AD9243AS datasheet(21/25 Pages) AD

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AD9243

REV. A

–20–

APPLICATIONS

DIRECT IF DOWN CONVERSION USING THE AD9243

As previously noted, the AD9243’s performance in the differen-

tial mode of operation extends well beyond its baseband region

and into several Nyquist zone regions. Hence, the AD9243 may

be well suited as a mix down converter in both narrow and

wideband applications. Various IF frequencies exist over the

frequency range in which the AD9243 maintains excellent dy-

namic performance (e.g., refer to Figure 5 and 6). The IF sig-

nal will be aliased to the ADC’s baseband region due to the

sampling process in a similar manner that a mixer will down

convert an IF signal. For signals in various Nyquist zones, the

following equation may be used to determine the final frequency

after aliasing.

f1 NYQUIST = fSIGNAL

f2 NYQUIST = fSAMPLE – fSIGNAL

f3 NYQUIST = abs (fSAMPLE – fSIGNAL)

f4 NYQUIST = 2

× f

SAMPLE – fSIGNAL

f5 NYQUIST = abs (2

× f

SAMPLE – fSIGNAL)

There are several potential benefits in using the ADC to alias

(i.e., mix) down a narrowband or wideband IF signal. First and

foremost is the elimination of a complete mixer stage with its

associated amplifiers and filters, reducing cost and power dissi-

pation. Second is the ability to apply various DSP techniques to

perform such functions as filtering, channel selection, quadra-

ture demodulation, data reduction, and detection.

One common example is the digitization of a 10.7 MHz IF using a

low jitter 2.5 MHz sample clock. Using the equation above for

the fifth Nyquist zone, the resultant frequency after sampling is

700 kHz. Figure 49 shows the typical performance of the

AD9243 operating under these conditions. Figure 50 demon-

strates how the AD9243 is still able to maintain a high degree of

linearity and SFDR over a wide amplitude.

FREQUENCY – MHz

–150

1.25

–15

–75

–105

–135

–30

–45

–90

–120

–60

Figure 49. IF Sampling a 10.7 MHz Input Using the

AD9243 (VCM = 2.5 V, Input Span = 2 V p-p)

AIN – dBFS

110

–60

–50

–40

–30

–20

–10

100

SFDR – dBc

SFDR – dBFS

Figure 50. AD9243 Differential Input SNR/SFDR vs.

Input Amplitude (AIN) @ 10.7 MHz

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