EVAL-AD7891-2CB Datasheet, PDF (8/12 Pages) Analog Devices – Evaluation Board for Single Supply, 12-Bit 500 kSPS ADC

EVAL-AD7891-2CB
F5: Save. This saves a set of samples to a file for use either
at a later date or with other software. The samples can
be saved either as "volts", "ints" or "binary". The
format of all these files is ASCII text. Note that the
AD7891 software can only load files saved in the "ints"
format. Files saved in the "volts" and "ints" formats can
be used with packages such as Mathcad. Files saved in
the "binary" format are for viewing purposes only.
F6: Load. This allows the user to load data from a file
with a .DAT extension. Only data that was saved as
ints can be loaded and analyzed. A configuration file
must be loaded via the "F2 Setup" menu before the
data file can be analyzed. If there is no EVAL-
CONTROL BOARD connected to the PC, then the
"no prog" button in the "F2 Setup" menu must be on.
Once a configuration file has been loaded, the data
loaded from the .DAT file is analyzed according to the
settings in the "F2 Setup" menu.
F7: Reset. Choosing this option resets the EVAL-
CONTROL BOARD.
F10: Quit. This quits the AD7891-2 evaluation software
and returns control to the operating system.
Information Windows
There are three information windows at the top of the main
screen. The leftmost window is the configuration window and
gives details about part being evaluated. It shows the name of
the program which has been downloaded to the EVAL-CON-
TROL BOARD, the sampling frequency, the number of bits,
the analog input range of the part and the output code format of
the part. The rightmost large window is the Status window.
This window provides feedback to the user as to what opera-
tions are currently being performed by the software and also
displays error messages. Directly underneath the status window
is a small window that shows the maximum and minimum
values of the most recently captured samples.
Test Mode
At the top right of the main screen are the Test Mode buttons.
These buttons determine what sort of testing is done on the
samples captured by the software. Both an ac analysis and dc
analysis can be performed. The function of these buttons are:
fft plot
Choosing this button causes the Digital Spectrum
Analyzer to appear at the bottom of the screen.
histogram: Choosing this button causes the Histogram
Analyzer to be displayed at the bottom of the
screen.
There is one other button near the top of the screen, beside the
"F10 Quit" button. This is:
blackman-harris: When performing a fourier transform of the
sampled data, this button determines whether or not
the data is windowed by a blackman-harris window
before the transform. When this button is on, the data
is windowed. When this button is off, the data isn't
windowed. See the Digital Spectrum Analyzer section
for more details.
Digital Storage Oscilliscope.
When samples of data are captured, they are displayed on the
Digital Storage Oscilliscope. If the blackman-harris button is
turned on then the windowed data is also displayed on the
oscilliscope. The 'scope has been designed to act in a similar
way as a conventional oscilliscope. To the right of the
oscilliscope are several buttons that control the manner in which
data is displayed on the 'scope. The timebase for the
oscilliscope is automatically chosen by the software if the Time/
Div "Auto" button is on. The user can also select the timebase
by clicking in the Time/Div window and scrolling up and down
through the possible timebases. Similarly, the vertical scale of
the oscilliscope is chosen automatically if the Volt/Div "Auto"
button is on. The user also has the option of selecting the
desired vertical scale in a similar manner to selecting the
timebase.
The other buttons associated with the oscilliscope are:
grid This button toggles the grid display of the oscilliscope
on and off.
axis This button toggles the axis display of the oscilliscope
on and off
text This button toggles the text displayed on the
oscilliscope screen on and off.
line When the line button is on, the displayed samples are
joined together by lines. When this button is off, the
samples are displayed as points.
ac
When this button is on, the dc component of the
sampled signal is removed and the signal is displayed.
This has the effect of centering the signal vertically on
the oscilliscope screen. When this button is on, the dc
component is not removed and the signal is displayed
with its horizontal axis corresponding to a code of 0.
The ac display option is useful for zooming in on a
low-level signal that has a large dc offset.
dual When the "dual" button is on, the oscilliscope screen is
divided into two parts with the sampled data display
centered on one horizontal axis and the windowed data
display centered on another. When the "dual" button
is off, both traces are centered on the same horizontal
axis.
1
This button toggles the sampled data trace on and off
2
This button toggles the windowed data trace on and
off.
Histogram Analyzer
The histogram analyzer counts the number of occurrences of
each code in the captured samples and displays a histogram of
these counts (see Fig. 5). The most frequently occurring code is
displayed in the center of the histogram. The analyzer is
normally used with a dc input signal and calculates the mean
and the standard deviation of the sampled data. The mean and
standard deviation are displayed in both volts and in units of the
lsb size of the converter. The histogram gives a good indication
of the dc noise performance of the ADC. The standard
deviation shows directly the noise introduced in the conversion
process.
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REV. A