5

dc1908af

DEMO MANUAL DC1908A

DC1908A SETUP

There are a number of scenarios that can produce mis-

leading results when evaluating an ADC. One that is

common is feeding the converter with a 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.

Layout

As with any high performance ADC, this part is sensitive

to layout. The area immediately surrounding the ADC on

theDC1908Ashouldbeusedasaguidelineforplacement,

and routing of the various components associated with

the ADC. Here are some things to remember when lay-

ing out a board for the LTC2338-18. A ground plane is

necessarytoobtainmaximumperformance.Keepbypass

capacitors as close to supply pins as possible. Use indi-

vidual low impedance returns for all bypass capacitors.

Use of a symmetrical layout around the analog inputs will

minimize the effects of parasitic elements. Shield analog

input traces with ground to minimize coupling from other

traces. Keep traces as short as possible.

Component Selection

When driving a low noise, low distortion ADC such as

the LTC2338-18, component selection is important so

as to not degrade performance. Resistors should have

low values to minimize noise and distortion. Metal film

resistors are recommended to reduce distortion caused

by self heating. Because of their low voltage coefficients,

to further reduce distortion NPO or silver mica capacitors

should be used. Any buffer used to drive the LTC2338-18

should have low distortion, low noise and a fast settling

time such as the LT1469.