National Semiconductor ADC14071, Инструкция по эксплуатации

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If the displayed waveform appears to be garbage, or if the
FFT plot shows nothing but noise with no apparent signal,
reset the evaluation board by pressing button S1 and try
again. 5.1.1 Evaluating a Sine Wave
Set the ADC clock frequency to 7MSPS as follows:
1. Be sure that a 14MHz oscillator (Y1) is in its socket, or
use an external frequency source.
If the performance is significantly better with the op-amp
circuit than it is with the transformer, the transformer may
need to be replaced. We have found that only one out of 10 of
these transformers perform satisfactorily.
2. Select the Procedures pull-down menu and the Configure
Board sub menu, or type
CTRL
P.
3. Be sure that the lower left of the dialog box indicates your
oscillator frequency. Check this each time you enter this
dialog box.
5.0 Exploring the Waveform 4. Set the clock divide by number to 2 by typing
CTRL
-P and
changing the "Board to ADC Clock Ratio" to 2 (choices are
2 and 4 for the ADC14071).
WaveVision software and the ADC14071 Evaluation Board
add a new tool to the designer's toolbox. The software and
evaluation board can be used to capture a signal. The
captured data can then be displayed on a computer monitor
and performance parameters can be estimated.
5. Select the number of samples you wish to take. It takes
longer for the computer to operate on a very large number
samples, but FFT accuracy is better with more samples. If
you are performing an FFT on the data, take at least 4096
samples.
After the ADC14071 Evaluation Board has uploaded a
captured waveform to the PC, WaveVision displays this
waveform on the computer monitor. You should realize that
any amplifier used before the ADC14071 can affect the
apparent performance of the ADC because most available
amplifiers exhibit more distortion than does the ADC14071.
The characteristics of any transformer you may use can also
affect the overall circuit performance.
6. Click on OK.
7. Connect a signal generator to input BNC J1.
8. Adjust the generator to provide a 5kHz to 6kHz output.
Adjust the level so LEDs D3 and D4 do not come on.
9. Capture the signal (Procedure Execute or
CTRL
X) and
wait for it to be displayed on your monitor.
See the Appendix for WaveVision screen drawings of
software operation.
You may select a small portion of the waveform by clicking
and dragging across it.
5.1 Signal Purity
5.1.2 Low Frequency Triangle Wave Input
A low frequency (about 1KHz) triangle wave will provide
general information on ADC performance. If you are looking
for triangle wave symmetry, compare the ADC output
symmetry with that of the generator output. Many triangle
wave generators do not produce a symmetrical output.
Most sine wave generators can not produce a signal pure
enough to adequately evaluate a 14-bit ADC. Since the
SINAD of a perfect 14 bit ADC is 86dB, any input signal
should be at least 6dB better than this, or have a SINAD of
92dB or better! Even very expensive, high-performance signal
generators, generally, can not produce such a clean signal.
5.1.2.1 Monotonicity and Uncertainty
When a voltage ramp is digitized, the code sequence shows
increasing codes up to the peak level, or decreasing codes to
the minimum level, depending upon whether the slope is
positive or negative. A monotonic condition is one where the
code sequence does not show any reversals, as in Figure 5a.
To ensure that the input signal is clean, you should insert a
low pass filter in series with the signal input. Comparing the
dynamic response with and without this filter is an education
in itself.
The elliptic filter of
Figure 4 is an example of a suitable filter.
It has attenuation of about 3dB at 800 kHz, 32dB at 1MHz and
64 dB at 1.5 MHz.
a. Monotonic Signal b. Non-Monotonic Signal
Figure 5. Monotonicity means codes are continually increasing or
decreasing.
1500pF 1500pF 4700pF 4700pF
220pF 560pF 470pF
22uH 22uH 22uH 75
75
Figure 4. This elliptic filter should be driven by a generator of 50 to
75 Ohms source impedance and terminated with 50 to 75 Ohms.
The input resistor shown here is normally included in the
generator.
A converter that has one or more instances of codes going in
the wrong direction,
always at the same point(s), is said to be
non-monotonic. Code progression reversal at sporadic points
in the waveform indicates noise in the system and not a non-
monotonic condition.
When digitizing signals with rise and fall times slow enough
to result in more than one conversion of the same code in
sequence, it is normal to have some code uncertainty when
the input is at a code transition point. See Figure 6.
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