AD-DPGIOZ datasheet(36/61 Pages) AD

36 / 61 page

AD9961/AD9963

Data Sheet

Rev. A | Page 36 of 60

200Ω

1kΩ

200Ω

33Ω

0.1µF

ADA4937

VOCM

RXIP

RXIN

AD9961/

AD9963

0.1µF

VCC

+VIN

–VIN

Figure 41. Differential Input Configuration, AC-Coupled

The output common-mode voltage of the ADA4937 is set to

match the common-mode voltage required by the ADC by

connecting the RXCML output to the VOCM input of the

amplifier. The RXCML output nominally is at 1.4 V. Bypassing

the RXCML output to analog ground maintains the stability of

the output buffer and lowers the noise.

200Ω

33Ω

0.1µF

ADA4937

VOCM

RXIP

RXIN

AD9961/

AD9963

+VIN

–VIN

RXCML

Figure 42. Differential Input Configuration, DC-Coupled

At higher input frequencies, the amplifiers required to maintain

the full dynamic power of the AD9963 requires considerable

supply current. For higher frequency power sensitive applications,

differential transformer coupling is the recommended input

configuration. The signal characteristics must be considered

when selecting a transformer. Most RF transformers saturate at

frequencies below a few megahertz, and excessive signal power

can also cause core saturation, which leads to distortion.

In any configuration, the value of the shunt capacitor, C, is

dependent on the input frequency and may need to be reduced

or removed.

*CDIFF IS OPTIONAL.

1.25V p-p

33Ω

*CDIFF

50Ω

0.1μF

ADT1-1WT

1:1 Z RATIO

ADC

AD9963

RXIP

0.1µF

RXIN

Figure 43. Differential Transformer—Coupled Configuration

Single-Ended Input Configuration

Driving the Rx inputs with a single-ended signal typically limits

the achievable ADC performance. When using this configuration,

best performance is achieved by maintaining a balanced

impedance off each of the Rx inputs as shown in Figure 44.

*CDIFF IS OPTIONAL.

1.25V p-p

33Ω

49.9Ω

33Ω

25Ω

*CDIFF

49.9Ω

0.1μF

ADC

AD9963

RXIP

0.1µF

RXIN

Figure 44. Single-Ended Input Configuration

Interfacing to the ADF4602 Rx Baseband Outputs

The ADF4602 is an RF transceiver suitable for femtocell and

other wireless communications applications. The ADF4602

Rx baseband outputs have a nominal output common-mode

voltage that can be set to 1.4 V. The ADF4602 can be dc-

coupled to the AD9963. It is recommended that a first-order

low-pass filter be placed between the two devices to reject

unwanted high frequency signals that may alias into the desired

baseband signal.

100Ω

68pF

ADC

AD9963

ADF4602

RXIP

RXIN

RXBBI

RXBBIB

Figure 45. ADF4602 to AD9963 Receive Interface Circuit

In this application, the ADF4602 is setting the common-mode

input voltage of the AD9963 ADCs. The input common-mode

buffer of the AD9963 should be disabled (set Register 0x7E,

Bit 1 = 1) to avoid contention with the ADF4602 output driver.

DECIMATION FILTER AND DIGITAL OFFSET

Decimation Filter

The I and Q receive paths each have a bypassable 2× decimating

low-pass filter. The half-band digital filter reduces the output

sample rate by a factor of 2 while rejecting aliases that fall into

the band of interest. These low-pass filters provide >7 dB of

stop-band rejection for 40% of the output data rate. When used

with quadrature signals, the complex output band is 80% of the

quadrature output data rate. A graph of the pass-band response

of the decimation filter is shown in Figure 46.

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