AD6676EBZ datasheet(53/90 Pages) AD

53 / 90 page

Data Sheet

AD6676

Rev. A | Page 53 of 90

APPLICATIONS INFORMATION

ANALOG INPUT CONSIDERATIONS

Equivalent Input Impedance and S11

The AD6676 benign input structure along with its low drive

level requirements facilitates interfacing it to external driver

circuitry. Figure 128 shows the equivalent parallel impedance

for attenuator settings of 0 dB and 6 dB. Note that the slight

variation in impedance between the different attenuator settings

is an error source affecting the absolute accuracy of the attenuator

settings. The AD6676 input also displays excellent S11 return

loss over a wide frequency range, as shown in Figure 94.

100

200

300

400

500

600

800

1000

700

900

FREQUENCY (MHz)

SHUNT R WITH ATTENUATOR = 0dB

SHUNT R WITH ATTENUATOR = 6dB

SHUNT C WITH ATTENUATOR = 0dB

SHUNT C WITH ATTENUATOR = 6dB

Figure 128. Typical Equivalent Parallel Impedance of AIN for Attenuator = 0

and 6 dB Settings

Input Driver and Filter Considerations

The input driver requirements, along with any additional

filtering, are application dependent. Additional filtering maybe

considered if any large signal content or blockers falling above

or below the IF pass band of interest can cause desensitization

by either increasing the ADC noise or spur floor. Below the IF

pass band, the AD6676 is most sensitive to second harmonic

content that is typically induced by the driver stage itself due

to its limited IP2 performance. The AD6676 second-order

nonlinearity contribution is typically on par with a balanced

mixer and well below the contribution of a single-ended amplifier

stage (with output balun) used for VHF applications. Table 20

shows the measured f1 + f2 spurious level and equivalent IIP2

for different IFs when dual tones are injected at −6 dBFS levels

and at IF/2.

Table 20. Harmonic Levels When Dual Tones = −6 dBFS of

PIN_0dBFS Level is Situated at IF/2

(MHz)

LEXT

(nH)

PIN_0dBFS

(dBm)

Dual Tone

Input Power

(dBm)

f1 + f2

Spur

(dBc)

Equivalent

IP2 (dBm)

200

−2.5

−8.5

−69.5

250

−2.2

−8.2

−68.3

300

−2.2

−8.2

−73

350

−2.2

−8.2

−66.3

58.5

400

−2.2

−8.2

−68.5

Above the IF pass band, the AD6676 is sensitive to high frequency

blockers that can increase the noise floor due to jitter or generate an

image component that falls back into the pass band. The AD6676 is

also fairly insensitive to spurious tones falling in the alias regions

occurring at FADC ± FIF because the AD6676 provides over 50 dB

of alias rejection. Table 21 shows the typical alias rejection for

different FADC and IF combinations. Because mixers often produce

fixed large spurious at M × LO as well as its sum term of LO +

FRF, determine if any of these spurs can fall in the alias regions and

if so, add the appropriate level of filtering to suppress them below

the receivers required spurious level.

Table 21. Typical Alias Rejection for Different IF and ADC

Combinations

ADC (MHz)

IF (MHz)

ADC − IF Alias

Rejection (dBc)

ADC + IF Alias

Rejection (dBc)

2000

150

2400

200

2800

300

3200

400

Because the required attenuation of out-of-band signal signals is

application dependent, evaluate the AD6676 under the desired

application conditions to understand the effects and determine

what amount of filtering is required. In practice, a simple third-

order low-pass roofing filter can provide adequate additional

suppression against spurs falling in the alias regions as well as

large signal signals falling a few 100 MHz above the IF pass

band. Note that the AD6676EBZ includes an optional 500 MHz

third-order low-pass filter (TDK MEA1210D501R) that may

suffice for many applications. This small, 0302 size differential

filter is also available with lower frequency options. Its effect on

the pass band flatness is mimimal but provides provides additional

suppression beyond 700 MHz. as shown in Figure 129 as well as

in the alias region as shown in Table 22.

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