Datasheet

AD9042
Rev. B | Page 20 of 24
Assuming that the C/N ratio must be 6 dB or better for accurate
demodulation, one of the eight signals can be reduced by 48.8 dB
before demodulation becomes unreliable. At this point, the
input signal power would be 40.6 μV rms on the ADC input or
−74.8 dBm. Referenced to the antenna, this is −104.8 dBm.
IF SAMPLING, USING THE AD9042 AS A MIX-
DOWN STAGE
Because the performance of the AD9042 extends beyond the
baseband region into the second and third Nyquist zone, the
converter may find many uses as a mix-down converter in both
narrow-band and wideband applications. Many common IF
frequencies exist in this range of frequencies. If the ADC is used
to sample these signals, they are aliased down to baseband during
the sampling process in much the same manner that a mixer
downconverts a signal. For signals in various Nyquist zones, the
following equations may be used to determine the final
frequency after aliasing.
To improve sensitivity, several things can be done. First, the
noise figure of the receiver can be reduced. Because front-end
noise dominates the 0.529 mV rms, each dB reduction in noise
figure translates to an additional dBc of sensitivity. Second,
providing broadband AGC can improve sensitivity by the range
of the AGC. However, the AGC only provides useful
improvements if all in-band signals are kept to an absolute
minimal power level so that AGC can be kept near the
maximum gain.
f
1NYQUISTS
= f
SAMPLE
− f
SIGNAL
f
2NYQUISTS
= abs (f
SAMPLE
− f
SIGNAL
)
f
3NYQUISTS
= 2 × (f
SAMPLE
− f
SIGNAL
)
f
4NYQUISTS
= abs (2 × f
SAMPLE
− f
SIGNAL
)
This noise-limited example does not adequately demonstrate
the true limitations in a wideband receiver. Other limitations
such as SFDR are more restrictive than SNR and noise. Assume
that the ADC has an SFDR specification of −80 dBFS or −76
dBm (full scale = 4 dBm). Also assume that a tolerable carrier-
to-interferer (C/I) (different from C/N) ratio is 18 dB (C/I is the
ratio of signal to in-band interfere). This means that the
minimum signal level is −62 dBFS (−80 plus 18) or −58 dBm.
At the antenna, this is −88 dBm. Therefore, as can be seen,
SFDR (single or multitone) would limit receiver performance in
this example. However, SFDR can be greatly improved through
the use of dither (see Figure 15 and Figure 18). In many cases,
the addition of the out-of-band dither can improve receiver
sensitivity nearly to that limited by thermal noise.
Using the converter to alias down these narrow-band or
wideband signals has many potential benefits. First and
foremost is the elimination of a complete mixer stage, along
with amplifiers, filters, and other devices, reducing cost and
power dissipation.
One common example is the digitization of a 21.4 MHz IF using a
10 MSPS sample clock. Using the equation for the fifth Nyquist
zone, the resultant frequency after sampling is 1.4 MHz. Figure 44
shows performance under these conditions. Even under these
conditions, the AD9042 typically maintains better than 80 dB
SFDR.
FREQUENCY (MHz)
0
–80
–120
–40
–100
–20
–60
POWER RELATIVE TO ADC FULL SCALE (dB)
dc12345
8 7 8 6 2 5 3 4
ENCODE = 10.0MSPS
AIN = 21.4MHz
00554-062
Multitone Performance
Figure 43 shows the AD9042 in a worst-case scenario of four
strong tones spaced fairly close together. In this plot, no dither
was used, and the converter still maintains 85 dBFS of spurious-
free range. As noted in the Overcoming Static Nonlinearities
with Dither section, a modest amount of dither introduced out-
of-band can be used to lower the nonlinear components.
FREQUENCY (MHz)
0
–80
–120
–40
–100
–20
–60
POWER RELATIVE TO ADC FULL SCALE (dB)
3 6 9 7 4 2 5 8
dc 20.516.412.38.24.1
ENCODE = 41MSPS
00554-061
Figure 44. IF Sampling at 21.4 MHz Input
Figure 43. Multitone Performance