National Semiconductor ADC08200, Инструкция по эксплуатации, Страница: 8 / 16

1. Perform steps 1, 2 and 5 of section 5.2.1.1, above. 9. With the mouse, you may click and drag to select a
portion of the displayed waveform for better
examination.
2. Monitor the ADC08200 output at 96-pin connector
J1 pins B16 through B19 and C16 through C19 (see
appendix for pin assignments).
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.
4. Clock the data out with a TTL clock of any speed up
to 200 MHz at pin B15 of 96-pin connector J1.
The FFT data will provide a measurement of SINAD,
SNR, THD and SFDR, easing the performance
verification of the ADC08200. Note that the readings may
not
accurately reflect the ADC08200's actual
performance unless an input filter is used, as explained in
sections 4.1 and 5.0, and unless the sampling is
coherent, as explained below.
5.2.2 Quick Check of Analog Functions
Refer to Figure 1 for locations of connectors, test points
and jumpers on the board. If at any time the expected
response is not obtained, see section 5.2.6 on
Troubleshooting.
1. Perform steps 1 through 6 (steps 3 and 4 are
optional here) of Section 5.2.1.1.
5.2.4 Getting Consistent Readings
2. Checl for the presence of correct d.c. voltages at
power connector J4, as called for in section 4.6.
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 the
measurements would indicate.
3. Check TP7 or either terminal of L1 for the presence
of a voltage between 2.7V and 3.3V.
4. JP1 - Short the two pins closest to Q1 (pins 2 & 3)
to NOT divide the on-board clock oscillator by 2.
5. Adjust RV1 for a voltage of 1.87V to 1.93V at TP2. 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.
6. Adjust RV2 for a voltage of 0.27V to 0.33V at TP4.
7. Adjust the signal source at Analog Input J3 for a
signal amplitude of approximately 1.6VP-P and
check for the presence of that signal at TP1.
This completes the testing of the analog portion of the
evaluation board.
5.2.3 Quick Check of Software and Computer
Interface Operation
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
1. Perform steps 1 through 5 of Section 5.2.1.1, above.
2. Supply a 1.6Vp-p sine wave of 1 MHz to 50MHz at
Analog Input BNC J3.
3. Be sure there is an interconnecting cable between
the board and your computer serial port.
CY
SS
f
in
f
s
=
4. R
UN
program WAVEVSN2.EXE.
5. After turning on the power, be sure to wait for yellow
LED D4 on the Digital Interface Board to go out
(about 5 seconds) before trying to acquire data or
the board will "freeze" and you will have to cycle the
power.
CY, the number of cycles in the data record, must be an
integer number and SS, the number of samples in the
data record, must be a factor of 2 integer. For optimum
results, CY should also be a prime number.
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.
6. Acquire data by clicking on the ACQUIRE icon or by
pressing
ALT
, P, A or
CTRL
-X. Data transfer can take
a few seconds.
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.
Windowing (an FFT Option under WaveVision) should be
turned off for coherent sampling.
8. Double click on the data window and change the
"Sampling Rate of this data (MHz)" to 200. This
must be done each time another data capture is
done or the frequency information in the FFT will not
be correct.
5.2.5 Jumper Information
Table 1 indicates the function and use of the jumpers on
the ADC08200 evaluation board. JP2, JP7 and JP8 are
hard-wired for 2 memory chips.
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