AD9240

REV. A

–21–

the bandlimited IF signal aliases back into the center of the ADC’s

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

frequency inversion that may easily be corrected for in the digital

domain.

200

0.1 F

AD9240

VINA

CML

VINB

50

AD8009

50

280

93.1

AD8009

200

22.1

50

SAW FILTER

OUTPUT

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

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.

40

–90

–30

–80

–70

–60

–40

100

90

80

60

50

70

30

20

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