference performance, band-pass filters similar to the
items referenced in Section 2.2 should be used. The absence of these filters on the input signal or external clock
will result in sub-standard performance. This is because most signal generators also generate harmonics which, if
not filtered out, will reduce the system performance.
The displayed units can be in dBc or dBFS.
2.6.4 External Clock Source
It is also possible to apply a high-quality external signal source to the clock input rather than using the on-board
LMX2541 clock synthesizer. This will help quantify the LMX2541's performance in an ultra-high-speed signal-
path such as this one. Also, it enables the user to sample at a frequency other than that which the LMX2541 is
programmed to.
If it is attempted to make a data capture while the External Clock source is selected, but the external source signal
generator is not turned on, then the software will hang until a clock source is provided.
The external clock source should be disconnected or turned off when the on-board clock source is selected.
Failure to do so will result in poor performance due to the mixing of the on-board clock and the small
amount of external clock signal leaking through the internal/external clock relay. When the external clock
