AD9689 datasheet(32/134 Pages) AD

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AD9689

Data Sheet

Rev. A | Page 32 of 134

VREF PIN

AND VFS

CONTROL

INPUT

FULL-SCALE

ADJUST

VREF

INPUT

0.1µF

VOUT

VIN

ADC

INTERNAL

0.5V

REFERENCE

GENERATOR

SET

GND

ADR130

Figure 86. External Reference Using the ADR130

DC OFFSET CALIBRATION

The AD9689 contains a digital filter to remove the dc offset

from the output of the ADC. For ac-coupled applications, this

filter can be enabled by writing 0x86 to Register 0x0701. The

filter computes the average dc signal and it is digitally subtracted

from the ADC output. As a result, the dc offset is improved to

better than 70 dBFS at the output. Because the filter does not

distinguish between the source of dc signals, this feature can be

used when the signal content at dc is not of interest. The filter

corrects dc up to ±512 codes and saturates beyond this value.

CLOCK INPUT CONSIDERATIONS

For optimum performance, drive the AD9689 sample clock

inputs (CLK+ and CLK−) with a differential signal. This signal is

ac-coupled to the CLK+ and CLK− pins via a transformer or clock

drivers. These pins are biased internally and require no

additional biasing.

Figure 87 shows the differential input return loss curve for the

clock inputs across a frequency range of 100 MHz to 6 GHz.

The reference impedance is 100 Ω.

5.0

–5.0

2.0

1.0

–1.0

FREQUENCY (100MHz TO 10GHz)

–2.0

0.5

–0.5

0.2

–0.2

FREQUENCY = 2.001GHz

SDD11 = 0.274/–156.496

IMPEDANCE = Z0 × (0.586 – j0.139)

FREQUENCY = 2.602GHz

SDD11 = 0.319/–176.549

IMPEDANCE = Z0 × (0.516 – j0.022)

FREQUENCY = 2.996GHz

SDD11 = 0.337/169.383

IMPEDANCE = Z0 × (0.499 – j0.070)

FREQUENCY = 4.001GHz

SDD11 = 0.360/139.617

IMPEDANCE = Z0 × (0.518 + j0.278)

FREQUENCY = 5.202GHz

SDD11 = 0.364/139.617

IMPEDANCE = Z0 × (0.761 + j0.639)

Figure 87. Differential Input Return Loss for the CLK± Inputs

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