Linear Technology DC2520A Demo Manual page 4

DEMO MANUAL DC2520A
DC2520A SETUP
Reference
The default reference is the LTC2344 internal 4.096V
reference. Alternatively, if a higher reference voltage is
desired, the LTC6655-5 reference (U7) can be used by
setting the REF jumper (JP1) to the EXT position and
installing a 0Ω resistor in the R7 position. This should
result in better SNR performance but may slightly degrade
the THD performance of the LTC2345.
Analog Inputs
All eight inputs have the same driver circuitry. The circuit of
Figure 2 shows the driver for AIN0. It provides a DC coupled
single-ended to fully differential output to the analog inputs
of the LTC2344 with a maximum 0V-4.096V input signal.
DC890 Data Collection
For SINAD, THD or SNR testing a low noise, low distor-
tion generator such as the B&K Type 1051 or Stanford
Research SR1 should be used. A low jitter RF oscillator
such as the Rohde & Schwarz SMB100A is used to drive
the clock input. This demo board is tested in house by
attempting to duplicate the FFT plot shown in Typical
Performance Characteristics section of the LTC2344 data
sheet. This involves using a 68.8MHz clock source, along
with a sinusoidal generator at a frequency of approximately
2kHz. The input signal level is approximately –1dBFS. A
typical FFT obtained with DC2520A is shown in Figure 3.
Note that to calculate the real SNR, the signal level (F1
amplitude = –1.081dB) has to be added back to the SNR
that PScope displays. With the example shown in Figure 3
this means that the actual SNR would be 93.59dB instead
of the 92.51dB that PScope displays. Taking the RMS sum
of the recalculated SNR and the THD yields a SINAD of
93.50dB which is fairly close to the typical number for
this ADC.
4
To change the default settings for the LTC2344 in PScope,
click on the Set Demo Bd Options button in the PScope
tool bar shown in Figure 4. This will open the Configure
Channels menu of Figure 5. In this menu it is possible to
set the input signal range setting for each channel. There
is also a button to return PScope to the default DC2520A
settings which are optimized for the default hardware
settings of the DC2520A.
There are a number of scenarios that can produce mislead-
ing results when evaluating an ADC. One that is common
is feeding the converter with an input frequency that is
a sub-multiple of the sample rate, and which will only
exercise a small subset of the possible output codes.
The proper method is to pick an M/N frequency for the
input sine wave frequency. N is the number of samples
in the FFT. M is a prime number between one and N/2.
Multiply M/N by the sample rate to obtain the input sine
wave frequency. Another scenario that can yield poor
results is if you do not have a signal generator capable of
ppm frequency accuracy or if it cannot be locked to the
clock frequency. You can use an FFT with windowing to
reduce the "leakage" or spreading of the fundamental, to
get a close approximation of the ADC performance. If an
amplifier or clock source with poor phase noise is used,
the windowing will not improve the SNR.
DC590/DC2026 Data Collection
Due to the relatively low and somewhat unpredictable
sample rate of the DC590/DC2026 its usefulness is lim-
ited to noise measurement and data collection of slowly
moving signals. A typical data capture and histogram are
shown in Figure 6. To change the default settings for the
LTC2344 in QuikEval, click on the Sequence Config but-
ton. This will open the Config Dialog menu of Figure 7.
In this menu it is possible to set the input signal range for
each sequence. There is also a button to return QuikEval
to the default DC2520A settings which are optimized for
the default hardware settings of the DC2520A.
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