Note:
Be sure to use a band pass filter between the
signal source and this board for accurate dynamic
performance measurement.
5.2.5 Troubleshooting
If nothing happens when F1 is pressed, Select Settings,
then Capture Board Settings and look at top for "Board
Properties" If you see "No WaveVision hardware is
present", be sure that the WaveVision Capture Board is
connected to an USB port and has power, that the
ADC121S101 evaluation board has power, and that the
ADC121S101 evaluation board is properly connected to
and properly seated with the WaveVision Capture Board.
5.2.4 Getting Consistent Readings
Artifacts can result when we perform an FFT on a
digitized waveform, producing inconsistent results when
testing repeatedly. The presence of these artifacts means
that the ADC under test may perform better than our
measurements would indicate. Windowing is a common
method of improving FFT results of finite data.
If there is no output from the ADC121S101, perform the
following:
We can eliminate the need for windowing and get more
consistent results if we observe the proper ratios between
the input and sampling frequencies, forcing the data to
cleanly "wrap around" itself, providing coherent sampling.
This eliminates the distortion that would otherwise be
present in an FFT and greatly increases its spectral
resolution. This, in turn, allows us to more accurately
evaluate the spectral response of the A/D converter.
•
Be sure that a shorting jumper is appropriately
placed on J4.
•
Be sure that the proper voltage and polarity is
present at Power Connector J6.
•
Check to see that the ADC121S101 input signal
does not go below ground or above the
ADC121S101 supply voltage.
When we do this, however, we must be sure that the
input signal has high spectral purity and stability and that
the sampling clock signal is extremely stable with
minimal jitter. Coherent sampling of a periodic waveform
occurs when an integer number of cycles exists in the
sample window. The relationship between the number of
cycles sampled (CY), the number of samples taken (SS),
the signal input frequency (fin) and the sample rate (fs),
for coherent sampling, is
•
Be sure there is a clock signal is present at
ADC121S101 pin 5.
If the PC displayed waveform appears to be noisy, or if
the FFT plot shows nothing but noise with no apparent
signal:
•
Be sure a shorting jumper is appropriately on J4.
•
Check to see that the ADC121S101 input signal
does not go below ground or above the
ADC121S101 analog supply voltage.
CY
SS
f
in
f
s
=
CY, the number of cycles in the data record, and SS, the
number of samples taken, can not have a common
divisor that would yield a whole number for both. The
easiest way to ensure this is to be sure that CY is a prime
integer and SS, the number of samples in the record, is a
even number. For evaluation with an FFT (as opposed to
a DFT), SS must be a power of 2 integer.
•
Be sure that a minimum of +2.7V is at J6 connector
pin 1.
6.0 Evaluation Board Specifications
Board Size:
2.8" x 2.8" (7.2 cm x 7.2 cm)
Power Requirements:
+ 2.9V to 5.7 @ 5 mA
Further, fin (signal input frequency) and fs (sampling rate)
should be locked to each other to ensure the proper ratio.
Clock Frequency Range:
1 MHz to 20 MHz
Analog Input
Windowing (an FFT Option under WaveVision) should
not be used for coherent sampling.
Nominal Voltage:
2.7 VP-P to 5.25 VP-P
Impedance:
50 Ohms
7
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