1.
Perform steps 1 through 7 of Section 5.2.1.1. Adjust
the input signal at J2 for 1.6VP-P.
We can eliminate the need for windowing and get more
consistent results if we observe the proper ratios between
the input and sampling frequencies. This greatly
increases the spectral resolution of the FFT, allowing us
to more accurately evaluate the spectral response of the
A/D converter. 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
2.
JP4 - Short the two pins farthest from Y1 (pins 1 &
2) to divide the on-board clock oscillator by 2
(Default position).
3.
Turn on the power to the board.
4.
Adjust VR1 for a voltage of 1.9V at TP2.
5.
Adjust VR2 for a voltage of 0.3V at TP4.
6.
Scope TP7 to be sure the input signal is present.
7.
Adjust the signal source at Analog Input J1 for a
signal amplitude of approximately 1.6VP-P.
This completes the testing of the analog portion of the
evaluation board.
CY
SS
f
in
f
s
=
5.2.3 Quick Check of Software and Computer
Interface Operation
CY, the number of cycles in the data record, must be an
integer number and SS, the number of samples in the
record, must be a factor of 2 integer. For optimum
results, CY should also be a prime number.
1.
Perform steps 1 through 7 of Section 5.2.1.1, above.
2.
Supply a 1.6Vp-p sine wave of about 10 MHz at
Analog Input BNC J2.
3.
Be sure there is an interconnecting cable between
the board and your computer serial port.
Further, fin (signal input frequency) and fs (sampling rate)
should be locked to each other. If they come from the
same generator,
whatever frequency instability (jitter) is
present in the two signals will cancel each other.
4.
R
UN
program WAVEVSN2.EXE.
5.
After turning on power, be sure to wait for yellow
LED D4 on the Digital Interface Board to go out
before trying to acquire data or the board will
"freeze" and you will have to cycle the power.
Windowing (an FFT Option under WaveVision) should be
turned off for coherent sampling.
5.2.5 Jumper Information
6.
Acquire data by clicking on the ACQUIRE icon or by
pressing
ALT
, P, A or
CTRL
-X. Data transfer and
calculations can take a few seconds.
Table 1 indicates the function and use of the jumpers on
the ADC08100 evaluation board. Note that which pins of
J5 are shorted (or whether any are shorted) is a "don't
care" when 1 memory chip is used, which is the normal
case for this board.
7.
When transfer is complete, the data window should
show many sine waves. The display may show a
nearly solid area of red, which is O.K.
JUMPER
FUNCTION
PINS 1 & 2
SHORTED
PINS 2 & 3
SHORTED
8.
Double click on the data window and change the
"Sampling Rate of this data (MHz)" to 100. This
must be done each time another data capture is
done or the frequency information in the FFT will not
be correct.
J1
(Hard wired)
No of Mem Chips
1 Mem Chip
2 Mem Chips
J2
Input BNC
-
-
9.
With the mouse, you may click and drag to select a
portion of the displayed waveform for better
examination.
J3
not used
-
-
10. Click on the FFT icon or type
ALT
, P, F or
CTRL
-F to
calculate the FFT of the data and display a
frequency domain plot.
J4
Clock Select
Divide Clock
by 2
Do not Divide
Clock
J5
(Hard wired)
DIV_EN
(No. of Mem Chips)
2 Mem Chips
1 Mem Chip
The FFT data will provide a measurement of SINAD,
SNR, THD and SFDR, easing the performance
verification of the ADC08100.
Table 1. Jumper settings.
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.
8
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