AD9240
REV.
–21–
the bandlimited IF signal aliases back into the center of the ADC’s
baseband region (i.e. F
S
/4). At a sample rate of 10 MSPS, an
image of the IF signal centered at 37.5 MHz will be aliased back
to 2.5 MHz which corresponds to one quarter of the sample rate
(i.e. F
S
/4). Note, the IF signal in this case will have undergone a
frequency inversion that may easily be corrected for in the digital
domain.
200V
0.1mF
AD9240
VINA
CML
VINB
50V
AD8009
50V
280V
93.1V
AD8009
200V
22.1V
50V
SAW FILTER
OUTPUT
G
1
= 20dB
G
2
= 12dB
MINI-CIRCUITS
T4-6T
Figure 52. Simplified AD9240 IF Sampling Circuit
To maximize its distortion performance, the AD9240 is config-
ured in the differential mode using a transformer. Preceding the
AD9240 is a bandpass filter and a 32 dB gain stage. A large
gain stage may be required to compensate for the high insertion
losses of a SAW filter used for image rejection. The gain stage
will also provide adequate isolation for the SAW filter from the
transient currents associated with AD9240’s input stage.
The gain stage can be realized using one or two cascaded AD8009
op amps. The AD8009 is a low cost current-feedback op amp
having a third order intercept of 33 dB for a gain of +10 MHz
at 37.5 MHz. A passive bandpass filter is required after the
AD8009 to reduce the resulting second order distortion prod-
ucts and limit its out-of-band noise. The specifications of this
filter are application dependent and will affect both the total
distortion and noise performance of this circuit.
Figure 53 shows the single-tone SNR and SFDR performance of
the AD9240 configured in the 2 V and 5 V span without using
the AD8009 gain stage. Only a slight degradation in SNR perfor-
mance (i.e., 1 dB) was noted with the inclusion of the AD8009
gain stage and a bandpass filter. Note, the tradeoff in SNR and
SFDR (dBFS) performance between the 5 V and 2 V spans at
different signal levels.
INPUT POWER LEVEL – dBFS
40
–30–80 –70 –60 –40
80
60
50
70
30
20
WORST CASE SPURIOUS AND SNR – dBc
10
0
–50 –20 –10 0
SFDR w/5V SPAN
SFDR w/2V SPAN
SNR w/5V SPAN
SNR w/2V SPAN
Figure 53. Single-Tone SNR/SFDR vs. Input Amplitude
@ 37.45 MHz
Figure 54 compares the two tone SFDR performance of the
AD9240 in the 2 V span with and without the use of the AD8009
gain stage. No degradation in distortion performance was noted
with the inclusion of the AD8009 gain stage provided that the
AD8009 2nd order distortion products are sufficiently attenu-
ated by the bandpass filter.
INPUT POWER LEVEL (f
1
= f
2
) – dBFS
40
–90 –30–80 –70 –60 –40
100
90
80
60
50
70
30
20
WORST CASE SPURIOUS – dBc AND dBFS
10
0
–50 –20 –10 0
w/o AD8009 – dBFS
w/AD8009 – dBFS
w/o AD8009 – dBc
w/AD8009 – dBc
85 dB REFERENCE LINE
Figure 54. Two Tone SFDR vs. Input Amplitude
@ f
1
= 36.40 MHz and f
2
= 38.60 MHz
