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dc2581af

DEMO MANUAL DC2581A

Reference

The default reference is the LTC2341 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 LTC2341.

Analog Inputs

Both inputs have the same driver circuitry. The circuit of

Figure2showsthedriverforAIN0.ItprovidesaDCcoupled

single-ended to fully differential output to the analog inputs

of the LTC2341 with a maximum 0V-4.096V input signal.

DC890 Data Collection

For SINAD, THD or SNR testing a low noise, low distortion

generatorsuchastheB&KType1051orStanfordResearch

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 LTC2341 data sheet. This

involves using a 60MHz clock source, along with a sinu-

soidalgeneratoratafrequencyofapproximately2kHz.The

input signal level is approximately –1dBFS. A typical FFT

obtained with DC2581A is shown in Figure 3. Note that

to calculate the real SNR, the signal level (F1 amplitude =

–1.133dB) 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 94.63dB instead of the

93.50dB that PScope displays. Taking the RMS sum of the

recalculated SNR and the THD yields a SINAD of 94.58dB

which is fairly close to the typical number for this ADC.

DC2581A SETUP

To change the default settings for the LTC2341 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 DC2581A

settings which are optimized for the default hardware

settings of the DC2581A.

Thereareanumberofscenariosthatcanproducemislead-

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

LTC2341 in QuikEval, click on the Channel 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 DC2581A settings which are optimized for

the default hardware settings of the DC2581A.